Java Design Patterns v1.21.0

* fix must override a superclass method

* fix must override a superclass method

.sonarcloud.properties

@@ -0,0 +1,15 @@
+# Path to sources
+#sonar.sources=.
+#sonar.exclusions=
+#sonar.inclusions=
+
+# Path to tests
+#sonar.tests=
+#sonar.test.exclusions=
+#sonar.test.inclusions=
+
+# Source encoding
+#sonar.sourceEncoding=UTF-8
+
+# Exclusions for copy-paste detection
+#sonar.cpd.exclusions=

.travis.yml

@@ -1,4 +1,5 @@
 language: java
+dist: trusty   # Xenial build environment won't allow installation of Java 8
 jdk:
 - oraclejdk8
 

CODE_COVERAGE.md

@@ -1,13 +1,13 @@
 # Code Coverage Report generation
 
 To generate the code coverage report, execute the following command:
-> mvn clean verify
+> mvn clean verify jacoco:report
 
 This will generate code coverage report in each of the modules. In order to view the same, open the following file in your browser.
 > target/site/jacoco/index.html 
 
 Please note that the above folder is created under each of the modules. For example:
 * adapter/target/site/jacoco/index.html 
-* busniess-delegate/target/site/jacoco/index.html 
+* business-delegate/target/site/jacoco/index.html 
 
 

README.md

@@ -7,18 +7,17 @@
 [![Build status](https://travis-ci.org/iluwatar/java-design-patterns.svg?branch=master)](https://travis-ci.org/iluwatar/java-design-patterns)
 [![License MIT](https://img.shields.io/badge/license-MIT-blue.svg)](https://raw.githubusercontent.com/iluwatar/java-design-patterns/master/LICENSE.md)
 [![Join the chat at https://gitter.im/iluwatar/java-design-patterns](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/iluwatar/java-design-patterns?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
-[![Quality Gate](https://sonarcloud.io/api/project_badges/measure?project=com.iluwatar%3Ajava-design-patterns&metric=alert_status)](https://sonarcloud.io/dashboard/index/com.iluwatar%3Ajava-design-patterns)
-[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/1503/badge)](https://bestpractices.coreinfrastructure.org/projects/1503)
+[![Sonarcloud Status](https://sonarcloud.io/api/project_badges/measure?project=iluwatar_java-design-patterns&metric=alert_status)](https://sonarcloud.io/dashboard?id=iluwatar_java-design-patterns) 
 
 # Introduction
 
-Design patterns are formalized best practices that the programmer can use to
+Design patterns are the best formalized practices a programmer can use to
 solve common problems when designing an application or system.
 
 Design patterns can speed up the development process by providing tested, proven
 development paradigms.
 
-Reusing design patterns helps to prevent subtle issues that can cause major
+Reusing design patterns help prevent subtle issues which cause major
 problems, and it also improves code readability for coders and architects who
 are familiar with the patterns.
 

abstract-document/pom.xml

@@ -29,19 +29,14 @@
     <parent>
         <artifactId>java-design-patterns</artifactId>
         <groupId>com.iluwatar</groupId>
-        <version>1.19.0</version>
+        <version>1.21.0</version>
     </parent>
     <artifactId>abstract-document</artifactId>
     <dependencies>
-        <dependency>
-            <groupId>org.junit.jupiter</groupId>
-            <artifactId>junit-jupiter-api</artifactId>
-            <scope>test</scope>
-        </dependency>
         <dependency>
             <groupId>org.junit.jupiter</groupId>
             <artifactId>junit-jupiter-engine</artifactId>
             <scope>test</scope>
         </dependency>
     </dependencies>
-</project>
+</project>

abstract-document/src/main/java/com/iluwatar/abstractdocument/AbstractDocument.java

@@ -54,17 +54,16 @@ public abstract class AbstractDocument implements Document {
 
   @Override
   public <T> Stream<T> children(String key, Function<Map<String, Object>, T> constructor) {
-    Optional<List<Map<String, Object>>> any = Stream.of(get(key)).filter(el -> el != null)
+    Optional<List<Map<String, Object>>> any = Stream.of(get(key)).filter(Objects::nonNull)
         .map(el -> (List<Map<String, Object>>) el).findAny();
-    return any.isPresent() ? any.get().stream().map(constructor) : Stream.empty();
+    return any.map(maps -> maps.stream().map(constructor)).orElseGet(Stream::empty);
   }
 
   @Override
   public String toString() {
     StringBuilder builder = new StringBuilder();
     builder.append(getClass().getName()).append("[");
-    properties.entrySet()
-        .forEach(e -> builder.append("[").append(e.getKey()).append(" : ").append(e.getValue()).append("]"));
+    properties.forEach((key, value) -> builder.append("[").append(key).append(" : ").append(value).append("]"));
     builder.append("]");
     return builder.toString();
   }

abstract-document/src/main/java/com/iluwatar/abstractdocument/App.java

@@ -22,18 +22,16 @@
  */
 package com.iluwatar.abstractdocument;
 
-import com.iluwatar.abstractdocument.domain.Car;
-import com.iluwatar.abstractdocument.domain.HasModel;
-import com.iluwatar.abstractdocument.domain.HasParts;
-import com.iluwatar.abstractdocument.domain.HasPrice;
-import com.iluwatar.abstractdocument.domain.HasType;
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
-
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.Map;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import com.iluwatar.abstractdocument.domain.Car;
+import com.iluwatar.abstractdocument.domain.enums.Property;
+
 /**
  * The Abstract Document pattern enables handling additional, non-static
  * properties. This pattern uses concept of traits to enable type safety and
@@ -55,20 +53,20 @@ public class App {
     LOGGER.info("Constructing parts and car");
 
     Map<String, Object> carProperties = new HashMap<>();
-    carProperties.put(HasModel.PROPERTY, "300SL");
-    carProperties.put(HasPrice.PROPERTY, 10000L);
+    carProperties.put(Property.MODEL.toString(), "300SL");
+    carProperties.put(Property.PRICE.toString(), 10000L);
 
     Map<String, Object> wheelProperties = new HashMap<>();
-    wheelProperties.put(HasType.PROPERTY, "wheel");
-    wheelProperties.put(HasModel.PROPERTY, "15C");
-    wheelProperties.put(HasPrice.PROPERTY, 100L);
+    wheelProperties.put(Property.TYPE.toString(), "wheel");
+    wheelProperties.put(Property.MODEL.toString(), "15C");
+    wheelProperties.put(Property.PRICE.toString(), 100L);
 
     Map<String, Object> doorProperties = new HashMap<>();
-    doorProperties.put(HasType.PROPERTY, "door");
-    doorProperties.put(HasModel.PROPERTY, "Lambo");
-    doorProperties.put(HasPrice.PROPERTY, 300L);
+    doorProperties.put(Property.TYPE.toString(), "door");
+    doorProperties.put(Property.MODEL.toString(), "Lambo");
+    doorProperties.put(Property.PRICE.toString(), 300L);
 
-    carProperties.put(HasParts.PROPERTY, Arrays.asList(wheelProperties, doorProperties));
+    carProperties.put(Property.PARTS.toString(), Arrays.asList(wheelProperties, doorProperties));
 
     Car car = new Car(carProperties);
 

abstract-document/src/main/java/com/iluwatar/abstractdocument/domain/HasModel.java

@@ -25,16 +25,15 @@ package com.iluwatar.abstractdocument.domain;
 import java.util.Optional;
 
 import com.iluwatar.abstractdocument.Document;
+import com.iluwatar.abstractdocument.domain.enums.Property;
 
 /**
  * HasModel trait for static access to 'model' property
  */
 public interface HasModel extends Document {
 
-  String PROPERTY = "model";
-
   default Optional<String> getModel() {
-    return Optional.ofNullable((String) get(PROPERTY));
+    return Optional.ofNullable((String) get(Property.MODEL.toString()));
   }
 
 }

abstract-document/src/main/java/com/iluwatar/abstractdocument/domain/HasParts.java

@@ -25,16 +25,16 @@ package com.iluwatar.abstractdocument.domain;
 import java.util.stream.Stream;
 
 import com.iluwatar.abstractdocument.Document;
+import com.iluwatar.abstractdocument.domain.enums.Property;
 
 /**
  * HasParts trait for static access to 'parts' property
  */
 public interface HasParts extends Document {
 
-  String PROPERTY = "parts";
 
   default Stream<Part> getParts() {
-    return children(PROPERTY, Part::new);
+    return children(Property.PARTS.toString(), Part::new);
   }
 
 }

abstract-document/src/main/java/com/iluwatar/abstractdocument/domain/HasPrice.java

@@ -25,16 +25,16 @@ package com.iluwatar.abstractdocument.domain;
 import java.util.Optional;
 
 import com.iluwatar.abstractdocument.Document;
+import com.iluwatar.abstractdocument.domain.enums.Property;
 
 /**
  * HasPrice trait for static access to 'price' property
  */
 public interface HasPrice extends Document {
 
-  String PROPERTY = "price";
 
   default Optional<Number> getPrice() {
-    return Optional.ofNullable((Number) get(PROPERTY));
+    return Optional.ofNullable((Number) get(Property.PRICE.toString()));
   }
 
 }

abstract-document/src/main/java/com/iluwatar/abstractdocument/domain/HasType.java

@@ -22,19 +22,19 @@
  */
 package com.iluwatar.abstractdocument.domain;
 
-import com.iluwatar.abstractdocument.Document;
-
 import java.util.Optional;
 
+import com.iluwatar.abstractdocument.Document;
+import com.iluwatar.abstractdocument.domain.enums.Property;
+
 /**
  * HasType trait for static access to 'type' property
  */
 public interface HasType extends Document {
 
-  String PROPERTY = "type";
 
   default Optional<String> getType() {
-    return Optional.ofNullable((String) get(PROPERTY));
+    return Optional.ofNullable((String) get(Property.TYPE.toString()));
   }
 
 }

abstract-document/src/main/java/com/iluwatar/abstractdocument/domain/enums/Property.java

@@ -0,0 +1,33 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.abstractdocument.domain.enums;
+
+/**
+ * 
+ * Enum To Describe Property type
+ *
+ */
+public enum Property {
+
+  PARTS, TYPE, PRICE, MODEL
+}

abstract-document/src/test/java/com/iluwatar/abstractdocument/AbstractDocumentTest.java

@@ -32,6 +32,7 @@ import java.util.stream.Stream;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 /**
  * AbstractDocument test class
@@ -81,8 +82,8 @@ public class AbstractDocumentTest {
     Map<String, Object> props = new HashMap<>();
     props.put(KEY, VALUE);
     DocumentImplementation document = new DocumentImplementation(props);
-    assertNotNull(document.toString().contains(KEY));
-    assertNotNull(document.toString().contains(VALUE));
+    assertTrue(document.toString().contains(KEY));
+    assertTrue(document.toString().contains(VALUE));
   }
 
 }

abstract-document/src/test/java/com/iluwatar/abstractdocument/DomainTest.java

@@ -22,19 +22,17 @@
  */
 package com.iluwatar.abstractdocument;
 
-import com.iluwatar.abstractdocument.domain.Car;
-import com.iluwatar.abstractdocument.domain.HasModel;
-import com.iluwatar.abstractdocument.domain.HasParts;
-import com.iluwatar.abstractdocument.domain.HasPrice;
-import com.iluwatar.abstractdocument.domain.HasType;
-import com.iluwatar.abstractdocument.domain.Part;
-import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.assertEquals;
 
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.Map;
 
-import static org.junit.jupiter.api.Assertions.assertEquals;
+import org.junit.jupiter.api.Test;
+
+import com.iluwatar.abstractdocument.domain.Car;
+import com.iluwatar.abstractdocument.domain.Part;
+import com.iluwatar.abstractdocument.domain.enums.Property;
 
 /**
  * Test for Part and Car
@@ -51,9 +49,9 @@ public class DomainTest {
   @Test
   public void shouldConstructPart() {
     Map<String, Object> partProperties = new HashMap<>();
-    partProperties.put(HasType.PROPERTY, TEST_PART_TYPE);
-    partProperties.put(HasModel.PROPERTY, TEST_PART_MODEL);
-    partProperties.put(HasPrice.PROPERTY, TEST_PART_PRICE);
+    partProperties.put(Property.TYPE.toString(), TEST_PART_TYPE);
+    partProperties.put(Property.MODEL.toString(), TEST_PART_MODEL);
+    partProperties.put(Property.PRICE.toString(), TEST_PART_PRICE);
     Part part = new Part(partProperties);
 
     assertEquals(TEST_PART_TYPE, part.getType().get());
@@ -64,9 +62,9 @@ public class DomainTest {
   @Test
   public void shouldConstructCar() {
     Map<String, Object> carProperties = new HashMap<>();
-    carProperties.put(HasModel.PROPERTY, TEST_CAR_MODEL);
-    carProperties.put(HasPrice.PROPERTY, TEST_CAR_PRICE);
-    carProperties.put(HasParts.PROPERTY, Arrays.asList(new HashMap<>(), new HashMap<>()));
+    carProperties.put(Property.MODEL.toString(), TEST_CAR_MODEL);
+    carProperties.put(Property.PRICE.toString(), TEST_CAR_PRICE);
+    carProperties.put(Property.PARTS.toString(), Arrays.asList(new HashMap<>(), new HashMap<>()));
     Car car = new Car(carProperties);
 
     assertEquals(TEST_CAR_MODEL, car.getModel().get());

abstract-factory/etc/presentation.html

@@ -181,7 +181,7 @@ Use the Abstract Factory pattern when:
 * Source code http://java-design-patterns.com/patterns/abstract-factory/
 
     </textarea>
-    <script src="https://gnab.github.io/remark/downloads/remark-latest.min.js">
+    <script src="https://remarkjs.com/downloads/remark-latest.min.js">
     </script>
     <script>
       var slideshow = remark.create();

abstract-factory/pom.xml

@@ -29,15 +29,10 @@
   <parent>
     <groupId>com.iluwatar</groupId>
     <artifactId>java-design-patterns</artifactId>
-    <version>1.19.0</version>
+    <version>1.21.0</version>
   </parent>
   <artifactId>abstract-factory</artifactId>
   <dependencies>
-    <dependency>
-      <groupId>org.junit.jupiter</groupId>
-      <artifactId>junit-jupiter-api</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.junit.jupiter</groupId>
       <artifactId>junit-jupiter-engine</artifactId>

abstract-factory/src/main/java/com/iluwatar/abstractfactory/ElfKingdomFactory.java

@@ -29,14 +29,17 @@ package com.iluwatar.abstractfactory;
  */
 public class ElfKingdomFactory implements KingdomFactory {
 
+  @Override
   public Castle createCastle() {
     return new ElfCastle();
   }
 
+  @Override
   public King createKing() {
     return new ElfKing();
   }
 
+  @Override
   public Army createArmy() {
     return new ElfArmy();
   }

abstract-factory/src/main/java/com/iluwatar/abstractfactory/OrcKingdomFactory.java

@@ -29,14 +29,17 @@ package com.iluwatar.abstractfactory;
  */
 public class OrcKingdomFactory implements KingdomFactory {
 
+  @Override
   public Castle createCastle() {
     return new OrcCastle();
   }
 
+  @Override
   public King createKing() {
     return new OrcKing();
   }
 
+  @Override
   public Army createArmy() {
     return new OrcArmy();
   }

acyclic-visitor/README.md

@@ -0,0 +1,39 @@
+---
+layout: pattern
+title: Acyclic Visitor
+folder: acyclic-visitor
+permalink: /patterns/acyclic-visitor/
+categories: Behavioral
+tags:
+ - Java
+ - Difficulty-Intermediate
+---
+
+![alt text](./etc/acyclic-visitor.png "Acyclic Visitor")
+
+## Intent
+Allow new functions to be added to existing class hierarchies without affecting those hierarchies, and without creating the troublesome dependency cycles that are inherent to the GOF VISITOR Pattern.
+
+## Applicability
+This pattern can be used:
+* When you need to add a new function to an existing hierarchy without the need to alter or affect that hierarchy.
+* When there are functions that operate upon a hierarchy, but which do not belong in the hierarchy itself. e.g. the ConfigureForDOS / ConfigureForUnix / ConfigureForX issue.
+* When you need to perform very different operations on an object depending upon its type.
+* When the visited class hierarchy will be frequently extended with new derivatives of the Element class.
+* When the recompilation, relinking, retesting or redistribution of the derivatives of Element is very expensive.
+
+## Consequences
+The good:
+* No dependency cycles between class hierarchies.
+* No need to recompile all the visitors if a new one is added.
+* Does not cause compilation failure in existing visitors if class hierarchy has a new member.
+
+The bad:
+* Violates the principle of least surprise or Liskov's Substitution principle by showing that it can accept all visitors but actually only being interested in particular visitors.
+* Parallel hierarchy of visitors has to be created for all members in visitable class hierarchy.
+
+## Related patterns
+* [Visitor Pattern](../visitor/README.md)
+
+## Credits
+* [Acyclic Visitor](http://condor.depaul.edu/dmumaugh/OOT/Design-Principles/acv.pdf)

acyclic-visitor/etc/Acyclic Visitor.ucls

@@ -0,0 +1,115 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<class-diagram version="1.2.2" icons="true" always-add-relationships="false" generalizations="true" realizations="true" 
+  associations="true" dependencies="false" nesting-relationships="true" router="FAN">  
+  <interface id="1" language="java" name="com.iluwatar.acyclicvisitor.ModemVisitor" project="acyclic-visitor" 
+    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ModemVisitor.java" binary="false" 
+    corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="860" y="67"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </interface>  
+  <class id="2" language="java" name="com.iluwatar.acyclicvisitor.Modem" project="acyclic-visitor" 
+    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/Modem.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="327" y="77"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="3" language="java" name="com.iluwatar.acyclicvisitor.ConfigureForUnixVisitor" project="acyclic-visitor" 
+    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ConfigureForUnixVisitor.java" binary="false" 
+    corner="BOTTOM_RIGHT">    
+    <position height="124" width="196" x="647" y="225"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="4" language="java" name="com.iluwatar.acyclicvisitor.Zoom" project="acyclic-visitor" 
+    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/Zoom.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="203" y="305"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <interface id="5" language="java" name="com.iluwatar.acyclicvisitor.HayesVisitor" project="acyclic-visitor" 
+    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/HayesVisitor.java" binary="false" 
+    corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="1019" y="468"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </interface>  
+  <interface id="6" language="java" name="com.iluwatar.acyclicvisitor.ZoomVisitor" project="acyclic-visitor" 
+    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ZoomVisitor.java" binary="false" 
+    corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="758" y="467"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </interface>  
+  <class id="7" language="java" name="com.iluwatar.acyclicvisitor.Hayes" project="acyclic-visitor" 
+    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/Hayes.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="479" y="307"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="8" language="java" name="com.iluwatar.acyclicvisitor.ConfigureForDosVisitor" project="acyclic-visitor" 
+    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ConfigureForDosVisitor.java" binary="false" 
+    corner="BOTTOM_RIGHT">    
+    <position height="142" width="192" x="883" y="225"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <generalization id="9">    
+    <end type="SOURCE" refId="7"/>    
+    <end type="TARGET" refId="2"/>  
+  </generalization>  
+  <realization id="10">    
+    <end type="SOURCE" refId="8"/>    
+    <end type="TARGET" refId="6"/>  
+  </realization>  
+  <realization id="11">    
+    <end type="SOURCE" refId="8"/>    
+    <end type="TARGET" refId="5"/>  
+  </realization>  
+  <realization id="12">    
+    <end type="SOURCE" refId="3"/>    
+    <end type="TARGET" refId="6"/>  
+  </realization>  
+  <realization id="13">    
+    <end type="SOURCE" refId="3"/>    
+    <end type="TARGET" refId="1"/>  
+  </realization>  
+  <realization id="14">    
+    <end type="SOURCE" refId="8"/>    
+    <end type="TARGET" refId="1"/>  
+  </realization>  
+  <generalization id="15">    
+    <end type="SOURCE" refId="4"/>    
+    <end type="TARGET" refId="2"/>  
+  </generalization>  
+  <classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+    sort-features="false" accessors="true" visibility="true">    
+    <attributes public="true" package="true" protected="true" private="true" static="true"/>    
+    <operations public="true" package="true" protected="true" private="true" static="true"/>  
+  </classifier-display>  
+  <association-display labels="true" multiplicity="true"/>
+</class-diagram>

acyclic-visitor/pom.xml

@@ -0,0 +1,71 @@
+<!--
+
+    The MIT License
+    Copyright (c) 2014 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+	
+	<parent>
+		<groupId>com.iluwatar</groupId>
+		<artifactId>java-design-patterns</artifactId>
+		<version>1.21.0</version>
+	</parent>
+	
+	<artifactId>acyclic-visitor</artifactId>
+
+	<properties>
+		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+		<maven.compiler.source>1.8</maven.compiler.source>
+		<maven.compiler.target>1.8</maven.compiler.target>
+	</properties>
+
+	<dependencies>
+		<!-- https://mvnrepository.com/artifact/org.assertj/assertj-core -->
+		<dependency>
+			<groupId>org.assertj</groupId>
+			<artifactId>assertj-core</artifactId>
+			<version>3.9.1</version>
+			<scope>test</scope>
+		</dependency>
+		<!-- https://mvnrepository.com/artifact/uk.org.lidalia/slf4j-test -->
+		<dependency>
+			<groupId>uk.org.lidalia</groupId>
+			<artifactId>slf4j-test</artifactId>
+			<version>1.0.0</version>
+			<scope>test</scope>
+		</dependency>
+    	<dependency>
+      		<groupId>org.junit.jupiter</groupId>
+      		<artifactId>junit-jupiter-engine</artifactId>
+      		<scope>test</scope>
+    	</dependency>
+		<dependency>
+			<groupId>org.mockito</groupId>
+			<artifactId>mockito-all</artifactId>
+			<version>1.9.5</version>
+			<scope>test</scope>
+		</dependency>
+	</dependencies>
+</project>

acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/AllModemVisitor.java

@@ -0,0 +1,31 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.acyclicvisitor;
+
+/**
+ * All ModemVisitor interface extends all visitor interfaces. This interface
+ * provides ease of use when a visitor needs to visit all modem types.
+ */
+public interface AllModemVisitor extends ZoomVisitor, HayesVisitor{
+
+}

acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/App.java

@@ -0,0 +1,55 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.acyclicvisitor;
+
+/**
+ * The Acyclic Visitor pattern allows new functions to be added to existing class 
+ * hierarchies without affecting those hierarchies, and without creating the dependency 
+ * cycles that are inherent to the GoF Visitor pattern, by making the Visitor base class 
+ * degenerate
+ * <p>
+ * In this example the visitor base class is {@link ModemVisitor}. The base class of the 
+ * visited hierarchy is {@link Modem} and has two children {@link Hayes} and {@link Zoom} 
+ * each one having its own visitor interface {@link HayesVisitor} and {@link ZoomVisitor} 
+ * respectively. {@link ConfigureForUnixVisitor} and {@link ConfigureForDosVisitor} 
+ * implement each derivative's visit method only if it is required 
+ */
+public class App {
+  
+  /**
+   * Program's entry point
+   */
+  
+  public static void main(String[] args) {  
+    ConfigureForUnixVisitor conUnix = new ConfigureForUnixVisitor();
+    ConfigureForDosVisitor conDos = new ConfigureForDosVisitor();
+    
+    Zoom zoom = new Zoom();
+    Hayes hayes = new Hayes();
+    
+    hayes.accept(conDos); // Hayes modem with Dos configurator
+    zoom.accept(conDos); // Zoom modem with Dos configurator
+    hayes.accept(conUnix); // Hayes modem with Unix configurator
+    zoom.accept(conUnix); // Zoom modem with Unix configurator   
+  }
+}

acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ConfigureForDosVisitor.java

@@ -0,0 +1,45 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.acyclicvisitor;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * ConfigureForDosVisitor class implements both zoom's and  hayes' visit method 
+ * for Dos manufacturer
+ */
+public class ConfigureForDosVisitor implements AllModemVisitor {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(ConfigureForDosVisitor.class);
+
+  @Override
+  public void visit(Hayes hayes) {
+    LOGGER.info(hayes + " used with Dos configurator.");
+  }
+
+  @Override
+  public void visit(Zoom zoom) {
+    LOGGER.info(zoom + " used with Dos configurator.");
+  }
+}

acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ConfigureForUnixVisitor.java

@@ -0,0 +1,41 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.acyclicvisitor;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * ConfigureForUnixVisitor class implements zoom's visit method for Unix 
+ * manufacturer, unlike traditional visitor pattern, this class may selectively implement
+ * visit for other modems.
+ */
+public class ConfigureForUnixVisitor implements ZoomVisitor {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(ConfigureForUnixVisitor.class);
+
+  @Override
+  public void visit(Zoom zoom) {
+    LOGGER.info(zoom + " used with Unix configurator.");
+  }
+}

acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/Hayes.java

@@ -0,0 +1,56 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.acyclicvisitor;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Hayes class implements its accept method
+ */
+public class Hayes extends Modem {
+  
+  private static final Logger LOGGER = LoggerFactory.getLogger(ConfigureForDosVisitor.class);
+
+  /**
+   * Accepts all visitors but honors only HayesVisitor
+   */
+  @Override
+  public void accept(ModemVisitor modemVisitor) {
+    if (modemVisitor instanceof HayesVisitor) {
+      ((HayesVisitor) modemVisitor).visit(this);
+    } else {
+      LOGGER.info("Only HayesVisitor is allowed to visit Hayes modem");
+    }
+
+  }
+  
+  /**
+   * Hayes' modem's toString
+   * method
+   */
+  @Override
+  public String toString() {
+    return "Hayes modem";
+  }
+}

acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/HayesVisitor.java

@@ -0,0 +1,30 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.acyclicvisitor;
+
+/**
+ * HayesVisitor interface
+ */
+public interface HayesVisitor extends ModemVisitor {
+  void visit(Hayes hayes);
+}

acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/Modem.java

@@ -0,0 +1,30 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.acyclicvisitor;
+
+/**
+ * Modem abstract class
+ */
+public abstract class Modem {
+  public abstract void accept(ModemVisitor modemVisitor);
+}

acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ModemVisitor.java

@@ -0,0 +1,32 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.acyclicvisitor;
+
+/**
+ * ModemVisitor interface does not contain any visit methods so that it does not
+ * depend on the visited hierarchy. Each derivative's visit method is declared in 
+ * its own visitor interface
+ */
+public interface ModemVisitor {
+  // Visitor is a degenerate base class for all visitors.
+}

acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/Zoom.java

@@ -0,0 +1,55 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.acyclicvisitor;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Zoom class implements its accept method
+ */
+public class Zoom extends Modem {
+  
+  private static final Logger LOGGER = LoggerFactory.getLogger(ConfigureForDosVisitor.class);
+
+  /**
+   * Accepts all visitors but honors only ZoomVisitor
+   */
+  @Override
+  public void accept(ModemVisitor modemVisitor) {
+    if (modemVisitor instanceof  ZoomVisitor) {
+      ((ZoomVisitor) modemVisitor).visit(this);
+    } else {
+      LOGGER.info("Only ZoomVisitor is allowed to visit Zoom modem");
+    }
+  }
+  
+  /**
+   * Zoom modem's toString
+   * method
+   */
+  @Override
+  public String toString() {
+    return "Zoom modem";
+  }
+}

acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ZoomVisitor.java

@@ -20,16 +20,11 @@
  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  * THE SOFTWARE.
  */
-package com.iluwatar.iterator;
+package com.iluwatar.acyclicvisitor;
 
 /**
- * 
- * ItemIterator interface.
- * 
+ * ZoomVisitor interface
  */
-public interface ItemIterator {
-
-  boolean hasNext();
-
-  Item next();
+public interface ZoomVisitor extends ModemVisitor {
+  void visit(Zoom zoom);
 }

acyclic-visitor/src/test/java/com/iluwatar/acyclicvisitor/AppTest.java

@@ -0,0 +1,39 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.acyclicvisitor;
+
+import org.junit.jupiter.api.Test;
+
+import com.iluwatar.acyclicvisitor.App;
+
+/**
+ * Tests that the Acyclic Visitor example runs without errors.
+ */
+public class AppTest {
+  
+  @Test
+  public void test() {
+    String[] args = {};
+    App.main(args);  
+  }
+}

acyclic-visitor/src/test/java/com/iluwatar/acyclicvisitor/ConfigureForDosVisitorTest.java

@@ -0,0 +1,75 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.acyclicvisitor;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.groups.Tuple.tuple;
+import static org.mockito.Mockito.mock;
+import static uk.org.lidalia.slf4jext.Level.INFO;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Test;
+
+import com.iluwatar.acyclicvisitor.ConfigureForDosVisitor;
+import com.iluwatar.acyclicvisitor.Hayes;
+import com.iluwatar.acyclicvisitor.HayesVisitor;
+import com.iluwatar.acyclicvisitor.Zoom;
+import com.iluwatar.acyclicvisitor.ZoomVisitor;
+
+import uk.org.lidalia.slf4jtest.TestLogger;
+import uk.org.lidalia.slf4jtest.TestLoggerFactory;
+
+/**
+ * ConfigureForDosVisitor test class
+ */
+public class ConfigureForDosVisitorTest {
+
+  TestLogger logger = TestLoggerFactory.getTestLogger(ConfigureForDosVisitor.class);
+  
+  @Test
+  public void testVisitForZoom() {    
+    ConfigureForDosVisitor conDos = new ConfigureForDosVisitor();
+    Zoom zoom = new Zoom();
+    
+    conDos.visit(zoom);
+    
+    assertThat(logger.getLoggingEvents()).extracting("level", "message").contains(
+        tuple(INFO, zoom + " used with Dos configurator."));
+  }
+  
+  @Test
+  public void testVisitForHayes() {
+    ConfigureForDosVisitor conDos = new ConfigureForDosVisitor();
+    Hayes hayes = new Hayes();
+    
+    conDos.visit(hayes);
+    
+    assertThat(logger.getLoggingEvents()).extracting("level", "message").contains(
+        tuple(INFO, hayes + " used with Dos configurator."));
+  }
+  
+  @AfterEach
+  public void clearLoggers() {
+    TestLoggerFactory.clear();
+  }
+}

acyclic-visitor/src/test/java/com/iluwatar/acyclicvisitor/ConfigureForUnixVisitorTest.java

@@ -0,0 +1,57 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.acyclicvisitor;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.groups.Tuple.tuple;
+import static uk.org.lidalia.slf4jext.Level.INFO;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Test;
+
+import uk.org.lidalia.slf4jtest.TestLogger;
+import uk.org.lidalia.slf4jtest.TestLoggerFactory;
+
+/**
+ * ConfigureForUnixVisitor test class
+ */
+public class ConfigureForUnixVisitorTest {
+  
+  private static final TestLogger LOGGER = TestLoggerFactory.getTestLogger(ConfigureForUnixVisitor.class);
+  
+  @AfterEach
+  public void clearLoggers() {
+    TestLoggerFactory.clear();
+  }
+  
+  @Test
+  public void testVisitForZoom() {
+    ConfigureForUnixVisitor conUnix = new ConfigureForUnixVisitor();
+    Zoom zoom = new Zoom();
+    
+    conUnix.visit(zoom);
+    
+    assertThat(LOGGER.getLoggingEvents()).extracting("level", "message").contains(
+        tuple(INFO, zoom + " used with Unix configurator."));
+  }
+}

acyclic-visitor/src/test/java/com/iluwatar/acyclicvisitor/HayesTest.java

@@ -0,0 +1,60 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.acyclicvisitor;
+
+import static org.mockito.Matchers.eq;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.verifyZeroInteractions;
+
+import org.junit.jupiter.api.Test;
+
+import com.iluwatar.acyclicvisitor.ConfigureForDosVisitor;
+import com.iluwatar.acyclicvisitor.ConfigureForUnixVisitor;
+import com.iluwatar.acyclicvisitor.Hayes;
+import com.iluwatar.acyclicvisitor.HayesVisitor;
+
+/**
+ * Hayes test class
+ */
+public class HayesTest {
+
+  @Test
+  public void testAcceptForDos() {  
+    Hayes hayes = new Hayes();
+    ConfigureForDosVisitor mockVisitor = mock(ConfigureForDosVisitor.class);
+    
+    hayes.accept(mockVisitor);
+    verify((HayesVisitor)mockVisitor).visit(eq(hayes));
+  }
+  
+  @Test
+  public void testAcceptForUnix() {    
+    Hayes hayes = new Hayes();
+    ConfigureForUnixVisitor mockVisitor = mock(ConfigureForUnixVisitor.class);
+    
+    hayes.accept(mockVisitor);
+    
+    verifyZeroInteractions(mockVisitor);
+  }
+}

acyclic-visitor/src/test/java/com/iluwatar/acyclicvisitor/ZoomTest.java

@@ -0,0 +1,59 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.acyclicvisitor;
+
+
+import static org.mockito.Matchers.eq;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.mock;
+
+import org.junit.jupiter.api.Test;
+
+import com.iluwatar.acyclicvisitor.ConfigureForDosVisitor;
+import com.iluwatar.acyclicvisitor.ConfigureForUnixVisitor;
+import com.iluwatar.acyclicvisitor.Zoom;
+import com.iluwatar.acyclicvisitor.ZoomVisitor;
+
+/**
+ * Zoom test class
+ */
+public class ZoomTest {
+  
+  @Test
+  public void testAcceptForDos() {  
+    Zoom zoom = new Zoom();
+    ConfigureForDosVisitor mockVisitor = mock(ConfigureForDosVisitor.class);
+    
+    zoom.accept(mockVisitor);
+    verify((ZoomVisitor)mockVisitor).visit(eq(zoom));
+  }
+  
+  @Test
+  public void testAcceptForUnix() {
+    Zoom zoom = new Zoom();
+    ConfigureForUnixVisitor mockVisitor = mock(ConfigureForUnixVisitor.class);
+    
+    zoom.accept(mockVisitor);
+    verify((ZoomVisitor)mockVisitor).visit(eq(zoom));
+  }
+}

adapter/pom.xml

@@ -29,15 +29,10 @@
   <parent>
     <groupId>com.iluwatar</groupId>
     <artifactId>java-design-patterns</artifactId>
-    <version>1.19.0</version>
+    <version>1.21.0</version>
   </parent>
   <artifactId>adapter</artifactId>
   <dependencies>
-    <dependency>
-      <groupId>org.junit.jupiter</groupId>
-      <artifactId>junit-jupiter-api</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.junit.jupiter</groupId>
       <artifactId>junit-jupiter-engine</artifactId>

aggregator-microservices/aggregator-service/pom.xml

@@ -29,7 +29,7 @@
     <parent>
         <artifactId>aggregator-microservices</artifactId>
         <groupId>com.iluwatar</groupId>
-        <version>1.19.0</version>
+        <version>1.21.0</version>
     </parent>
     <modelVersion>4.0.0</modelVersion>
 
@@ -53,11 +53,6 @@
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-web</artifactId>
         </dependency>
-        <dependency>
-            <groupId>org.junit.jupiter</groupId>
-            <artifactId>junit-jupiter-api</artifactId>
-            <scope>test</scope>
-        </dependency>
         <dependency>
             <groupId>org.junit.jupiter</groupId>
             <artifactId>junit-jupiter-engine</artifactId>
@@ -90,4 +85,4 @@
             </plugin>
         </plugins>
     </build>
-</project>
+</project>

aggregator-microservices/information-microservice/pom.xml

@@ -29,7 +29,7 @@
     <parent>
         <artifactId>aggregator-microservices</artifactId>
         <groupId>com.iluwatar</groupId>
-        <version>1.19.0</version>
+        <version>1.21.0</version>
     </parent>
     <modelVersion>4.0.0</modelVersion>
 
@@ -53,11 +53,6 @@
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-web</artifactId>
         </dependency>
-        <dependency>
-            <groupId>org.junit.jupiter</groupId>
-            <artifactId>junit-jupiter-api</artifactId>
-            <scope>test</scope>
-        </dependency>
         <dependency>
             <groupId>org.junit.jupiter</groupId>
             <artifactId>junit-jupiter-engine</artifactId>

aggregator-microservices/inventory-microservice/pom.xml

@@ -29,7 +29,7 @@
     <parent>
         <artifactId>aggregator-microservices</artifactId>
         <groupId>com.iluwatar</groupId>
-        <version>1.19.0</version>
+        <version>1.21.0</version>
     </parent>
     <modelVersion>4.0.0</modelVersion>
 
@@ -53,11 +53,6 @@
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-web</artifactId>
         </dependency>
-        <dependency>
-            <groupId>org.junit.jupiter</groupId>
-            <artifactId>junit-jupiter-api</artifactId>
-            <scope>test</scope>
-        </dependency>
         <dependency>
             <groupId>org.junit.jupiter</groupId>
             <artifactId>junit-jupiter-engine</artifactId>
@@ -81,4 +76,4 @@
             </plugin>
         </plugins>
     </build>
-</project>
+</project>

aggregator-microservices/inventory-microservice/src/test/java/com/iluwatar/inventory/microservice/InventoryControllerTest.java

@@ -31,7 +31,7 @@ import static org.junit.jupiter.api.Assertions.assertEquals;
  */
 public class InventoryControllerTest {
   @Test
-  public void testGetProductInventories() throws Exception {
+  public void testGetProductInventories() {
     InventoryController inventoryController = new InventoryController();
 
     int numberOfInventories = inventoryController.getProductInventories();

aggregator-microservices/pom.xml

@@ -29,7 +29,7 @@
     <parent>
         <artifactId>java-design-patterns</artifactId>
         <groupId>com.iluwatar</groupId>
-        <version>1.19.0</version>
+        <version>1.21.0</version>
     </parent>
     <modelVersion>4.0.0</modelVersion>
     <artifactId>aggregator-microservices</artifactId>

ambassador/README.md

@@ -0,0 +1,178 @@
+---
+layout: pattern
+title: Ambassador
+folder: ambassador
+permalink: /patterns/ambassador/
+categories: Structural
+tags:
+ - Java
+ - Difficulty-Intermediate
+---
+
+## Intent
+Provide a helper service instance on a client and offload common functionality away from a shared resource.
+
+## Explanation
+Real world example
+
+> A remote service has many clients accessing a function it provides. The service is a legacy application and is impossible to update. Large numbers of requests from users are causing connectivity issues. New rules for request frequency should be implemented along with latency checks and client-side logging.
+
+In plain words
+
+> Using the ambassador pattern, we can implement less-frequent polling from clients along with latency checks and logging.
+
+Microsoft documentation states
+
+> An ambassador service can be thought of as an out-of-process proxy that is co-located with the client.
+  This pattern can be useful for offloading common client connectivity tasks such as monitoring, logging, routing, security (such as TLS), and resiliency patterns in a language agnostic way. It is often used with legacy applications, or other applications that are difficult to modify, in order to extend their networking capabilities. It can also enable a specialized team to implement those features.
+
+**Programmatic Example**
+
+With the above example in mind we will imitate the functionality in a simple manner. We have an interface implemented by the remote service as well as the ambassador service:
+
+```java
+interface RemoteServiceInterface {
+
+    long doRemoteFunction(int value) throws Exception;
+}
+```
+
+A remote services represented as a singleton.
+
+```java
+public class RemoteService implements RemoteServiceInterface {
+
+		private static final Logger LOGGER = LoggerFactory.getLogger(RemoteService.class);
+    private static RemoteService service = null;
+
+    static synchronized RemoteService getRemoteService() {
+        if (service == null) {
+            service = new RemoteService();
+        }
+        return service;
+    }
+
+    private RemoteService() {}
+
+    @Override
+    public long doRemoteFunction(int value) {
+
+        long waitTime = (long) Math.floor(Math.random() * 1000);
+
+        try {
+            sleep(waitTime);
+        } catch (InterruptedException e) {
+            LOGGER.error("Thread sleep interrupted", e)
+        }
+        return waitTime >= 200 ? value * 10 : -1;
+    }
+}
+```
+
+A service ambassador adding additional features such as logging, latency checks
+
+```java
+public class ServiceAmbassador implements RemoteServiceInterface {
+
+    private static final Logger LOGGER = LoggerFactory.getLogger(ServiceAmbassador.class);
+    private static final int RETRIES = 3;
+    private static final int DELAY_MS = 3000;
+
+    ServiceAmbassador() {}
+
+    @Override
+    public long doRemoteFunction(int value) {
+
+        return safeCall(value);
+    }
+
+    private long checkLatency(int value) {
+        RemoteService service = RemoteService.getRemoteService();
+        long startTime = System.currentTimeMillis();
+        long result = service.doRemoteFunction(value);
+        long timeTaken = System.currentTimeMillis() - startTime;
+
+        LOGGER.info("Time taken (ms): " + timeTaken);
+        return result;
+    }
+
+    private long safeCall(int value) {
+
+        int retries = 0;
+        long result = -1;
+
+        for (int i = 0; i < RETRIES; i++) {
+
+            if (retries >= RETRIES) {
+                return -1;
+            }
+
+            if ((result = checkLatency(value)) == -1) {
+                LOGGER.info("Failed to reach remote: (" + (i + 1) + ")");
+                retries++;
+                try {
+                    sleep(DELAY_MS);
+                } catch (InterruptedException e) {
+                    LOGGER.error("Thread sleep state interrupted", e);
+                }
+            } else {
+                break;
+            }
+        }
+        return result;
+    }
+}
+```
+
+A client has a local service ambassador used to interact with the remote service:
+
+```java
+public class Client {
+
+    private ServiceAmbassador serviceAmbassador;
+
+    Client() {
+        serviceAmbassador = new ServiceAmbassador();
+    }
+
+    long useService(int value) {
+        long result = serviceAmbassador.doRemoteFunction(value);
+        LOGGER.info("Service result: " + result)
+        return result;
+    }
+}
+```
+
+And here are two clients using the service.
+
+```java
+Client host1 = new Client();
+Client host2 = new Client();
+host1.useService(12);
+host2.useService(73);
+```
+
+## Applicability
+Ambassador is applicable when working with a legacy remote service that cannot
+be modified or would be extremely difficult to modify. Connectivity features can
+be implemented on the client avoiding the need for changes on the remote service.
+
+* Ambassador provides a local interface for a remote service.
+* Ambassador provides logging, circuit breaking, retries and security on the client.
+
+## Typical Use Case
+
+* Control access to another object
+* Implement logging
+* Implement circuit breaking
+* Offload remote service tasks
+* Facilitate network connection
+
+## Real world examples
+
+* [Kubernetes-native API gateway for microservices](https://github.com/datawire/ambassador)
+
+## Credits
+
+* [Ambassador pattern](https://docs.microsoft.com/en-us/azure/architecture/patterns/ambassador)
+* [Designing Distributed Systems: Patterns and Paradigms for Scalable, Reliable Services](https://books.google.co.uk/books?id=6BJNDwAAQBAJ&pg=PT35&lpg=PT35&dq=ambassador+pattern+in+real+world&source=bl&ots=d2e7GhYdHi&sig=Lfl_MDnCgn6lUcjzOg4GXrN13bQ&hl=en&sa=X&ved=0ahUKEwjk9L_18rrbAhVpKcAKHX_KA7EQ6AEIWTAI#v=onepage&q=ambassador%20pattern%20in%20real%20world&f=false)

ambassador/pom.xml

@@ -0,0 +1,43 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    The MIT License
+    Copyright (c) 2014-2016 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <parent>
+    <artifactId>java-design-patterns</artifactId>
+    <groupId>com.iluwatar</groupId>
+    <version>1.21.0</version>
+  </parent>
+  <modelVersion>4.0.0</modelVersion>
+  <artifactId>ambassador</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter-engine</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>

ambassador/src/main/java/com/iluwatar/ambassador/App.java

@@ -0,0 +1,53 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.ambassador;
+
+/**
+ *
+ * The ambassador pattern creates a helper service that sends network requests on behalf of a
+ * client. It is often used in cloud-based applications to offload features of a remote service.
+ *
+ * An ambassador service can be thought of as an out-of-process proxy that is co-located with
+ * the client. Similar to the proxy design pattern, the ambassador service provides an interface
+ * for another remote service. In addition to the interface, the ambassador provides extra
+ * functionality and features, specifically offloaded common connectivity tasks. This usually
+ * consists of monitoring, logging, routing, security etc. This is extremely useful in
+ * legacy applications where the codebase is difficult to modify and allows for improvements
+ * in the application's networking capabilities.
+ *
+ * In this example, we will the ({@link ServiceAmbassador}) class represents the ambassador while the
+ * ({@link RemoteService}) class represents a remote application.
+ *
+ */
+public class App {
+
+  /**
+  * Entry point
+  */
+  public static void main(String[] args) {
+    Client host1 = new Client();
+    Client host2 = new Client();
+    host1.useService(12);
+    host2.useService(73);
+  }
+}

ambassador/src/main/java/com/iluwatar/ambassador/Client.java

@@ -0,0 +1,42 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.ambassador;
+
+import org.slf4j.LoggerFactory;
+
+import org.slf4j.Logger;
+
+/**
+ * A simple Client
+ */
+public class Client {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(Client.class);
+  private final ServiceAmbassador serviceAmbassador = new ServiceAmbassador();
+
+  long useService(int value) {
+    long result = serviceAmbassador.doRemoteFunction(value);
+    LOGGER.info("Service result: " + result);
+    return result;
+  }
+}

ambassador/src/main/java/com/iluwatar/ambassador/RemoteService.java

@@ -0,0 +1,76 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.ambassador;
+
+import com.iluwatar.ambassador.util.RandomProvider;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import static java.lang.Thread.sleep;
+
+/**
+ * A remote legacy application represented by a Singleton implementation.
+ */
+public class RemoteService implements RemoteServiceInterface {
+  static final int THRESHOLD = 200;
+  private static final Logger LOGGER = LoggerFactory.getLogger(RemoteService.class);
+  private static RemoteService service = null;
+  private final RandomProvider randomProvider;
+
+  static synchronized RemoteService getRemoteService() {
+    if (service == null) {
+      service = new RemoteService();
+    }
+    return service;
+  }
+
+  private RemoteService() {
+    this(Math::random);
+  }
+
+  /**
+   * This constuctor is used for testing purposes only.
+   */
+  RemoteService(RandomProvider randomProvider) {
+    this.randomProvider = randomProvider;
+  }
+  /**
+   * Remote function takes a value and multiplies it by 10 taking a random amount of time.
+   * Will sometimes return -1. This imitates connectivity issues a client might have to account for.
+   * @param value integer value to be multiplied.
+   * @return if waitTime is less than {@link RemoteService#THRESHOLD}, it returns value * 10,
+   *     otherwise {@link RemoteServiceInterface#FAILURE}.
+   */
+  @Override
+  public long doRemoteFunction(int value) {
+
+    long waitTime = (long) Math.floor(randomProvider.random() * 1000);
+
+    try {
+      sleep(waitTime);
+    } catch (InterruptedException e) {
+      LOGGER.error("Thread sleep state interrupted", e);
+    }
+    return waitTime <= THRESHOLD ? value * 10 : FAILURE;
+  }
+}

ambassador/src/main/java/com/iluwatar/ambassador/RemoteServiceInterface.java

@@ -0,0 +1,32 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.ambassador;
+
+/**
+ * Interface shared by ({@link RemoteService}) and ({@link ServiceAmbassador}).
+ */
+interface RemoteServiceInterface {
+  int FAILURE = -1;
+
+  long doRemoteFunction(int value) throws Exception;
+}

ambassador/src/main/java/com/iluwatar/ambassador/ServiceAmbassador.java

@@ -0,0 +1,84 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.ambassador;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import static java.lang.Thread.sleep;
+
+/**
+ *
+ * ServiceAmbassador provides an interface for a ({@link Client}) to access ({@link RemoteService}).
+ * The interface adds logging, latency testing and usage of the service in a safe way that will not
+ * add stress to the remote service when connectivity issues occur.
+ *
+ */
+public class ServiceAmbassador implements RemoteServiceInterface {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(ServiceAmbassador.class);
+  private static final int RETRIES = 3;
+  private static final int DELAY_MS = 3000;
+
+  ServiceAmbassador() {}
+
+  @Override
+  public long doRemoteFunction(int value) {
+    return safeCall(value);
+  }
+
+  private long checkLatency(int value) {
+    long startTime = System.currentTimeMillis();
+    long result = RemoteService.getRemoteService().doRemoteFunction(value);
+    long timeTaken = System.currentTimeMillis() - startTime;
+
+    LOGGER.info("Time taken (ms): " + timeTaken);
+    return result;
+  }
+
+  private long safeCall(int value) {
+
+    int retries = 0;
+    long result = FAILURE;
+
+    for (int i = 0; i < RETRIES; i++) {
+
+      if (retries >= RETRIES) {
+        return FAILURE;
+      }
+
+      if ((result = checkLatency(value)) == FAILURE) {
+        LOGGER.info("Failed to reach remote: (" + (i + 1) + ")");
+        retries++;
+        try {
+          sleep(DELAY_MS);
+        } catch (InterruptedException e) {
+          LOGGER.error("Thread sleep state interrupted", e);
+        }
+      } else {
+        break;
+      }
+    }
+    return result;
+  }
+}

ambassador/src/main/java/com/iluwatar/ambassador/util/RandomProvider.java

@@ -0,0 +1,30 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.ambassador.util;
+
+/**
+ * An interface for randomness. Useful for testing purposes.
+ */
+public interface RandomProvider {
+  double random();
+}

ambassador/src/test/java/com/iluwatar/ambassador/AppTest.java

@@ -0,0 +1,36 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.ambassador;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Application test
+ */
+public class AppTest {
+
+  @Test
+  public void test() {
+    App.main(new String[]{});
+  }
+}

ambassador/src/test/java/com/iluwatar/ambassador/ClientTest.java

@@ -0,0 +1,42 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.ambassador;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * Test for {@link Client}
+ */
+public class ClientTest {
+
+  @Test
+  public void test() {
+
+    Client client = new Client();
+    long result = client.useService(10);
+
+    assertTrue(result == 100 || result == RemoteService.FAILURE);
+  }
+}

ambassador/src/test/java/com/iluwatar/ambassador/RemoteServiceTest.java

@@ -0,0 +1,64 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.ambassador;
+
+import com.iluwatar.ambassador.util.RandomProvider;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * Test for {@link RemoteService}
+ */
+public class RemoteServiceTest {
+
+  @Test
+  public void testFailedCall() {
+    RemoteService remoteService = new RemoteService(
+        new StaticRandomProvider(0.21));
+    long result = remoteService.doRemoteFunction(10);
+    assertEquals(RemoteServiceInterface.FAILURE, result);
+  }
+
+  @Test
+  public void testSuccessfulCall() {
+    RemoteService remoteService = new RemoteService(
+        new StaticRandomProvider(0.2));
+    long result = remoteService.doRemoteFunction(10);
+    assertEquals(100, result);
+  }
+
+  private class StaticRandomProvider implements RandomProvider {
+    private double value;
+
+    StaticRandomProvider(double value) {
+      this.value = value;
+    }
+
+    @Override
+    public double random() {
+      return value;
+    }
+  }
+}

ambassador/src/test/java/com/iluwatar/ambassador/ServiceAmbassadorTest.java

@@ -0,0 +1,39 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.ambassador;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * Test for {@link ServiceAmbassador}
+ */
+public class ServiceAmbassadorTest {
+
+  @Test
+  public void test() {
+    long result = new ServiceAmbassador().doRemoteFunction(10);
+    assertTrue(result == 100 || result == RemoteServiceInterface.FAILURE);
+  }
+}

api-gateway/api-gateway-service/pom.xml

@@ -29,7 +29,7 @@
   <parent>
     <artifactId>api-gateway</artifactId>
     <groupId>com.iluwatar</groupId>
-    <version>1.19.0</version>
+    <version>1.21.0</version>
   </parent>
   <modelVersion>4.0.0</modelVersion>
   <artifactId>api-gateway-service</artifactId>
@@ -49,13 +49,12 @@
       <artifactId>spring-webmvc</artifactId>
     </dependency>
     <dependency>
-      <groupId>org.springframework.boot</groupId>
-      <artifactId>spring-boot-starter-web</artifactId>
+      <groupId>org.apache.httpcomponents</groupId>
+      <artifactId>httpclient</artifactId>
     </dependency>
     <dependency>
-      <groupId>org.junit.jupiter</groupId>
-      <artifactId>junit-jupiter-api</artifactId>
-      <scope>test</scope>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-web</artifactId>
     </dependency>
     <dependency>
       <groupId>org.junit.jupiter</groupId>
@@ -67,10 +66,6 @@
       <artifactId>mockito-core</artifactId>
       <scope>test</scope>
     </dependency>
-    <dependency>
-      <groupId>org.apache.httpcomponents</groupId>
-      <artifactId>httpclient</artifactId>
-    </dependency>
   </dependencies>
 
   <build>
@@ -89,4 +84,4 @@
       </plugin>
     </plugins>
   </build>
-</project>
+</project>

api-gateway/image-microservice/pom.xml

@@ -29,7 +29,7 @@
   <parent>
     <artifactId>api-gateway</artifactId>
     <groupId>com.iluwatar</groupId>
-    <version>1.19.0</version>
+    <version>1.21.0</version>
   </parent>
 
   <modelVersion>4.0.0</modelVersion>
@@ -53,11 +53,6 @@
       <groupId>org.springframework.boot</groupId>
       <artifactId>spring-boot-starter-web</artifactId>
     </dependency>
-    <dependency>
-      <groupId>org.junit.jupiter</groupId>
-      <artifactId>junit-jupiter-api</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.junit.jupiter</groupId>
       <artifactId>junit-jupiter-engine</artifactId>
@@ -81,4 +76,4 @@
       </plugin>
     </plugins>
   </build>
-</project>
+</project>

api-gateway/pom.xml

@@ -29,7 +29,7 @@
   <parent>
     <artifactId>java-design-patterns</artifactId>
     <groupId>com.iluwatar</groupId>
-    <version>1.19.0</version>
+    <version>1.21.0</version>
   </parent>
   <modelVersion>4.0.0</modelVersion>
   <artifactId>api-gateway</artifactId>

api-gateway/price-microservice/pom.xml

@@ -29,7 +29,7 @@
   <parent>
     <artifactId>api-gateway</artifactId>
     <groupId>com.iluwatar</groupId>
-    <version>1.19.0</version>
+    <version>1.21.0</version>
   </parent>
 
   <modelVersion>4.0.0</modelVersion>
@@ -53,11 +53,6 @@
       <groupId>org.springframework.boot</groupId>
       <artifactId>spring-boot-starter-web</artifactId>
     </dependency>
-    <dependency>
-      <groupId>org.junit.jupiter</groupId>
-      <artifactId>junit-jupiter-api</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.junit.jupiter</groupId>
       <artifactId>junit-jupiter-engine</artifactId>
@@ -81,4 +76,4 @@
       </plugin>
     </plugins>
   </build>
-</project>
+</project>

async-method-invocation/pom.xml

@@ -29,15 +29,10 @@
   <parent>
     <groupId>com.iluwatar</groupId>
     <artifactId>java-design-patterns</artifactId>
-    <version>1.19.0</version>
+    <version>1.21.0</version>
   </parent>
   <artifactId>async-method-invocation</artifactId>
   <dependencies>
-    <dependency>
-      <groupId>org.junit.jupiter</groupId>
-      <artifactId>junit-jupiter-api</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.junit.jupiter</groupId>
       <artifactId>junit-jupiter-engine</artifactId>

balking/pom.xml

@@ -29,17 +29,12 @@
   <parent>
     <artifactId>java-design-patterns</artifactId>
     <groupId>com.iluwatar</groupId>
-    <version>1.19.0</version>
+    <version>1.21.0</version>
   </parent>
   <modelVersion>4.0.0</modelVersion>
 
   <artifactId>balking</artifactId>
   <dependencies>
-    <dependency>
-      <groupId>org.junit.jupiter</groupId>
-      <artifactId>junit-jupiter-api</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.junit.jupiter</groupId>
       <artifactId>junit-jupiter-engine</artifactId>
@@ -48,4 +43,4 @@
   </dependencies>
 
 
-</project>
+</project>

balking/src/main/java/com/iluwatar/balking/DelayProvider.java

@@ -0,0 +1,32 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.balking;
+
+import java.util.concurrent.TimeUnit;
+
+/**
+ * An interface to simulate delay while executing some work.
+ */
+public interface DelayProvider {
+  void executeAfterDelay(long interval, TimeUnit timeUnit, Runnable task);
+}

balking/src/main/java/com/iluwatar/balking/WashingMachine.java

@@ -25,17 +25,38 @@ package com.iluwatar.balking;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import java.util.concurrent.TimeUnit;
+
 /**
  * Washing machine class
  */
 public class WashingMachine {
 
   private static final Logger LOGGER = LoggerFactory.getLogger(WashingMachine.class);
-
+  private final DelayProvider delayProvider;
   private WashingMachineState washingMachineState;
 
+  /**
+   * Creates a new instance of WashingMachine
+   */
   public WashingMachine() {
-    washingMachineState = WashingMachineState.ENABLED;
+    this((interval, timeUnit, task) -> {
+      try {
+        Thread.sleep(timeUnit.toMillis(interval));
+      } catch (InterruptedException ie) {
+        ie.printStackTrace();
+      }
+      task.run();
+    });
+  }
+
+  /**
+   * Creates a new instance of WashingMachine using provided delayProvider. This constructor is used only for
+   * unit testing purposes.
+   */
+  public WashingMachine(DelayProvider delayProvider) {
+    this.delayProvider = delayProvider;
+    this.washingMachineState = WashingMachineState.ENABLED;
   }
 
   public WashingMachineState getWashingMachineState() {
@@ -56,12 +77,8 @@ public class WashingMachine {
       washingMachineState = WashingMachineState.WASHING;
     }
     LOGGER.info("{}: Doing the washing", Thread.currentThread().getName());
-    try {
-      Thread.sleep(50);
-    } catch (InterruptedException ie) {
-      ie.printStackTrace();
-    }
-    endOfWashing();
+
+    this.delayProvider.executeAfterDelay(50, TimeUnit.MILLISECONDS, this::endOfWashing);
   }
 
   /**

balking/src/test/java/com/iluwatar/balking/WashingMachineTest.java

@@ -22,11 +22,8 @@
  */
 package com.iluwatar.balking;
 
-import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 
-import java.util.concurrent.ExecutorService;
-import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
@@ -36,32 +33,39 @@ import static org.junit.jupiter.api.Assertions.assertEquals;
  */
 public class WashingMachineTest {
 
-  private volatile WashingMachineState machineStateGlobal;
+  private FakeDelayProvider fakeDelayProvider = new FakeDelayProvider();
 
-  @Disabled
   @Test
-  public void wash() throws Exception {
-    WashingMachine washingMachine = new WashingMachine();
-    ExecutorService executorService = Executors.newFixedThreadPool(2);
-    executorService.execute(washingMachine::wash);
-    executorService.execute(() -> {
-      washingMachine.wash();
-      machineStateGlobal = washingMachine.getWashingMachineState();
-    });
-    executorService.shutdown();
-    try {
-      executorService.awaitTermination(10, TimeUnit.SECONDS);
-    } catch (InterruptedException ie) {
-      ie.printStackTrace();
-    }
+  public void wash() {
+    WashingMachine washingMachine = new WashingMachine(fakeDelayProvider);
+
+    washingMachine.wash();
+    washingMachine.wash();
+
+    WashingMachineState machineStateGlobal = washingMachine.getWashingMachineState();
+
+    fakeDelayProvider.task.run();
+
+    // washing machine remains in washing state
     assertEquals(WashingMachineState.WASHING, machineStateGlobal);
+
+    // washing machine goes back to enabled state
+    assertEquals(WashingMachineState.ENABLED, washingMachine.getWashingMachineState());
   }
 
   @Test
-  public void endOfWashing() throws Exception {
+  public void endOfWashing() {
     WashingMachine washingMachine = new WashingMachine();
     washingMachine.wash();
     assertEquals(WashingMachineState.ENABLED, washingMachine.getWashingMachineState());
   }
 
+  private class FakeDelayProvider implements DelayProvider {
+    private Runnable task;
+
+    @Override
+    public void executeAfterDelay(long interval, TimeUnit timeUnit, Runnable task) {
+      this.task = task;
+    }
+  }
 }

bridge/pom.xml

@@ -29,15 +29,10 @@
   <parent>
     <groupId>com.iluwatar</groupId>
     <artifactId>java-design-patterns</artifactId>
-    <version>1.19.0</version>
+    <version>1.21.0</version>
   </parent>
   <artifactId>bridge</artifactId>
   <dependencies>
-    <dependency>
-      <groupId>org.junit.jupiter</groupId>
-      <artifactId>junit-jupiter-api</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.junit.jupiter</groupId>
       <artifactId>junit-jupiter-engine</artifactId>

bridge/src/test/java/com/iluwatar/bridge/HammerTest.java

@@ -37,7 +37,7 @@ public class HammerTest extends WeaponTest {
    * underlying weapon implementation.
    */
   @Test
-  public void testHammer() throws Exception {
+  public void testHammer() {
     final Hammer hammer = spy(new Hammer(mock(FlyingEnchantment.class)));
     testBasicWeaponActions(hammer);
   }

bridge/src/test/java/com/iluwatar/bridge/SwordTest.java

@@ -37,7 +37,7 @@ public class SwordTest extends WeaponTest {
    * underlying weapon implementation.
    */
   @Test
-  public void testSword() throws Exception {
+  public void testSword() {
     final Sword sword = spy(new Sword(mock(FlyingEnchantment.class)));
     testBasicWeaponActions(sword);
   }

builder/pom.xml

@@ -29,15 +29,10 @@
   <parent>
     <groupId>com.iluwatar</groupId>
     <artifactId>java-design-patterns</artifactId>
-    <version>1.19.0</version>
+    <version>1.21.0</version>
   </parent>
   <artifactId>builder</artifactId>
   <dependencies>
-    <dependency>
-      <groupId>org.junit.jupiter</groupId>
-      <artifactId>junit-jupiter-api</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.junit.jupiter</groupId>
       <artifactId>junit-jupiter-engine</artifactId>

business-delegate/pom.xml

@@ -30,15 +30,10 @@
 	<parent>
 		<groupId>com.iluwatar</groupId>
 		<artifactId>java-design-patterns</artifactId>
-		<version>1.19.0</version>
+		<version>1.21.0</version>
 	</parent>
 	<artifactId>business-delegate</artifactId>
 	<dependencies>
-		<dependency>
-			<groupId>org.junit.jupiter</groupId>
-			<artifactId>junit-jupiter-api</artifactId>
-			<scope>test</scope>
-		</dependency>
 		<dependency>
 			<groupId>org.junit.jupiter</groupId>
 			<artifactId>junit-jupiter-engine</artifactId>

bytecode/README.md

@@ -0,0 +1,25 @@
+---
+layout: pattern
+title: Bytecode
+folder: bytecode
+permalink: /patterns/bytecode/
+categories: Behavioral
+tags:
+ - Java
+ - Difficulty-Beginner
+---
+
+## Intent
+Allows to encode behaviour as instructions for virtual machine.
+
+## Applicability
+Use the Bytecode pattern when you have a lot of behavior you need to define and your
+game’s implementation language isn’t a good fit because:
+
+* it’s too low-level, making it tedious or error-prone to program in.
+* iterating on it takes too long due to slow compile times or other tooling issues.
+* it has too much trust. If you want to ensure the behavior being defined can’t break the game, you need to sandbox it from the rest of the codebase.
+
+## Credits
+
+* [Game programming patterns](http://gameprogrammingpatterns.com/bytecode.html)

bytecode/etc/bytecode.ucls

@@ -0,0 +1,49 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<class-diagram version="1.2.3" icons="true" always-add-relationships="false" generalizations="true" realizations="true" 
+  associations="true" dependencies="false" nesting-relationships="true" router="FAN">  
+  <class id="1" language="java" name="com.iluwatar.bytecode.VirtualMachine" project="bytecode" 
+    file="/bytecode/src/main/java/com/iluwatar/bytecode/VirtualMachine.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="455" y="173"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="2" language="java" name="com.iluwatar.bytecode.App" project="bytecode" 
+    file="/bytecode/src/main/java/com/iluwatar/bytecode/App.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="148" y="110"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="3" language="java" name="com.iluwatar.bytecode.Wizard" project="bytecode" 
+    file="/bytecode/src/main/java/com/iluwatar/bytecode/Wizard.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="148" y="416"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <association id="4">    
+    <end type="SOURCE" refId="1" navigable="false" variant="ASSOCIATION">      
+      <attribute id="5" name="wizards">        
+        <position height="18" width="48" x="296" y="291"/>      
+      </attribute>      
+      <multiplicity id="6" minimum="0" maximum="2147483647">        
+        <position height="0" width="0" x="-327" y="-27"/>      
+      </multiplicity>    
+    </end>    
+    <end type="TARGET" refId="3" navigable="true" variant="ASSOCIATION"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+    sort-features="false" accessors="true" visibility="true">    
+    <attributes public="true" package="true" protected="true" private="true" static="true"/>    
+    <operations public="true" package="true" protected="true" private="true" static="true"/>  
+  </classifier-display>  
+  <association-display labels="true" multiplicity="true"/>
+</class-diagram>

bytecode/pom.xml

@@ -0,0 +1,45 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    The MIT License
+    Copyright (c) 2019 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <parent>
+        <artifactId>java-design-patterns</artifactId>
+        <groupId>com.iluwatar</groupId>
+        <version>1.21.0</version>
+    </parent>
+    <modelVersion>4.0.0</modelVersion>
+
+    <artifactId>bytecode</artifactId>
+    <dependencies>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+</project>

bytecode/src/main/java/com/iluwatar/bytecode/App.java

@@ -0,0 +1,79 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bytecode;
+
+import com.iluwatar.bytecode.util.InstructionConverterUtil;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * The intention of Bytecode pattern is to give behavior the flexibility of data by encoding it as instructions
+ * for a virtual machine.
+ * An instruction set defines the low-level operations that can be performed. A series of instructions is encoded as
+ * a sequence of bytes. A virtual machine executes these instructions one at a time,
+ * using a stack for intermediate values. By combining instructions, complex high-level behavior can be defined.
+ *
+ * This pattern should be used when there is a need to define high number of behaviours and implementation engine
+ * is not a good choice because
+ * It is too lowe level
+ * Iterating on it takes too long due to slow compile times or other tooling issues.
+ * It has too much trust. If you want to ensure the behavior being defined can’t break the game,
+ * you need to sandbox it from the rest of the codebase.
+ *
+ */
+public class App {
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
+  /**
+   * Main app method
+   * @param args command line args
+   */
+  public static void main(String[] args) {
+    VirtualMachine vm = new VirtualMachine();
+
+    Wizard wizard = new Wizard();
+    wizard.setHealth(45);
+    wizard.setAgility(7);
+    wizard.setWisdom(11);
+    vm.getWizards()[0] = wizard;
+
+    interpretInstruction("LITERAL 0", vm);
+    interpretInstruction( "LITERAL 0", vm);
+    interpretInstruction( "GET_HEALTH", vm);
+    interpretInstruction( "LITERAL 0", vm);
+    interpretInstruction( "GET_AGILITY", vm);
+    interpretInstruction( "LITERAL 0", vm);
+    interpretInstruction( "GET_WISDOM ", vm);
+    interpretInstruction( "ADD", vm);
+    interpretInstruction( "LITERAL 2", vm);
+    interpretInstruction( "DIVIDE", vm);
+    interpretInstruction( "ADD", vm);
+    interpretInstruction( "SET_HEALTH", vm);
+  }
+
+  private static void interpretInstruction(String instruction, VirtualMachine vm) {
+    InstructionConverterUtil converter = new InstructionConverterUtil();
+    vm.execute(converter.convertToByteCode(instruction));
+    LOGGER.info(instruction + String.format("%" + (12 - instruction.length()) + "s", "" ) + vm.getStack());
+  }
+}

bytecode/src/main/java/com/iluwatar/bytecode/Instruction.java

@@ -0,0 +1,65 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bytecode;
+
+/**
+ * Representation of instructions understandable by virtual machine
+ */
+public enum Instruction {
+
+  LITERAL(1),
+  SET_HEALTH(2),
+  SET_WISDOM (3),
+  SET_AGILITY(4),
+  PLAY_SOUND(5),
+  SPAWN_PARTICLES(6),
+  GET_HEALTH(7),
+  GET_AGILITY(8),
+  GET_WISDOM(9),
+  ADD(10),
+  DIVIDE (11);
+
+  private int value;
+
+  Instruction(int value) {
+    this.value = value;
+  }
+
+  public int getIntValue() {
+    return value;
+  }
+
+  /**
+   * Converts integer value to Instruction
+   * @param value value of instruction
+   * @return representation of the instruction
+   */
+  public static Instruction getInstruction(int value) {
+    for (int i = 0; i < Instruction.values().length; i++) {
+      if (Instruction.values()[i].getIntValue() == value) {
+        return Instruction.values()[i];
+      }
+    }
+    throw new IllegalArgumentException("Invalid instruction value");
+  }
+}

bytecode/src/main/java/com/iluwatar/bytecode/VirtualMachine.java

@@ -0,0 +1,142 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bytecode;
+
+import java.util.Stack;
+
+/**
+ * Implementation of virtual machine
+ */
+public class VirtualMachine {
+
+  private Stack<Integer> stack = new Stack();
+
+  private Wizard[] wizards = new Wizard[2];
+
+  /**
+   * Constructor
+   */
+  public VirtualMachine() {
+    for (int i = 0; i < wizards.length; i++) {
+      wizards[i] = new Wizard();
+    }
+  }
+
+  /**
+   * Executes provided bytecode
+   * @param bytecode to execute
+   */
+  public void execute(int[] bytecode) {
+    for (int i = 0; i < bytecode.length; i++) {
+      Instruction instruction = Instruction.getInstruction(bytecode[i]);
+      int wizard;
+      int amount;
+      switch (instruction) {
+        case LITERAL:
+          // Read the next byte from the bytecode.
+          int value = bytecode[++i];
+          stack.push(value);
+          break;
+        case SET_AGILITY:
+          amount = stack.pop();
+          wizard = stack.pop();
+          setAgility(wizard, amount);
+          break;
+        case SET_WISDOM:
+          amount = stack.pop();
+          wizard = stack.pop();
+          setWisdom(wizard, amount);
+          break;
+        case SET_HEALTH:
+          amount = stack.pop();
+          wizard = stack.pop();
+          setHealth(wizard, amount);
+          break;
+        case GET_HEALTH:
+          wizard = stack.pop();
+          stack.push(getHealth(wizard));
+          break;
+        case GET_AGILITY:
+          wizard = stack.pop();
+          stack.push(getAgility(wizard));
+          break;
+        case GET_WISDOM:
+          wizard = stack.pop();
+          stack.push(getWisdom(wizard));
+          break;
+        case ADD:
+          int a = stack.pop();
+          int b = stack.pop();
+          stack.push(a + b);
+          break;
+        case DIVIDE:
+          a = stack.pop();
+          b = stack.pop();
+          stack.push(b / a);
+          break;
+        case PLAY_SOUND:
+          wizard = stack.pop();
+          getWizards()[wizard].playSound();
+          break;
+        case SPAWN_PARTICLES:
+          wizard = stack.pop();
+          getWizards()[wizard].spawnParticles();
+          break;
+        default:
+          throw new IllegalArgumentException("Invalid instruction value");
+      }
+    }
+  }
+
+  public Stack<Integer> getStack() {
+    return stack;
+  }
+
+  public void setHealth(int wizard, int amount) {
+    wizards[wizard].setHealth(amount);
+  }
+
+  public void setWisdom(int wizard, int amount) {
+    wizards[wizard].setWisdom(amount);
+  }
+
+  public void setAgility(int wizard, int amount) {
+    wizards[wizard].setAgility(amount);
+  }
+
+  public int getHealth(int wizard) {
+    return wizards[wizard].getHealth();
+  }
+
+  public int getWisdom(int wizard) {
+    return wizards[wizard].getWisdom();
+  }
+
+  public int getAgility(int wizard) {
+    return wizards[wizard].getAgility();
+  }
+
+  public Wizard[] getWizards() {
+    return wizards;
+  }
+}

bytecode/src/main/java/com/iluwatar/bytecode/Wizard.java

@@ -0,0 +1,83 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bytecode;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * This class represent game objects which properties can be changed by instructions interpreted by virtual machine
+ */
+public class Wizard {
+  private static final Logger LOGGER = LoggerFactory.getLogger(Wizard.class);
+
+  private int health;
+
+  private int agility;
+  private int wisdom;
+
+  private int numberOfPlayedSounds;
+  private int numberOfSpawnedParticles;
+
+  public int getHealth() {
+    return health;
+  }
+
+  public void setHealth(int health) {
+    this.health = health;
+  }
+
+  public int getAgility() {
+    return agility;
+  }
+
+  public void setAgility(int agility) {
+    this.agility = agility;
+  }
+
+  public int getWisdom() {
+    return wisdom;
+  }
+
+  public void setWisdom(int wisdom) {
+    this.wisdom = wisdom;
+  }
+
+  public void playSound() {
+    LOGGER.info("Playing sound");
+    numberOfPlayedSounds++;
+  }
+
+  public void spawnParticles() {
+    LOGGER.info("Spawning particles");
+    numberOfSpawnedParticles++;
+  }
+
+  public int getNumberOfPlayedSounds() {
+    return numberOfPlayedSounds;
+  }
+
+  public int getNumberOfSpawnedParticles() {
+    return numberOfSpawnedParticles;
+  }
+}

bytecode/src/main/java/com/iluwatar/bytecode/util/InstructionConverterUtil.java

@@ -0,0 +1,76 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bytecode.util;
+
+import com.iluwatar.bytecode.Instruction;
+
+/**
+ * Utility class used for instruction validation and conversion
+ */
+public class InstructionConverterUtil {
+  /**
+   * Converts instructions represented as String
+   *
+   * @param instructions to convert
+   * @return array of int representing bytecode
+   */
+  public static int[] convertToByteCode(String instructions) {
+    if (instructions == null || instructions.trim().length() == 0) {
+      return new int[0];
+    }
+
+    String[] splitedInstructions = instructions.trim().split(" ");
+    int[] bytecode = new int[splitedInstructions.length];
+    for (int i = 0; i < splitedInstructions.length; i++) {
+      if (isValidInstruction(splitedInstructions[i])) {
+        bytecode[i] = Instruction.valueOf(splitedInstructions[i]).getIntValue();
+      } else if (isValidInt(splitedInstructions[i])) {
+        bytecode[i] = Integer.valueOf(splitedInstructions[i]);
+      } else {
+        throw new IllegalArgumentException("Invalid instruction or number: " + splitedInstructions[i]);
+      }
+    }
+
+    return bytecode;
+  }
+
+  private static boolean isValidInstruction(String instruction) {
+    try {
+      Instruction.valueOf(instruction);
+      return true;
+    } catch (IllegalArgumentException e) {
+      return false;
+    }
+  }
+
+  private static boolean isValidInt(String value) {
+    try {
+      Integer.parseInt(value);
+      return true;
+    } catch (NumberFormatException e) {
+      return false;
+    }
+  }
+
+
+}

bytecode/src/test/java/com/iluwatar/bytecode/AppTest.java

@@ -0,0 +1,37 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bytecode;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Application test
+ */
+public class AppTest {
+
+  @Test
+  public void test() {
+    String[] args = {};
+    App.main(args);
+  }
+}

bytecode/src/test/java/com/iluwatar/bytecode/VirtualMachineTest.java

@@ -0,0 +1,154 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bytecode;
+
+import org.junit.jupiter.api.Test;
+
+import static com.iluwatar.bytecode.Instruction.*;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+/**
+ * Test for {@Link VirtualMachine}
+ */
+public class VirtualMachineTest {
+
+  @Test
+  public void testLiteral() {
+    int[] bytecode = new int[2];
+    bytecode[0] = LITERAL.getIntValue();
+    bytecode[1] = 10;
+
+    VirtualMachine vm = new VirtualMachine();
+    vm.execute(bytecode);
+
+    assertEquals(1, vm.getStack().size());
+    assertEquals(Integer.valueOf(10), vm.getStack().pop());
+  }
+
+  @Test
+  public void testSetHealth() {
+    int wizardNumber = 0;
+    int[] bytecode = new int[5];
+    bytecode[0] = LITERAL.getIntValue();
+    bytecode[1] = wizardNumber;
+    bytecode[2] = LITERAL.getIntValue();
+    bytecode[3] = 50;                        // health amount
+    bytecode[4] = SET_HEALTH.getIntValue();
+
+    VirtualMachine vm = new VirtualMachine();
+    vm.execute(bytecode);
+
+    assertEquals(50, vm.getWizards()[wizardNumber].getHealth());
+  }
+
+  @Test
+  public void testSetAgility() {
+    int wizardNumber = 0;
+    int[] bytecode = new int[5];
+    bytecode[0] = LITERAL.getIntValue();
+    bytecode[1] = wizardNumber;
+    bytecode[2] = LITERAL.getIntValue();
+    bytecode[3] = 50;                        // agility amount
+    bytecode[4] = SET_AGILITY.getIntValue();
+
+    VirtualMachine vm = new VirtualMachine();
+    vm.execute(bytecode);
+
+    assertEquals(50, vm.getWizards()[wizardNumber].getAgility());
+  }
+
+  @Test
+  public void testSetWisdom() {
+    int wizardNumber = 0;
+    int[] bytecode = new int[5];
+    bytecode[0] = LITERAL.getIntValue();
+    bytecode[1] = wizardNumber;
+    bytecode[2] = LITERAL.getIntValue();
+    bytecode[3] = 50;                        // wisdom amount
+    bytecode[4] = SET_WISDOM.getIntValue();
+
+    VirtualMachine vm = new VirtualMachine();
+    vm.execute(bytecode);
+
+    assertEquals(50, vm.getWizards()[wizardNumber].getWisdom());
+  }
+
+  @Test
+  public void testGetHealth() {
+    int wizardNumber = 0;
+    int[] bytecode = new int[8];
+    bytecode[0] = LITERAL.getIntValue();
+    bytecode[1] = wizardNumber;
+    bytecode[2] = LITERAL.getIntValue();
+    bytecode[3] = 50;                        // health amount
+    bytecode[4] = SET_HEALTH.getIntValue();
+    bytecode[5] = LITERAL.getIntValue();;
+    bytecode[6] = wizardNumber;
+    bytecode[7] = GET_HEALTH.getIntValue();
+
+    VirtualMachine vm = new VirtualMachine();
+    vm.execute(bytecode);
+
+    assertEquals(Integer.valueOf(50), vm.getStack().pop());
+  }
+
+  @Test
+  public void testPlaySound() {
+    int wizardNumber = 0;
+    int[] bytecode = new int[3];
+    bytecode[0] = LITERAL.getIntValue();
+    bytecode[1] = wizardNumber;
+    bytecode[2] = PLAY_SOUND.getIntValue();
+
+    VirtualMachine vm = new VirtualMachine();
+    vm.execute(bytecode);
+
+    assertEquals(0, vm.getStack().size());
+    assertEquals(1, vm.getWizards()[0].getNumberOfPlayedSounds());
+  }
+
+  @Test
+  public void testSpawnParticles() {
+    int wizardNumber = 0;
+    int[] bytecode = new int[3];
+    bytecode[0] = LITERAL.getIntValue();
+    bytecode[1] = wizardNumber;
+    bytecode[2] = SPAWN_PARTICLES.getIntValue();
+
+    VirtualMachine vm = new VirtualMachine();
+    vm.execute(bytecode);
+
+    assertEquals(0, vm.getStack().size());
+    assertEquals(1, vm.getWizards()[0].getNumberOfSpawnedParticles());
+  }
+
+  @Test
+  public void testInvalidInstruction() {
+    int[] bytecode = new int[1];
+    bytecode[0] = 999;
+    VirtualMachine vm = new VirtualMachine();
+
+    assertThrows(IllegalArgumentException.class, () -> vm.execute(bytecode));
+  }
+}

bytecode/src/test/java/com/iluwatar/bytecode/util/InstructionConverterUtilTest.java

@@ -0,0 +1,63 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bytecode.util;
+
+import com.iluwatar.bytecode.Instruction;
+import com.iluwatar.bytecode.util.InstructionConverterUtil;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Test for {@Link InstructionConverterUtil}
+ */
+public class InstructionConverterUtilTest {
+  @Test
+  public void testEmptyInstruction() {
+    String instruction = "";
+
+    int[] bytecode = InstructionConverterUtil.convertToByteCode(instruction);
+
+    Assertions.assertEquals(0, bytecode.length);
+  }
+
+  @Test
+  public void testInstructions() {
+    String instructions =
+        "LITERAL 35 SET_HEALTH SET_WISDOM SET_AGILITY PLAY_SOUND SPAWN_PARTICLES GET_HEALTH ADD DIVIDE";
+
+    int[] bytecode = InstructionConverterUtil.convertToByteCode(instructions);
+
+    Assertions.assertEquals(10, bytecode.length);
+    Assertions.assertEquals(Instruction.LITERAL.getIntValue(), bytecode[0]);
+    Assertions.assertEquals(35, bytecode[1]);
+    Assertions.assertEquals(Instruction.SET_HEALTH.getIntValue(), bytecode[2]);
+    Assertions.assertEquals(Instruction.SET_WISDOM.getIntValue(), bytecode[3]);
+    Assertions.assertEquals(Instruction.SET_AGILITY.getIntValue(), bytecode[4]);
+    Assertions.assertEquals(Instruction.PLAY_SOUND.getIntValue(), bytecode[5]);
+    Assertions.assertEquals(Instruction.SPAWN_PARTICLES.getIntValue(), bytecode[6]);
+    Assertions.assertEquals(Instruction.GET_HEALTH.getIntValue(), bytecode[7]);
+    Assertions.assertEquals(Instruction.ADD.getIntValue(), bytecode[8]);
+    Assertions.assertEquals(Instruction.DIVIDE.getIntValue(), bytecode[9]);
+  }
+
+}

caching/pom.xml

@@ -29,15 +29,10 @@
   <parent>
     <groupId>com.iluwatar</groupId>
     <artifactId>java-design-patterns</artifactId>
-    <version>1.19.0</version>
+    <version>1.21.0</version>
   </parent>
   <artifactId>caching</artifactId>
   <dependencies>
-    <dependency>
-      <groupId>org.junit.jupiter</groupId>
-      <artifactId>junit-jupiter-api</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.junit.jupiter</groupId>
       <artifactId>junit-jupiter-engine</artifactId>

caching/src/main/java/com/iluwatar/caching/DbManager.java

@@ -28,6 +28,7 @@ import java.util.Map;
 
 import org.bson.Document;
 
+import com.iluwatar.caching.constants.CachingConstants;
 import com.mongodb.MongoClient;
 import com.mongodb.client.FindIterable;
 import com.mongodb.client.MongoDatabase;
@@ -90,12 +91,12 @@ public final class DbManager {
       }
     }
     FindIterable<Document> iterable =
-        db.getCollection("user_accounts").find(new Document("userID", userId));
+        db.getCollection(CachingConstants.USER_ACCOUNT).find(new Document(CachingConstants.USER_ID, userId));
     if (iterable == null) {
       return null;
     }
     Document doc = iterable.first();
-    return new UserAccount(userId, doc.getString("userName"), doc.getString("additionalInfo"));
+    return new UserAccount(userId, doc.getString(CachingConstants.USER_NAME), doc.getString(CachingConstants.ADD_INFO));
   }
 
   /**
@@ -113,9 +114,9 @@ public final class DbManager {
         e.printStackTrace();
       }
     }
-    db.getCollection("user_accounts").insertOne(
-        new Document("userID", userAccount.getUserId()).append("userName",
-            userAccount.getUserName()).append("additionalInfo", userAccount.getAdditionalInfo()));
+    db.getCollection(CachingConstants.USER_ACCOUNT).insertOne(
+        new Document(CachingConstants.USER_ID ,userAccount.getUserId()).append(CachingConstants.USER_NAME,
+            userAccount.getUserName()).append(CachingConstants.ADD_INFO, userAccount.getAdditionalInfo()));
   }
 
   /**
@@ -133,10 +134,10 @@ public final class DbManager {
         e.printStackTrace();
       }
     }
-    db.getCollection("user_accounts").updateOne(
-        new Document("userID", userAccount.getUserId()),
-        new Document("$set", new Document("userName", userAccount.getUserName()).append(
-            "additionalInfo", userAccount.getAdditionalInfo())));
+    db.getCollection(CachingConstants.USER_ACCOUNT).updateOne(
+        new Document(CachingConstants.USER_ID, userAccount.getUserId()),
+        new Document("$set", new Document(CachingConstants.USER_NAME, userAccount.getUserName())
+            .append(CachingConstants.ADD_INFO, userAccount.getAdditionalInfo())));
   }
 
   /**
@@ -155,10 +156,12 @@ public final class DbManager {
         e.printStackTrace();
       }
     }
-    db.getCollection("user_accounts").updateOne(
-        new Document("userID", userAccount.getUserId()),
-        new Document("$set", new Document("userID", userAccount.getUserId()).append("userName",
-            userAccount.getUserName()).append("additionalInfo", userAccount.getAdditionalInfo())),
+    db.getCollection(CachingConstants.USER_ACCOUNT).updateOne(
+        new Document(CachingConstants.USER_ID, userAccount.getUserId()),
+        new Document("$set",
+            new Document(CachingConstants.USER_ID, userAccount.getUserId())
+                .append(CachingConstants.USER_NAME, userAccount.getUserName()).append(CachingConstants.ADD_INFO,
+                    userAccount.getAdditionalInfo())),
         new UpdateOptions().upsert(true));
   }
 }

caching/src/main/java/com/iluwatar/caching/constants/CachingConstants.java

@@ -0,0 +1,37 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.caching.constants;
+
+/**
+ * 
+ * Constant class for defining constants
+ *
+ */
+public class CachingConstants {
+
+  public static final String USER_ACCOUNT = "user_accounts";
+  public static final String USER_ID = "userID";
+  public static final String USER_NAME = "userName";
+  public static final String ADD_INFO = "additionalInfo";
+
+}

callback/pom.xml

@@ -29,15 +29,10 @@
   <parent>
     <groupId>com.iluwatar</groupId>
     <artifactId>java-design-patterns</artifactId>
-    <version>1.19.0</version>
+    <version>1.21.0</version>
   </parent>
   <artifactId>callback</artifactId>
   <dependencies>
-    <dependency>
-      <groupId>org.junit.jupiter</groupId>
-      <artifactId>junit-jupiter-api</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.junit.jupiter</groupId>
       <artifactId>junit-jupiter-engine</artifactId>

callback/src/main/java/com/iluwatar/callback/App.java

@@ -41,12 +41,7 @@ public class App {
    */
   public static void main(String[] args) {
     Task task = new SimpleTask();
-    Callback callback = new Callback() {
-      @Override
-      public void call() {
-        LOGGER.info("I'm done now.");
-      }
-    };
+    Callback callback = () -> LOGGER.info("I'm done now.");
     task.executeWith(callback);
   }
 }

callback/src/test/java/com/iluwatar/callback/CallbackTest.java

@@ -38,29 +38,6 @@ public class CallbackTest {
 
   @Test
   public void test() {
-    Callback callback = new Callback() {
-      @Override
-      public void call() {
-        callingCount++;
-      }
-    };
-
-    Task task = new SimpleTask();
-
-    assertEquals(new Integer(0), callingCount, "Initial calling count of 0");
-
-    task.executeWith(callback);
-
-    assertEquals(new Integer(1), callingCount, "Callback called once");
-
-    task.executeWith(callback);
-
-    assertEquals(new Integer(2), callingCount, "Callback called twice");
-
-  }
-
-  @Test
-  public void testWithLambdasExample() {
     Callback callback = () -> callingCount++;
 
     Task task = new SimpleTask();

chain/pom.xml

@@ -29,15 +29,10 @@
   <parent>
     <groupId>com.iluwatar</groupId>
     <artifactId>java-design-patterns</artifactId>
-    <version>1.19.0</version>
+    <version>1.21.0</version>
   </parent>
   <artifactId>chain</artifactId>
   <dependencies>
-    <dependency>
-      <groupId>org.junit.jupiter</groupId>
-      <artifactId>junit-jupiter-api</artifactId>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.junit.jupiter</groupId>
       <artifactId>junit-jupiter-engine</artifactId>

chain/src/main/java/com/iluwatar/chain/OrcCommander.java

@@ -35,7 +35,7 @@ public class OrcCommander extends RequestHandler {
 
   @Override
   public void handleRequest(Request req) {
-    if (req.getRequestType().equals(RequestType.DEFEND_CASTLE)) {
+    if (RequestType.DEFEND_CASTLE == req.getRequestType()) {
       printHandling(req);
       req.markHandled();
     } else {

chain/src/main/java/com/iluwatar/chain/OrcOfficer.java

@@ -35,7 +35,7 @@ public class OrcOfficer extends RequestHandler {
 
   @Override
   public void handleRequest(Request req) {
-    if (req.getRequestType().equals(RequestType.TORTURE_PRISONER)) {
+    if (RequestType.TORTURE_PRISONER == req.getRequestType()) {
       printHandling(req);
       req.markHandled();
     } else {

chain/src/main/java/com/iluwatar/chain/OrcSoldier.java

@@ -35,7 +35,7 @@ public class OrcSoldier extends RequestHandler {
 
   @Override
   public void handleRequest(Request req) {
-    if (req.getRequestType().equals(RequestType.COLLECT_TAX)) {
+    if (RequestType.COLLECT_TAX == req.getRequestType()) {
       printHandling(req);
       req.markHandled();
     } else {

chain/src/test/java/com/iluwatar/chain/OrcKingTest.java

@@ -43,7 +43,7 @@ public class OrcKingTest {
   };
 
   @Test
-  public void testMakeRequest() throws Exception {
+  public void testMakeRequest() {
     final OrcKing king = new OrcKing();
 
     for (final Request request : REQUESTS) {

collection-pipeline/README.md

@@ -0,0 +1,30 @@
+---
+layout: pattern
+title: Collection Pipeline
+folder: collection-pipeline
+permalink: /patterns/collection-pipeline/
+categories: Functional
+tags:
+ - Java
+ - Difficulty-Beginner
+ - Functional
+---
+
+## Intent
+Collection Pipeline introduces Function Composition and Collection Pipeline, two functional-style patterns that you can combine to iterate collections in your code. 
+In functional programming, it's common to sequence complex operations through a series of smaller modular functions or operations. The series is called a composition of functions, or a function composition. When a collection of data flows through a function composition, it becomes a collection pipeline. Function Composition and Collection Pipeline are two design patterns frequently used in functional-style programming.
+
+![alt text](./etc/collection-pipeline.png "Collection Pipeline")
+
+## Applicability
+Use the Collection Pipeline pattern when
+
+* When you want to perform a sequence of operations where one operation's collected output is fed into the next
+* When you use a lot of statements in your code
+* When you use a lot of loops in your code
+
+## Credits
+
+* [Function composition and the Collection Pipeline pattern](https://www.ibm.com/developerworks/library/j-java8idioms2/index.html)
+* [Martin Fowler](https://martinfowler.com/articles/collection-pipeline/)
+* [Java8 Streams](https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html)

collection-pipeline/etc/collection-pipeline.ucls

@@ -0,0 +1,98 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<class-diagram version="1.2.2" icons="true" always-add-relationships="false" generalizations="true" realizations="true" 
+  associations="true" dependencies="false" nesting-relationships="true" router="FAN">  
+  <class id="1" language="java" name="com.iluwatar.collectionpipeline.App" project="collection-pipeline" 
+    file="/collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/App.java" binary="false" 
+    corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="150" y="100"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="2" language="java" name="com.iluwatar.collectionpipeline.ImperativeProgramming" 
+    project="collection-pipeline" 
+    file="/collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/ImperativeProgramming.java" binary="false" 
+    corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="437" y="109"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="3" language="java" name="com.iluwatar.collectionpipeline.Car" project="collection-pipeline" 
+    file="/collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/Car.java" binary="false" 
+    corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="724" y="339"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="4" language="java" name="com.iluwatar.collectionpipeline.CarFactory" project="collection-pipeline" 
+    file="/collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/CarFactory.java" binary="false" 
+    corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="899" y="91"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="5" language="java" name="com.iluwatar.collectionpipeline.FunctionalProgramming" 
+    project="collection-pipeline" 
+    file="/collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/FunctionalProgramming.java" binary="false" 
+    corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="1187" y="109"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <enumeration id="6" language="java" name="com.iluwatar.collectionpipeline.Category" project="collection-pipeline" 
+    file="/collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/Category.java" binary="false" 
+    corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="486" y="339"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="true" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </enumeration>  
+  <class id="7" language="java" name="com.iluwatar.collectionpipeline.Person" project="collection-pipeline" 
+    file="/collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/Person.java" binary="false" 
+    corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="723" y="91"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <association id="8">    
+    <end type="SOURCE" refId="7" navigable="false">      
+      <attribute id="9" name="cars"/>      
+      <multiplicity id="10" minimum="0" maximum="2147483647"/>    
+    </end>    
+    <end type="TARGET" refId="3" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <association id="11">    
+    <end type="SOURCE" refId="3" navigable="false">      
+      <attribute id="12" name="category"/>      
+      <multiplicity id="13" minimum="0" maximum="1"/>    
+    </end>    
+    <end type="TARGET" refId="6" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+    sort-features="false" accessors="true" visibility="true">    
+    <attributes public="true" package="true" protected="true" private="true" static="true"/>    
+    <operations public="true" package="true" protected="true" private="true" static="true"/>  
+  </classifier-display>  
+  <association-display labels="true" multiplicity="true"/>
+</class-diagram>

collection-pipeline/pom.xml

@@ -0,0 +1,40 @@
+<!--
+
+    The MIT License
+    Copyright (c) 2014 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.21.0</version>
+  </parent>
+  <artifactId>collection-pipeline</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter-engine</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>

collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/App.java

@@ -0,0 +1,76 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.collectionpipeline;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Map;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * In imperative-style programming, it is common to use for and while loops for
+ * most kinds of data processing. Function composition is a simple technique
+ * that lets you sequence modular functions to create more complex operations.
+ * When you run data through the sequence, you have a collection pipeline.
+ * Together, the Function Composition and Collection Pipeline patterns enable
+ * you to create sophisticated programs where data flow from upstream to
+ * downstream and is passed through a series of transformations.
+ *
+ */
+public class App {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
+  /**
+   * Program entry point.
+   * 
+   * @param args
+   *            command line args
+   */
+  public static void main(String[] args) {
+
+    List<Car> cars = CarFactory.createCars();
+    
+    List<String> modelsImperative = ImperativeProgramming.getModelsAfter2000(cars);
+    LOGGER.info(modelsImperative.toString());
+
+    List<String> modelsFunctional = FunctionalProgramming.getModelsAfter2000(cars);
+    LOGGER.info(modelsFunctional.toString());
+    
+    Map<Category, List<Car>> groupingByCategoryImperative = ImperativeProgramming.getGroupingOfCarsByCategory(cars);
+    LOGGER.info(groupingByCategoryImperative.toString());
+
+    Map<Category, List<Car>> groupingByCategoryFunctional = FunctionalProgramming.getGroupingOfCarsByCategory(cars);
+    LOGGER.info(groupingByCategoryFunctional.toString());
+    
+    Person john = new Person(cars);
+    
+    List<Car> sedansOwnedImperative = ImperativeProgramming.getSedanCarsOwnedSortedByDate(Arrays.asList(john));
+    LOGGER.info(sedansOwnedImperative.toString());
+
+    List<Car> sedansOwnedFunctional = FunctionalProgramming.getSedanCarsOwnedSortedByDate(Arrays.asList(john));
+    LOGGER.info(sedansOwnedFunctional.toString());
+  }
+}

collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/Car.java

@@ -0,0 +1,109 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.collectionpipeline;
+
+/**
+ * A Car class that has the properties of make, model, year and category.
+ */
+public class Car {
+  private final String make;
+  private final String model;
+  private final int year;
+  private final Category category;
+
+  /**
+   * Constructor to create an instance of car.
+   * @param make the make of the car
+   * @param model the model of the car
+   * @param yearOfMake the year of built of the car
+   * @param category the {@link Category} of the car
+   */
+  public Car(String make, String model, int yearOfMake, Category category) {
+    this.make = make;
+    this.model = model;
+    this.year = yearOfMake;
+    this.category = category;
+  }
+
+  @Override
+  public int hashCode() {
+    final int prime = 31;
+    int result = 1;
+    result = prime * result + ((category == null) ? 0 : category.hashCode());
+    result = prime * result + ((make == null) ? 0 : make.hashCode());
+    result = prime * result + ((model == null) ? 0 : model.hashCode());
+    result = prime * result + year;
+    return result;
+  }
+
+  @Override
+  public boolean equals(Object obj) {
+    if (this == obj) {
+      return true;
+    }
+    if (obj == null) {
+      return false;
+    }
+    if (getClass() != obj.getClass()) {
+      return false;
+    }
+    Car other = (Car) obj;
+    if (category != other.category) {
+      return false;
+    }
+    if (make == null) {
+      if (other.make != null) {
+        return false;
+      }
+    } else if (!make.equals(other.make)) {
+      return false;
+    }
+    if (model == null) {
+      if (other.model != null) {
+        return false;
+      }
+    } else if (!model.equals(other.model)) {
+      return false;
+    }
+    if (year != other.year) {
+      return false;
+    }
+    return true;
+  }
+
+  public String getMake() {
+    return make;
+  }
+
+  public String getModel() {
+    return model;
+  }
+
+  public int getYear() {
+    return year;
+  }
+  
+  public Category getCategory() {
+    return category;
+  }
+}

collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/CarFactory.java

@@ -0,0 +1,47 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.collectionpipeline;
+
+import java.util.Arrays;
+import java.util.List;
+
+/**
+ * A factory class to create a collection of {@link Car} instances.
+ */
+public class CarFactory {
+  private CarFactory() {
+  }
+
+  /**
+   * Factory method to create a {@link List} of {@link Car} instances.
+   * @return {@link List} of {@link Car}
+   */
+  public static List<Car> createCars() {
+    return Arrays.asList(new Car("Jeep", "Wrangler", 2011, Category.JEEP),
+        new Car("Jeep", "Comanche", 1990, Category.JEEP),
+        new Car("Dodge", "Avenger", 2010, Category.SEDAN),
+        new Car("Buick", "Cascada", 2016, Category.CONVERTIBLE),
+        new Car("Ford", "Focus", 2012, Category.SEDAN),
+        new Car("Chevrolet", "Geo Metro", 1992, Category.CONVERTIBLE));
+  }
+}

collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/Category.java

@@ -0,0 +1,30 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.collectionpipeline;
+
+/**
+ * Enum for the category of car 
+ */
+public enum Category {
+  JEEP, SEDAN, CONVERTIBLE
+}