docs: update .all-contributorsrc [skip ci]

#1313 Add Separated Interface pattern

.all-contributorsrc

@@ -1082,7 +1082,8 @@
       "avatar_url": "https://avatars1.githubusercontent.com/u/10645273?v=4",
       "profile": "https://github.com/ravening",
       "contributions": [
-        "code"
+        "code",
+        "review"
       ]
     },
     {
@@ -1185,6 +1186,33 @@
       "contributions": [
         "code"
       ]
+    },
+    {
+      "login": "samilAyoub",
+      "name": "samilAyoub",
+      "avatar_url": "https://avatars0.githubusercontent.com/u/61546990?v=4",
+      "profile": "https://github.com/samilAyoub",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "vdlald",
+      "name": "Vladislav Golubinov",
+      "avatar_url": "https://avatars0.githubusercontent.com/u/29997701?v=4",
+      "profile": "https://github.com/vdlald",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "swarajsaaj",
+      "name": "Swaraj",
+      "avatar_url": "https://avatars2.githubusercontent.com/u/6285049?v=4",
+      "profile": "https://github.com/swarajsaaj",
+      "contributions": [
+        "code"
+      ]
     }
   ],
   "contributorsPerLine": 4,

README.md

@@ -10,7 +10,7 @@
 [![Coverage](https://sonarcloud.io/api/project_badges/measure?project=iluwatar_java-design-patterns&metric=coverage)](https://sonarcloud.io/dashboard?id=iluwatar_java-design-patterns)
 [![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)
 <!-- ALL-CONTRIBUTORS-BADGE:START - Do not remove or modify this section -->
-[![All Contributors](https://img.shields.io/badge/all_contributors-130-orange.svg?style=flat-square)](#contributors-)
+[![All Contributors](https://img.shields.io/badge/all_contributors-133-orange.svg?style=flat-square)](#contributors-)
 <!-- ALL-CONTRIBUTORS-BADGE:END -->
 
 # Introduction
@@ -246,7 +246,7 @@ This project is licensed under the terms of the MIT license.
   <tr>
     <td align="center"><a href="https://github.com/nishant"><img src="https://avatars2.githubusercontent.com/u/15331971?v=4" width="100px;" alt=""/><br /><sub><b>Nishant Arora</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=nishant" title="Code">💻</a></td>
     <td align="center"><a href="https://github.com/raja-peeyush-kumar-singh"><img src="https://avatars0.githubusercontent.com/u/5496024?v=4" width="100px;" alt=""/><br /><sub><b>Peeyush</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=raja-peeyush-kumar-singh" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/ravening"><img src="https://avatars1.githubusercontent.com/u/10645273?v=4" width="100px;" alt=""/><br /><sub><b>Rakesh</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ravening" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/ravening"><img src="https://avatars1.githubusercontent.com/u/10645273?v=4" width="100px;" alt=""/><br /><sub><b>Rakesh</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ravening" title="Code">💻</a> <a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3Aravening" title="Reviewed Pull Requests">👀</a></td>
     <td align="center"><a href="https://github.com/vINCENT8888801"><img src="https://avatars0.githubusercontent.com/u/8037883?v=4" width="100px;" alt=""/><br /><sub><b>Wei Seng</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=vINCENT8888801" title="Code">💻</a></td>
   </tr>
   <tr>
@@ -264,6 +264,11 @@ This project is licensed under the terms of the MIT license.
   <tr>
     <td align="center"><a href="https://www.stefan-birkner.de"><img src="https://avatars1.githubusercontent.com/u/711349?v=4" width="100px;" alt=""/><br /><sub><b>Stefan Birkner</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=stefanbirkner" title="Code">💻</a></td>
     <td align="center"><a href="https://github.com/fedorskvorcov"><img src="https://avatars3.githubusercontent.com/u/43882212?v=4" width="100px;" alt=""/><br /><sub><b>Fedor Skvorcov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=fedorskvorcov" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/samilAyoub"><img src="https://avatars0.githubusercontent.com/u/61546990?v=4" width="100px;" alt=""/><br /><sub><b>samilAyoub</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=samilAyoub" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/vdlald"><img src="https://avatars0.githubusercontent.com/u/29997701?v=4" width="100px;" alt=""/><br /><sub><b>Vladislav Golubinov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=vdlald" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/swarajsaaj"><img src="https://avatars2.githubusercontent.com/u/6285049?v=4" width="100px;" alt=""/><br /><sub><b>Swaraj</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=swarajsaaj" title="Code">💻</a></td>
   </tr>
 </table>
 

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

@@ -23,7 +23,6 @@
 
 package com.iluwatar.abstractfactory;
 
-import com.iluwatar.abstractfactory.App.FactoryMaker.KingdomType;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -41,84 +40,14 @@ import org.slf4j.LoggerFactory;
  * and its implementations ( {@link ElfKingdomFactory}, {@link OrcKingdomFactory}). The example uses
  * both concrete implementations to create a king, a castle and an army.
  */
-public class App {
+public class App implements Runnable {
 
-  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+  private static Logger log = LoggerFactory.getLogger(App.class);
 
-  private King king;
-  private Castle castle;
-  private Army army;
+  private final Kingdom kingdom = new Kingdom();
 
-  /**
-   * Creates kingdom.
-   */
-  public void createKingdom(final KingdomFactory factory) {
-    setKing(factory.createKing());
-    setCastle(factory.createCastle());
-    setArmy(factory.createArmy());
-  }
-
-  King getKing(final KingdomFactory factory) {
-    return factory.createKing();
-  }
-
-  public King getKing() {
-    return king;
-  }
-
-  private void setKing(final King king) {
-    this.king = king;
-  }
-
-  Castle getCastle(final KingdomFactory factory) {
-    return factory.createCastle();
-  }
-
-  public Castle getCastle() {
-    return castle;
-  }
-
-  private void setCastle(final Castle castle) {
-    this.castle = castle;
-  }
-
-  Army getArmy(final KingdomFactory factory) {
-    return factory.createArmy();
-  }
-
-  public Army getArmy() {
-    return army;
-  }
-
-  private void setArmy(final Army army) {
-    this.army = army;
-  }
-
-  /**
-   * The factory of kingdom factories.
-   */
-  public static class FactoryMaker {
-
-    /**
-     * Enumeration for the different types of Kingdoms.
-     */
-    public enum KingdomType {
-      ELF, ORC
-    }
-
-    /**
-     * The factory method to create KingdomFactory concrete objects.
-     */
-    public static KingdomFactory makeFactory(KingdomType type) {
-      switch (type) {
-        case ELF:
-          return new ElfKingdomFactory();
-        case ORC:
-          return new OrcKingdomFactory();
-        default:
-          throw new IllegalArgumentException("KingdomType not supported.");
-      }
-    }
+  public Kingdom getKingdom() {
+    return kingdom;
   }
 
   /**
@@ -127,19 +56,33 @@ public class App {
    * @param args command line args
    */
   public static void main(String[] args) {
-
     var app = new App();
+    app.run();
+  }
 
-    LOGGER.info("Elf Kingdom");
-    app.createKingdom(FactoryMaker.makeFactory(KingdomType.ELF));
-    LOGGER.info(app.getArmy().getDescription());
-    LOGGER.info(app.getCastle().getDescription());
-    LOGGER.info(app.getKing().getDescription());
+  @Override
+  public void run() {
+    log.info("Elf Kingdom");
+    createKingdom(Kingdom.FactoryMaker.KingdomType.ELF);
+    log.info(kingdom.getArmy().getDescription());
+    log.info(kingdom.getCastle().getDescription());
+    log.info(kingdom.getKing().getDescription());
 
-    LOGGER.info("Orc Kingdom");
-    app.createKingdom(FactoryMaker.makeFactory(KingdomType.ORC));
-    LOGGER.info(app.getArmy().getDescription());
-    LOGGER.info(app.getCastle().getDescription());
-    LOGGER.info(app.getKing().getDescription());
+    log.info("Orc Kingdom");
+    createKingdom(Kingdom.FactoryMaker.KingdomType.ORC);
+    log.info(kingdom.getArmy().getDescription());
+    log.info(kingdom.getCastle().getDescription());
+    log.info(kingdom.getKing().getDescription());
+  }
+
+  /**
+   * Creates kingdom.
+   * @param kingdomType type of Kingdom
+   */
+  public void createKingdom(final Kingdom.FactoryMaker.KingdomType kingdomType) {
+    final KingdomFactory kingdomFactory = Kingdom.FactoryMaker.makeFactory(kingdomType);
+    kingdom.setKing(kingdomFactory.createKing());
+    kingdom.setCastle(kingdomFactory.createCastle());
+    kingdom.setArmy(kingdomFactory.createArmy());
   }
 }

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

@@ -0,0 +1,59 @@
+package com.iluwatar.abstractfactory;
+
+public class Kingdom {
+
+  private King king;
+  private Castle castle;
+  private Army army;
+
+  public King getKing() {
+    return king;
+  }
+
+  public Castle getCastle() {
+    return castle;
+  }
+
+  public Army getArmy() {
+    return army;
+  }
+
+  public void setKing(King king) {
+    this.king = king;
+  }
+
+  public void setCastle(Castle castle) {
+    this.castle = castle;
+  }
+
+  public void setArmy(Army army) {
+    this.army = army;
+  }
+
+  /**
+   * The factory of kingdom factories.
+   */
+  public static class FactoryMaker {
+
+    /**
+     * Enumeration for the different types of Kingdoms.
+     */
+    public enum KingdomType {
+      ELF, ORC
+    }
+
+    /**
+     * The factory method to create KingdomFactory concrete objects.
+     */
+    public static KingdomFactory makeFactory(KingdomType type) {
+      switch (type) {
+        case ELF:
+          return new ElfKingdomFactory();
+        case ORC:
+          return new OrcKingdomFactory();
+        default:
+          throw new IllegalArgumentException("KingdomType not supported.");
+      }
+    }
+  }
+}

abstract-factory/src/test/java/com/iluwatar/abstractfactory/AbstractFactoryTest.java

@@ -23,65 +23,71 @@
 
 package com.iluwatar.abstractfactory;
 
+import org.junit.jupiter.api.Test;
+
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
-import com.iluwatar.abstractfactory.App.FactoryMaker;
-import com.iluwatar.abstractfactory.App.FactoryMaker.KingdomType;
-import org.junit.jupiter.api.BeforeEach;
-import org.junit.jupiter.api.Test;
-
 /**
  * Test for abstract factory.
  */
 public class AbstractFactoryTest {
 
   private final App app = new App();
-  private KingdomFactory elfFactory;
-  private KingdomFactory orcFactory;
-
-  @BeforeEach
-  public void setUp() {
-    elfFactory = FactoryMaker.makeFactory(KingdomType.ELF);
-    orcFactory = FactoryMaker.makeFactory(KingdomType.ORC);
-  }
 
   @Test
   public void king() {
-    final var elfKing = app.getKing(elfFactory);
+    app.createKingdom(Kingdom.FactoryMaker.KingdomType.ELF);
+    final var kingdom = app.getKingdom();
+
+    final var elfKing = kingdom.getKing();
     assertTrue(elfKing instanceof ElfKing);
     assertEquals(ElfKing.DESCRIPTION, elfKing.getDescription());
-    final var orcKing = app.getKing(orcFactory);
+
+    app.createKingdom(Kingdom.FactoryMaker.KingdomType.ORC);
+    final var orcKing = kingdom.getKing();
     assertTrue(orcKing instanceof OrcKing);
     assertEquals(OrcKing.DESCRIPTION, orcKing.getDescription());
   }
 
   @Test
   public void castle() {
-    final var elfCastle = app.getCastle(elfFactory);
+    app.createKingdom(Kingdom.FactoryMaker.KingdomType.ELF);
+    final var kingdom = app.getKingdom();
+
+    final var elfCastle = kingdom.getCastle();
     assertTrue(elfCastle instanceof ElfCastle);
     assertEquals(ElfCastle.DESCRIPTION, elfCastle.getDescription());
-    final var orcCastle = app.getCastle(orcFactory);
+
+    app.createKingdom(Kingdom.FactoryMaker.KingdomType.ORC);
+    final var orcCastle = kingdom.getCastle();
     assertTrue(orcCastle instanceof OrcCastle);
     assertEquals(OrcCastle.DESCRIPTION, orcCastle.getDescription());
   }
 
   @Test
   public void army() {
-    final var elfArmy = app.getArmy(elfFactory);
+    app.createKingdom(Kingdom.FactoryMaker.KingdomType.ELF);
+    final var kingdom = app.getKingdom();
+
+    final var elfArmy = kingdom.getArmy();
     assertTrue(elfArmy instanceof ElfArmy);
     assertEquals(ElfArmy.DESCRIPTION, elfArmy.getDescription());
-    final var orcArmy = app.getArmy(orcFactory);
+
+    app.createKingdom(Kingdom.FactoryMaker.KingdomType.ORC);
+    final var orcArmy = kingdom.getArmy();
     assertTrue(orcArmy instanceof OrcArmy);
     assertEquals(OrcArmy.DESCRIPTION, orcArmy.getDescription());
   }
 
   @Test
   public void createElfKingdom() {
-    app.createKingdom(elfFactory);
-    final var king = app.getKing();
-    final var castle = app.getCastle();
-    final var army = app.getArmy();
+    app.createKingdom(Kingdom.FactoryMaker.KingdomType.ELF);
+    final var kingdom = app.getKingdom();
+
+    final var king = kingdom.getKing();
+    final var castle = kingdom.getCastle();
+    final var army = kingdom.getArmy();
     assertTrue(king instanceof ElfKing);
     assertEquals(ElfKing.DESCRIPTION, king.getDescription());
     assertTrue(castle instanceof ElfCastle);
@@ -92,10 +98,12 @@ public class AbstractFactoryTest {
 
   @Test
   public void createOrcKingdom() {
-    app.createKingdom(orcFactory);
-    final var king = app.getKing();
-    final var castle = app.getCastle();
-    final var army = app.getArmy();
+    app.createKingdom(Kingdom.FactoryMaker.KingdomType.ORC);
+    final var kingdom = app.getKingdom();
+
+    final var king = kingdom.getKing();
+    final var castle = kingdom.getCastle();
+    final var army = kingdom.getArmy();
     assertTrue(king instanceof OrcKing);
     assertEquals(OrcKing.DESCRIPTION, king.getDescription());
     assertTrue(castle instanceof OrcCastle);

factory/README.md

@@ -0,0 +1,133 @@
+---
+layout: pattern
+title: Factory
+folder: factory
+permalink: /patterns/factory/
+categories: Creational
+tags:
+ - Gang of Four
+---
+
+## Also known as
+
+* Simple Factory
+* Static Factory Method
+
+## Intent
+
+Providing a static method encapsulated in a class called factory, in order to hide the 
+implementation logic and makes client code focus on usage rather then initialization new objects.
+
+## Explanation
+
+Real world example
+
+> Lets say we have a web application connected to SQLServer, but now we want to switch to Oracle. To 
+> do so without modifying existing source code, we need to implements Simple Factory pattern, in 
+> which a static method can be invoked to create connection to a given database.
+
+Wikipedia says
+
+> Factory is an object for creating other objects – formally a factory is a function or method that 
+> returns objects of a varying prototype or class.
+
+**Programmatic Example**
+
+We have an interface `Car` and two implementations `Ford` and `Ferrari`.
+
+```java
+public interface Car {
+  String getDescription();
+}
+
+public class Ford implements Car {
+
+  static final String DESCRIPTION = "This is Ford.";
+
+  @Override
+  public String getDescription() {
+    return DESCRIPTION;
+  }
+}
+
+public class Ferrari implements Car {
+   
+  static final String DESCRIPTION = "This is Ferrari.";
+
+  @Override
+  public String getDescription() {
+    return DESCRIPTION;
+  }
+}
+```
+
+Enumeration above represents types of cars that we support (`Ford` and `Ferrari`).
+
+```java
+public enum CarType {
+  
+  FORD(Ford::new), 
+  FERRARI(Ferrari::new);
+  
+  private final Supplier<Car> constructor; 
+  
+  CarType(Supplier<Car> constructor) {
+    this.constructor = constructor;
+  }
+  
+  public Supplier<Car> getConstructor() {
+    return this.constructor;
+  }
+}
+```
+Then we have the static method `getCar` to create car objects encapsulated in the factory class 
+`CarSimpleFactory`.
+
+```java
+public class CarsFactory {
+  
+  public static Car getCar(CarType type) {
+    return type.getConstructor().get();
+  }
+}
+```
+
+Now on the client code we can create different types of cars using the factory class.
+
+```java
+var car1 = CarsFactory.getCar(CarType.FORD);
+var car2 = CarsFactory.getCar(CarType.FERRARI);
+LOGGER.info(car1.getDescription());
+LOGGER.info(car2.getDescription());;
+```
+
+Program output:
+
+```java
+This is Ford.
+This Ferrari.
+```
+
+## Class Diagram
+
+![alt text](./etc/factory.urm.png "Factory pattern class diagram")
+
+## Applicability
+
+Use the Simple Factory pattern when you only care about the creation of a object, not how to create 
+and manage it.
+
+Pros
+
+* Allows keeping all objects creation in one place and avoid of spreading 'new' key value across codebase.
+* Allows to writs loosely coupled code. Some of its main advantages include better testability, easy-to-understand code, swappable components, scalability and isolated features.
+
+Cons
+
+* The code becomes more complicated than it should be. 
+
+## Related patterns
+
+* [Factory Method](https://java-design-patterns.com/patterns/factory-method/)
+* [Factory Kit](https://java-design-patterns.com/patterns/factory-kit/)
+* [Abstract Factory](https://java-design-patterns.com/patterns/abstract-factory/)

factory/etc/factory.urm.puml

@@ -0,0 +1,35 @@
+@startuml
+package com.iluwatar.factory {
+  class App {
+    - LOGGER : Logger {static}
+    + App()
+    + main(args : String[]) {static}
+  }
+  interface Car {
+    + getDescription() : String {abstract}
+  }
+  class CarsFactory {
+    + CarsFactory()
+    + getCar(type : CarType) : Car {static}
+  }
+  ~enum CarType {
+    + FERRARI {static}
+    + FORD {static}
+    + valueOf(name : String) : CarType {static}
+    + values() : CarType[] {static}
+  }
+  class Ferrari {
+    ~ DESCRIPTION : String {static}
+    + Ferrari()
+    + getDescription() : String
+  }
+  class Ford {
+    ~ DESCRIPTION : String {static}
+    + Ford()
+    + getDescription() : String
+  }
+}
+CarType ..+ CarsFactory
+Ferrari ..|> Car 
+Ford ..|> Car 
+@enduml

factory/pom.xml

@@ -0,0 +1,43 @@
+<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 https://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>1.24.0-SNAPSHOT</version>
+ </parent>
+ <artifactId>factory</artifactId>
+ <dependencies>
+ <dependency>
+  <groupId>org.junit.jupiter</groupId>
+  <artifactId>junit-jupiter-engine</artifactId>
+  <scope>test</scope>
+ </dependency>
+ <dependency>
+ <groupId>junit</groupId>
+  <artifactId>junit</artifactId>
+ </dependency>
+ </dependencies>
+ <build>
+ <plugins>
+  <!-- Maven assembly plugin is invoked with default setting which we have 
+  in parent pom and specifying the class having main method -->
+  <plugin>
+  <groupId>org.apache.maven.plugins</groupId>
+  <artifactId>maven-assembly-plugin</artifactId>
+  <executions>
+   <execution>
+   <configuration>
+    <archive>
+    <manifest>
+     <mainClass>com.iluwatar.factory.App</mainClass>
+    </manifest>
+    </archive>
+   </configuration>
+   </execution>
+  </executions>
+  </plugin>
+ </plugins>
+ </build>
+</project>

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

@@ -0,0 +1,51 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 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.
+ */
+
+package com.iluwatar.factory;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Factory is an object for creating other objects, it providing Providing a static method to 
+ * create and return objects of varying classes, in order to hide the implementation logic 
+ * and makes client code focus on usage rather then objects initialization and management.
+ * 
+ * <p>In this example the CarFactory is the factory class and it provides a static method to 
+ * create different cars.
+ */
+
+public class App {
+  
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+  
+  /**
+   * Program main entry point.
+   */
+  public static void main(String[] args) {
+    var car1 = CarsFactory.getCar(CarType.FORD);
+    var car2 = CarsFactory.getCar(CarType.FERRARI);
+    LOGGER.info(car1.getDescription());
+    LOGGER.info(car2.getDescription());
+  }
+}

factory/src/main/java/com/iluwatar/factory/Car.java

@@ -0,0 +1,10 @@
+package com.iluwatar.factory;
+
+/**
+ * Car interface.
+ */
+public interface Car {
+  
+  String getDescription();
+  
+}

factory/src/main/java/com/iluwatar/factory/CarType.java

@@ -0,0 +1,22 @@
+package com.iluwatar.factory;
+
+import java.util.function.Supplier;
+
+public enum CarType {
+  
+  /**
+   * Enumeration for different types of cars.
+   */
+  FORD(Ford::new), 
+  FERRARI(Ferrari::new);
+  
+  private final Supplier<Car> constructor; 
+  
+  CarType(Supplier<Car> constructor) {
+    this.constructor = constructor;
+  }
+  
+  public Supplier<Car> getConstructor() {
+    return this.constructor;
+  }
+}

factory/src/main/java/com/iluwatar/factory/CarsFactory.java

@@ -0,0 +1,14 @@
+package com.iluwatar.factory;
+
+/**
+ * Factory of cars.
+ */
+public class CarsFactory {
+  
+  /**
+   * Factory method takes as parameter a car type and initiate the appropriate class.
+   */
+  public static Car getCar(CarType type) {
+    return type.getConstructor().get();
+  }
+}

factory/src/main/java/com/iluwatar/factory/Ferrari.java

@@ -0,0 +1,14 @@
+package com.iluwatar.factory;
+
+/**
+ * Ferrari implementation.
+ */
+public class Ferrari implements Car {
+   
+  static final String DESCRIPTION = "This is Ferrari.";
+
+  @Override
+  public String getDescription() {
+    return DESCRIPTION;
+  }
+}

factory/src/main/java/com/iluwatar/factory/Ford.java

@@ -0,0 +1,14 @@
+package com.iluwatar.factory;
+
+/**
+ * Ford implementation.
+ */
+public class Ford implements Car {
+
+  static final String DESCRIPTION = "This is Ford.";
+
+  @Override
+  public String getDescription() {
+    return DESCRIPTION;
+  }
+}

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

@@ -0,0 +1,14 @@
+package com.iluwatar.factory;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+class AppTest {
+
+	@Test
+	void shouldExecuteWithoutExceptions() {
+		assertDoesNotThrow(() -> App.main(new String[]{}));
+	}
+
+}

factory/src/test/java/com/iluwatar/factory/CarsFactoryTest.java

@@ -0,0 +1,15 @@
+package com.iluwatar.factory;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+class CarsFactoryTest {
+
+	@Test
+	void shouldReturnFerrariInstance() {
+		final var ferrari = CarsFactory.getCar(CarType.FERRARI);
+		assertTrue(ferrari instanceof Ferrari);
+	}
+
+}

pom.xml

@@ -195,6 +195,8 @@
         <module>arrange-act-assert</module>
         <module>transaction-script</module>
         <module>filterer</module>
+        <module>factory</module>
+        <module>separated-interface</module>
     </modules>
 
     <repositories>

private-class-data/README.md

@@ -9,6 +9,7 @@ tags:
 ---
 
 ## Intent
+
 Private Class Data design pattern seeks to reduce exposure of attributes by limiting their 
 visibility. It reduces the number of class attributes by encapsulating them in single Data object.
 
@@ -124,9 +125,11 @@ immutableStew.mix();  // Mixing the immutable stew we find: 2 potatoes, 4 carrot
 ```
 
 ## Class diagram
+
 ![alt text](./etc/private-class-data.png "Private Class Data")
 
 ## Applicability
+
 Use the Private Class Data pattern when
 
-* You want to prevent write access to class data members
+* You want to prevent write access to class data members.

promise/README.md

@@ -14,23 +14,25 @@ CompletableFuture
 
 ## Intent
 
-A Promise represents a proxy for a value not necessarily known when the promise is created. It allows you to associate 
-dependent promises to an asynchronous action's eventual success value or failure reason. Promises are a way to write 
-async code that still appears as though it is executing in a synchronous way.
+A Promise represents a proxy for a value not necessarily known when the promise is created. It 
+allows you to associate dependent promises to an asynchronous action's eventual success value or 
+failure reason. Promises are a way to write async code that still appears as though it is executing 
+in a synchronous way.
 
 ## Explanation
 
-The Promise object is used for asynchronous computations. A Promise represents an operation that hasn't completed yet, 
-but is expected in the future.
+The Promise object is used for asynchronous computations. A Promise represents an operation that 
+hasn't completed yet, but is expected in the future.
 
 Promises provide a few advantages over callback objects:
- * Functional composition and error handling
- * Prevents callback hell and provides callback aggregation
+ * Functional composition and error handling.
+ * Prevents callback hell and provides callback aggregation.
 
 Real world example
 
-> We are developing a software solution that downloads files and calculates the number of lines and character 
-frequencies in those files. Promise is an ideal solution to make the code concise and easy to understand.
+> We are developing a software solution that downloads files and calculates the number of lines and 
+> character frequencies in those files. Promise is an ideal solution to make the code concise and 
+> easy to understand.
 
 In plain words
 
@@ -38,14 +40,15 @@ In plain words
 
 Wikipedia says
 
-> In computer science, future, promise, delay, and deferred refer to constructs used for synchronizing program 
-execution in some concurrent programming languages. They describe an object that acts as a proxy for a result that is 
-initially unknown, usually because the computation of its value is not yet complete.
+> In computer science, future, promise, delay, and deferred refer to constructs used for 
+> synchronizing program execution in some concurrent programming languages. They describe an object 
+> that acts as a proxy for a result that is initially unknown, usually because the computation of 
+> its value is not yet complete.
 
 **Programmatic Example**
 
-In the example a file is downloaded and its line count is calculated. The calculated line count is then consumed and 
-printed on console.
+In the example a file is downloaded and its line count is calculated. The calculated line count is 
+then consumed and printed on console.
 
 Let's first introduce a support class we need for implementation. Here's `PromiseSupport`.
 
@@ -195,7 +198,7 @@ public class Promise<T> extends PromiseSupport<T> {
 
   public <V> Promise<V> thenApply(Function<? super T, V> func) {
     Promise<V> dest = new Promise<>();
-    fulfillmentAction = new TransformAction<V>(this, dest, func);
+    fulfillmentAction = new TransformAction<>(this, dest, func);
     return dest;
   }
 
@@ -246,8 +249,8 @@ public class Promise<T> extends PromiseSupport<T> {
 }
 ```
 
-Now we can show the full example in action. Here's how to download and count the number of lines in a file using
-`Promise`.
+Now we can show the full example in action. Here's how to download and count the number of lines in 
+a file using `Promise`.
 
 ```java
   countLines().thenAccept(
@@ -280,8 +283,8 @@ Now we can show the full example in action. Here's how to download and count the
 
 ## Applicability
 
-Promise pattern is applicable in concurrent programming when some work needs to be done asynchronously
-and:
+Promise pattern is applicable in concurrent programming when some work needs to be done 
+asynchronously and:
 
 * Code maintainability and readability suffers due to callback hell.
 * You need to compose promises and need better error handling for asynchronous tasks.

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

@@ -81,7 +81,7 @@ public class App {
    * @throws InterruptedException if main thread is interrupted.
    * @throws ExecutionException   if an execution error occurs.
    */
-  public static void main(String[] args) throws InterruptedException, ExecutionException {
+  public static void main(String[] args) throws InterruptedException {
     var app = new App();
     try {
       app.promiseUsage();

promise/src/main/java/com/iluwatar/promise/Promise.java

@@ -141,7 +141,7 @@ public class Promise<T> extends PromiseSupport<T> {
    */
   public <V> Promise<V> thenApply(Function<? super T, V> func) {
     Promise<V> dest = new Promise<>();
-    fulfillmentAction = new TransformAction<V>(this, dest, func);
+    fulfillmentAction = new TransformAction<>(this, dest, func);
     return dest;
   }
 

promise/src/main/java/com/iluwatar/promise/PromiseSupport.java

@@ -26,7 +26,6 @@ package com.iluwatar.promise;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
-import java.util.concurrent.TimeoutException;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -114,4 +113,4 @@ class PromiseSupport<T> implements Future<T> {
     }
     throw new ExecutionException(exception);
   }
-}
+}

prototype/README.md

@@ -10,16 +10,19 @@ tags:
 ---
 
 ## Intent
-Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.
+
+Specify the kinds of objects to create using a prototypical instance, and create new objects by 
+copying this prototype.
 
 ## Explanation
 
-First it should be noted that Prototype pattern is not used to gain performance benefits. It's only used for creating
-new objects from prototype instance.
+First it should be noted that Prototype pattern is not used to gain performance benefits. It's only 
+used for creating new objects from prototype instance.
 
 Real world example
 
-> Remember Dolly? The sheep that was cloned! Lets not get into the details but the key point here is that it is all about cloning.
+> Remember Dolly? The sheep that was cloned! Lets not get into the details but the key point here is 
+> that it is all about cloning.
 
 In plain words
 
@@ -27,9 +30,12 @@ In plain words
 
 Wikipedia says
 
-> The prototype pattern is a creational design pattern in software development. It is used when the type of objects to create is determined by a prototypical instance, which is cloned to produce new objects.
+> The prototype pattern is a creational design pattern in software development. It is used when the 
+> type of objects to create is determined by a prototypical instance, which is cloned to produce new 
+> objects.
 
-In short, it allows you to create a copy of an existing object and modify it to your needs, instead of going through the trouble of creating an object from scratch and setting it up.
+In short, it allows you to create a copy of an existing object and modify it to your needs, instead 
+of going through the trouble of creating an object from scratch and setting it up.
 
 **Programmatic Example**
 
@@ -52,7 +58,7 @@ class Sheep implements Cloneable {
 }
 ```
 
-Then it can be cloned like below
+Then it can be cloned like below:
 
 ```java
 var original = new Sheep("Jolly");
@@ -65,15 +71,20 @@ System.out.println(cloned.getName()); // Dolly
 ```
 
 ## Class diagram
+
 ![alt text](./etc/prototype.urm.png "Prototype pattern class diagram")
 
 ## Applicability
-Use the Prototype pattern when a system should be independent of how its products are created, composed and represented; and
 
-* When the classes to instantiate are specified at run-time, for example, by dynamic loading
-* To avoid building a class hierarchy of factories that parallels the class hierarchy of products
-* When instances of a class can have one of only a few different combinations of state. It may be more convenient to install a corresponding number of prototypes and clone them rather than instantiating the class manually, each time with the appropriate state
-* When object creation is expensive compared to cloning
+Use the Prototype pattern when a system should be independent of how its products are created, 
+composed, represented and
+
+* When the classes to instantiate are specified at run-time, for example, by dynamic loading.
+* To avoid building a class hierarchy of factories that parallels the class hierarchy of products.
+* When instances of a class can have one of only a few different combinations of state. It may be 
+more convenient to install a corresponding number of prototypes and clone them rather than 
+instantiating the class manually, each time with the appropriate state.
+* When object creation is expensive compared to cloning.
 
 ## Real world examples
 

proxy/README.md

@@ -10,16 +10,20 @@ tags:
 ---
 
 ## Also known as
+
 Surrogate
 
 ## Intent
-Provide a surrogate or placeholder for another object to control
-access to it.
+
+Provide a surrogate or placeholder for another object to control access to it.
 
 ## Explanation
+
 Real world example
 
-> Imagine a tower where the local wizards go to study their spells. The ivory tower can only be accessed through a proxy which ensures that only the first three wizards can enter. Here the proxy represents the functionality of the tower and adds access control to it.
+> Imagine a tower where the local wizards go to study their spells. The ivory tower can only be 
+> accessed through a proxy which ensures that only the first three wizards can enter. Here the proxy 
+> represents the functionality of the tower and adds access control to it.
 
 In plain words
 
@@ -27,11 +31,17 @@ In plain words
 
 Wikipedia says
 
-> A proxy, in its most general form, is a class functioning as an interface to something else. A proxy is a wrapper or agent object that is being called by the client to access the real serving object behind the scenes. Use of the proxy can simply be forwarding to the real object, or can provide additional logic. In the proxy extra functionality can be provided, for example caching when operations on the real object are resource intensive, or checking preconditions before operations on the real object are invoked.
+> A proxy, in its most general form, is a class functioning as an interface to something else. 
+> A proxy is a wrapper or agent object that is being called by the client to access the real serving 
+> object behind the scenes. Use of the proxy can simply be forwarding to the real object, or can 
+> provide additional logic. In the proxy extra functionality can be provided, for example caching 
+> when operations on the real object are resource intensive, or checking preconditions before 
+> operations on the real object are invoked.
 
 **Programmatic Example**
 
-Taking our wizard tower example from above. Firstly we have the wizard tower interface and the ivory tower class
+Taking our wizard tower example from above. Firstly we have the `WizardTower` interface and the 
+`IvoryTower` class.
 
 ```java
 public interface WizardTower {
@@ -50,7 +60,7 @@ public class IvoryTower implements WizardTower {
 }
 ```
 
-Then a simple wizard class
+Then a simple `Wizard` class.
 
 ```java
 public class Wizard {
@@ -68,7 +78,7 @@ public class Wizard {
 }
 ```
 
-Then we have the proxy to add access control to wizard tower
+Then we have the `WizardTowerProxy` to add access control to `WizardTower`.
 
 ```java
 public class WizardTowerProxy implements WizardTower {
@@ -97,28 +107,41 @@ public class WizardTowerProxy implements WizardTower {
 }
 ```
 
-And here is tower entering scenario
+And here is the tower entering scenario.
 
 ```java
 var proxy = new WizardTowerProxy(new IvoryTower());
-proxy.enter(new Wizard("Red wizard")); // Red wizard enters the tower.
-proxy.enter(new Wizard("White wizard")); // White wizard enters the tower.
-proxy.enter(new Wizard("Black wizard")); // Black wizard enters the tower.
-proxy.enter(new Wizard("Green wizard")); // Green wizard is not allowed to enter!
-proxy.enter(new Wizard("Brown wizard")); // Brown wizard is not allowed to enter!
+proxy.enter(new Wizard("Red wizard"));
+proxy.enter(new Wizard("White wizard"));
+proxy.enter(new Wizard("Black wizard"));
+proxy.enter(new Wizard("Green wizard"));
+proxy.enter(new Wizard("Brown wizard"));
+```
+
+Program output:
+
+```
+Red wizard enters the tower.
+White wizard enters the tower.
+Black wizard enters the tower.
+Green wizard is not allowed to enter!
+Brown wizard is not allowed to enter!
 ```
 
 ## Class diagram
+
 ![alt text](./etc/proxy.urm.png "Proxy pattern class diagram")
 
 ## Applicability
-Proxy is applicable whenever there is a need for a more
-versatile or sophisticated reference to an object than a simple pointer. Here
-are several common situations in which the Proxy pattern is applicable
+
+Proxy is applicable whenever there is a need for a more versatile or sophisticated reference to an 
+object than a simple pointer. Here are several common situations in which the Proxy pattern is 
+applicable.
 
 * Remote proxy provides a local representative for an object in a different address space.
 * Virtual proxy creates expensive objects on demand.
-* Protection proxy controls access to the original object. Protection proxies are useful when objects should have different access rights.
+* Protection proxy controls access to the original object. Protection proxies are useful when 
+objects should have different access rights.
 
 ## Typical Use Case
 
@@ -136,7 +159,8 @@ are several common situations in which the Proxy pattern is applicable
 
 * [java.lang.reflect.Proxy](http://docs.oracle.com/javase/8/docs/api/java/lang/reflect/Proxy.html)
 * [Apache Commons Proxy](https://commons.apache.org/proper/commons-proxy/)
-* Mocking frameworks Mockito, Powermock, EasyMock
+* Mocking frameworks [Mockito](https://site.mockito.org/), 
+[Powermock](https://powermock.github.io/), [EasyMock](https://easymock.org/)
 
 ## Related patterns
 

proxy/src/main/java/com/iluwatar/proxy/IvoryTower.java

@@ -1,40 +1,40 @@
-/*
- * The MIT License
- * Copyright © 2014-2019 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.
- */
-
-package com.iluwatar.proxy;
-
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
-
-/**
- * The object to be proxyed.
- */
-public class IvoryTower implements WizardTower {
-
-  private static final Logger LOGGER = LoggerFactory.getLogger(IvoryTower.class);
-
-  public void enter(Wizard wizard) {
-    LOGGER.info("{} enters the tower.", wizard);
-  }
-
-}
+/*
+ * The MIT License
+ * Copyright © 2014-2019 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.
+ */
+
+package com.iluwatar.proxy;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * The object to be proxied.
+ */
+public class IvoryTower implements WizardTower {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(IvoryTower.class);
+
+  public void enter(Wizard wizard) {
+    LOGGER.info("{} enters the tower.", wizard);
+  }
+
+}

repository/README.md

@@ -9,26 +9,33 @@ tags:
 ---
 
 ## Intent
-Repository layer is added between the domain and data mapping layers to isolate domain objects from details of the 
-database access code and to minimize scattering and duplication of query code. The Repository pattern is especially 
-useful in systems where number of domain classes is large or heavy querying is utilized.
+
+Repository layer is added between the domain and data mapping layers to isolate domain objects from 
+details of the database access code and to minimize scattering and duplication of query code. The 
+Repository pattern is especially useful in systems where number of domain classes is large or heavy 
+querying is utilized.
 
 ## Explanation
+
 Real world example
 
-> Let's say we need a persistent data store for persons. Adding new persons and searching for them according to different criteria must be easy. 
+> Let's say we need a persistent data store for persons. Adding new persons and searching for them 
+> according to different criteria must be easy. 
 
 In plain words
 
-> Repository architectural pattern creates a uniform layer of data repositories that can be used for CRUD operations.
+> Repository architectural pattern creates a uniform layer of data repositories that can be used for 
+> CRUD operations.
 
 [Microsoft documentation](https://docs.microsoft.com/en-us/dotnet/architecture/microservices/microservice-ddd-cqrs-patterns/infrastructure-persistence-layer-design) says
 
-> Repositories are classes or components that encapsulate the logic required to access data sources. They centralize common data access functionality, providing better maintainability and decoupling the infrastructure or technology used to access databases from the domain model layer.
+> Repositories are classes or components that encapsulate the logic required to access data sources. 
+> They centralize common data access functionality, providing better maintainability and decoupling 
+> the infrastructure or technology used to access databases from the domain model layer.
 
 **Programmatic Example**
 
-Let's first look at the person class that we need to persist. 
+Let's first look at the person entity that we need to persist. 
 
 ```java
 @Entity
@@ -39,107 +46,23 @@ public class Person {
   private Long id;
   private String name;
   private String surname;
-
   private int age;
 
   public Person() {
   }
 
-  /**
-   * Constructor.
-   */
   public Person(String name, String surname, int age) {
     this.name = name;
     this.surname = surname;
     this.age = age;
   }
 
-  public Long getId() {
-    return id;
-  }
-
-  public void setId(Long id) {
-    this.id = id;
-  }
-
-  public String getName() {
-    return name;
-  }
-
-  public void setName(String name) {
-    this.name = name;
-  }
-
-  public String getSurname() {
-    return surname;
-  }
-
-  public void setSurname(String surname) {
-    this.surname = surname;
-  }
-
-  public int getAge() {
-    return age;
-  }
-
-  public void setAge(int age) {
-    this.age = age;
-  }
-
-  @Override
-  public String toString() {
-    return "Person [id=" + id + ", name=" + name + ", surname=" + surname + ", age=" + age + "]";
-  }
-
-  @Override
-  public int hashCode() {
-    final var prime = 31;
-    var result = 1;
-    result = prime * result + age;
-    result = prime * result + (id == null ? 0 : id.hashCode());
-    result = prime * result + (name == null ? 0 : name.hashCode());
-    result = prime * result + (surname == null ? 0 : surname.hashCode());
-    return result;
-  }
-
-  @Override
-  public boolean equals(Object obj) {
-    if (this == obj) {
-      return true;
-    }
-    if (obj == null) {
-      return false;
-    }
-    if (getClass() != obj.getClass()) {
-      return false;
-    }
-    var other = (Person) obj;
-    if (age != other.age) {
-      return false;
-    }
-    if (id == null) {
-      if (other.id != null) {
-        return false;
-      }
-    } else if (!id.equals(other.id)) {
-      return false;
-    }
-    if (name == null) {
-      if (other.name != null) {
-        return false;
-      }
-    } else if (!name.equals(other.name)) {
-      return false;
-    }
-    if (surname == null) {
-      return other.surname == null;
-    }
-    return surname.equals(other.surname);
-  }
+  // getters and setters ->
+  ...
 }
 ```
 
-We are using Spring Data to create the repository so it becomes really simple.
+We are using Spring Data to create the `PersonRepository` so it becomes really simple.
 
 ```java
 @Repository
@@ -150,7 +73,7 @@ public interface PersonRepository
 }
 ```
 
-Additionally we define a helper class for specification queries.
+Additionally we define a helper class `PersonSpecifications` for specification queries.
 
 ```java
 public class PersonSpecifications {
@@ -189,7 +112,7 @@ public class PersonSpecifications {
 }
 ```
 
-And here's the repository in action.
+And here's the repository example in action.
 
 ```java
     var peter = new Person("Peter", "Sagan", 17);
@@ -226,30 +149,36 @@ And here's the repository in action.
     persons.stream().map(Person::toString).forEach(LOGGER::info);
 
     repository.deleteAll();
-    
-    // Count Person records: 4
-    // Person [id=1, name=Peter, surname=Sagan, age=17]
-    // Person [id=2, name=Nasta, surname=Kuzminova, age=25]
-    // Person [id=3, name=John, surname=lawrence, age=35]
-    // Person [id=4, name=Terry, surname=Law, age=36]
-    // Find by id 2: Person [id=2, name=Barbora, surname=Spotakova, age=25]
-    // Count Person records: 3
-    // Find by John is Person [id=3, name=John, surname=lawrence, age=35]
-    // Find Person with age between 20,40: 
-    // Person [id=3, name=John, surname=lawrence, age=35]
-    // Person [id=4, name=Terry, surname=Law, age=36]
+```
+
+Program output:
+
+```
+Count Person records: 4
+Person [id=1, name=Peter, surname=Sagan, age=17]
+Person [id=2, name=Nasta, surname=Kuzminova, age=25]
+Person [id=3, name=John, surname=lawrence, age=35]
+Person [id=4, name=Terry, surname=Law, age=36]
+Find by id 2: Person [id=2, name=Barbora, surname=Spotakova, age=25]
+Count Person records: 3
+Find by John is Person [id=3, name=John, surname=lawrence, age=35]
+Find Person with age between 20,40: 
+Person [id=3, name=John, surname=lawrence, age=35]
+Person [id=4, name=Terry, surname=Law, age=36]
 ```
 
 ## Class diagram
+
 ![alt text](./etc/repository.png "Repository")
 
 ## Applicability
+
 Use the Repository pattern when
 
-* The number of domain objects is large
-* You want to avoid duplication of query code
-* You want to keep the database querying code in single place
-* You have multiple data sources
+* The number of domain objects is large.
+* You want to avoid duplication of query code.
+* You want to keep the database querying code in single place.
+* You have multiple data sources.
 
 ## Real world examples
 

repository/src/main/java/com/iluwatar/repository/AppConfig.java

@@ -23,7 +23,6 @@
 
 package com.iluwatar.repository;
 
-import java.sql.SQLException;
 import java.util.List;
 import java.util.Properties;
 import javax.sql.DataSource;
@@ -89,7 +88,7 @@ public class AppConfig {
    * Get transaction manager.
    */
   @Bean
-  public JpaTransactionManager transactionManager() throws SQLException {
+  public JpaTransactionManager transactionManager() {
     var transactionManager = new JpaTransactionManager();
     transactionManager.setEntityManagerFactory(entityManagerFactory().getObject());
     return transactionManager;

repository/src/main/java/com/iluwatar/repository/PersonSpecifications.java

@@ -60,7 +60,7 @@ public class PersonSpecifications {
    */
   public static class NameEqualSpec implements Specification<Person> {
 
-    public String name;
+    public final String name;
 
     public NameEqualSpec(String name) {
       this.name = name;

retry/README.md

@@ -10,39 +10,40 @@ tags:
 ---
 
 ## Intent
-Transparently retry certain operations that involve communication with external resources, particularly over the 
-network, isolating calling code from the retry implementation details.
+
+Transparently retry certain operations that involve communication with external resources, 
+particularly over the network, isolating calling code from the retry implementation details.
 
 ## Explanation
-Retry pattern consists retrying operations on remote resources over the 
-network a set number of times. It closely depends on both business and technical
-requirements: how much time will the business allow the end user to wait while
-the operation finishes? What are the performance characteristics of the 
-remote resource during peak loads as well as our application as more threads
-are waiting for the remote resource's availability? Among the errors returned
-by the remote service, which can be safely ignored in order to retry? Is the 
-operation [idempotent](https://en.wikipedia.org/wiki/Idempotence)?
 
-Another concern is the impact on the calling code by implementing the retry 
-mechanism. The retry mechanics should ideally be completely transparent to the
-calling code (service interface remains unaltered). There are two general 
-approaches to this problem: from an enterprise architecture standpoint 
-(strategic), and a shared library standpoint (tactical).
+Retry pattern consists retrying operations on remote resources over the network a set number of 
+times. It closely depends on both business and technical requirements: How much time will the 
+business allow the end user to wait while the operation finishes? What are the performance 
+characteristics of the remote resource during peak loads as well as our application as more threads
+are waiting for the remote resource's availability? Among the errors returned by the remote service, 
+which can be safely ignored in order to retry? Is the operation 
+[idempotent](https://en.wikipedia.org/wiki/Idempotence)?
 
-From a strategic point of view, this would be solved by having requests
-be redirected to a separate intermediary system, traditionally an 
-[ESB](https://en.wikipedia.org/wiki/Enterprise_service_bus), but more recently
-a [Service Mesh](https://medium.com/microservices-in-practice/service-mesh-for-microservices-2953109a3c9a).
+Another concern is the impact on the calling code by implementing the retry mechanism. The retry 
+mechanics should ideally be completely transparent to the calling code (service interface remains 
+unaltered). There are two general approaches to this problem: From an enterprise architecture 
+standpoint (strategic), and a shared library standpoint (tactical).
 
-From a tactical point of view, this would be solved by reusing shared libraries 
-like [Hystrix](https://github.com/Netflix/Hystrix) (please note that *Hystrix* is a complete implementation of 
-the [Circuit Breaker](https://java-design-patterns.com/patterns/circuit-breaker/) pattern, of which the Retry pattern 
-can be considered a subset of.). This is the type of solution showcased in the simple example that accompanies this 
-*README*.
+From a strategic point of view, this would be solved by having requests redirected to a separate 
+intermediary system, traditionally an [ESB](https://en.wikipedia.org/wiki/Enterprise_service_bus), 
+but more recently a [Service Mesh](https://medium.com/microservices-in-practice/service-mesh-for-microservices-2953109a3c9a).
+
+From a tactical point of view, this would be solved by reusing shared libraries like 
+[Hystrix](https://github.com/Netflix/Hystrix) (please note that Hystrix is a complete implementation 
+of the [Circuit Breaker](https://java-design-patterns.com/patterns/circuit-breaker/) pattern, of 
+which the Retry pattern can be considered a subset of). This is the type of solution showcased in 
+the simple example that accompanies this `README.md`.
 
 Real world example
 
-> Our application uses a service providing customer information. Once in a while the service seems to be flaky and can return errors or sometimes it just times out. To circumvent these problems we apply the retry pattern. 
+> Our application uses a service providing customer information. Once in a while the service seems 
+> to be flaky and can return errors or sometimes it just times out. To circumvent these problems we 
+> apply the retry pattern. 
 
 In plain words
 
@@ -50,11 +51,14 @@ In plain words
 
 [Microsoft documentation](https://docs.microsoft.com/en-us/azure/architecture/patterns/retry) says
 
-> Enable an application to handle transient failures when it tries to connect to a service or network resource, by transparently retrying a failed operation. This can improve the stability of the application.
+> Enable an application to handle transient failures when it tries to connect to a service or 
+> network resource, by transparently retrying a failed operation. This can improve the stability of 
+> the application.
 
 **Programmatic Example**
 
-In our hypothetical application, we have a generic interface for all operations on remote interfaces.
+In our hypothetical application, we have a generic interface for all operations on remote 
+interfaces.
 
 ```java
 public interface BusinessOperation<T> {
@@ -73,16 +77,14 @@ public final class FindCustomer implements BusinessOperation<String> {
 }
 ```
 
-Our `FindCustomer` implementation can be configured to throw 
-`BusinessException`s before returning the customer's ID, thereby simulating a
-'flaky' service that intermittently fails. Some exceptions, like the 
-`CustomerNotFoundException`, are deemed to be recoverable after some 
-hypothetical analysis because the root cause of the error stems from "some
-database locking issue". However, the `DatabaseNotAvailableException` is 
-considered to be a definite showstopper - the application should not attempt
-to recover from this error.
+Our `FindCustomer` implementation can be configured to throw `BusinessException`s before returning 
+the customer's ID, thereby simulating a flaky service that intermittently fails. Some exceptions, 
+like the `CustomerNotFoundException`, are deemed to be recoverable after some hypothetical analysis 
+because the root cause of the error stems from "some database locking issue". However, the 
+`DatabaseNotAvailableException` is considered to be a definite showstopper - the application should 
+not attempt to recover from this error.
 
-We can model a 'recoverable' scenario by instantiating `FindCustomer` like this:
+We can model a recoverable scenario by instantiating `FindCustomer` like this:
 
 ```java
 final var op = new FindCustomer(
@@ -93,15 +95,12 @@ final var op = new FindCustomer(
 );
 ```
 
-In this configuration, `FindCustomer` will throw `CustomerNotFoundException`
-three times, after which it will consistently return the customer's ID 
-(`12345`).
+In this configuration, `FindCustomer` will throw `CustomerNotFoundException` three times, after 
+which it will consistently return the customer's ID (`12345`).
 
-In our hypothetical scenario, our analysts indicate that this operation 
-typically fails 2-4 times for a given input during peak hours, and that each 
-worker thread in the database subsystem typically needs 50ms to 
-"recover from an error". Applying these policies would yield something like
-this:
+In our hypothetical scenario, our analysts indicate that this operation typically fails 2-4 times 
+for a given input during peak hours, and that each worker thread in the database subsystem typically 
+needs 50ms to "recover from an error". Applying these policies would yield something like this:
 
 ```java
 final var op = new Retry<>(
@@ -117,26 +116,27 @@ final var op = new Retry<>(
 );
 ```
 
-Executing `op` *once* would automatically trigger at most 5 retry attempts,
-with a 100 millisecond delay between attempts, ignoring any 
-`CustomerNotFoundException` thrown while trying. In this particular scenario,
-due to the configuration for `FindCustomer`, there will be 1 initial attempt 
+Executing `op` once would automatically trigger at most 5 retry attempts, with a 100 millisecond 
+delay between attempts, ignoring any `CustomerNotFoundException` thrown while trying. In this 
+particular scenario, due to the configuration for `FindCustomer`, there will be 1 initial attempt 
 and 3 additional retries before finally returning the desired result `12345`.
 
-If our `FindCustomer` operation were instead to throw a fatal 
-`DatabaseNotFoundException`, which we were instructed not to ignore, but 
-more importantly we did *not* instruct our `Retry` to ignore, then the operation
-would have failed immediately upon receiving the error, not matter how many 
-attempts were left.
+If our `FindCustomer` operation were instead to throw a fatal `DatabaseNotFoundException`, which we 
+were instructed not to ignore, but more importantly we did not instruct our `Retry` to ignore, then 
+the operation would have failed immediately upon receiving the error, not matter how many attempts 
+were left.
 
 ## Class diagram
+
 ![alt text](./etc/retry.png "Retry")
 
 ## Applicability
-Whenever an application needs to communicate with an external resource, particularly in a cloud environment, and if 
-the business requirements allow it.
+
+Whenever an application needs to communicate with an external resource, particularly in a cloud 
+environment, and if the business requirements allow it.
 
 ## Consequences
+
 **Pros:** 
 
 * Resiliency

separated-interface/README.md

@@ -0,0 +1,119 @@
+---
+layout: pattern
+title: Separated Interface
+folder: separated-interface
+permalink: /patterns/separated-interface/
+categories: Structural
+tags:
+ - Decoupling
+---
+
+
+## Intent
+Separate the interface definition and implementation in different packages. This allows the client to be completely unaware of the implementation.
+
+## Explanation
+
+Real world example
+
+> An Invoice generator may be created with ability to use different Tax calculators that may be added in the invoice depending upon type of purchase, region etc.         
+
+In plain words
+
+> Separated interface pattern encourages to keep the implementations of an interface decoupled from the client and its definition, so the client is not dependent on the implementation.
+
+A client code may abstract some specific functionality to an interface, and define the definition of the interface as an SPI ([Service Programming Interface](https://en.wikipedia.org/wiki/Service_provider_interface) is an API intended and open to be implemented or extended by a third party). Another package may implement this interface definition with a concrete logic, which will be injected into the client code at runtime (with a third class, injecting the implementation in the client) or at compile time (using Plugin pattern with some configurable file).
+
+**Programmatic Example**
+
+**Client** An Invoice generator class accepts the cost of the product and calculates the total amount payable inclusive of tax
+
+```java
+public class InvoiceGenerator {
+
+  private final TaxCalculator taxCalculator;
+
+  private final double amount;
+
+  public InvoiceGenerator(double amount, TaxCalculator taxCalculator) {
+    this.amount = amount;
+    this.taxCalculator = taxCalculator;
+  }
+
+  public double getAmountWithTax() {
+    return amount + taxCalculator.calculate(amount);
+  }
+
+}
+```
+The tax calculation logic is delegated to the ```TaxCalculator``` interface
+
+```java
+
+public interface TaxCalculator {
+
+  double calculate(double amount);
+
+}
+
+```
+
+**Implementation package**
+In another package (which the client is completely unaware of) there exist multiple implementations of the ```TaxCalculator``` interface
+```ForeignTaxCalculator``` which levies 60% tax for international products.
+```java
+public class ForeignTaxCalculator implements TaxCalculator {
+
+  public static final double TAX_PERCENTAGE = 60;
+
+  @Override
+  public double calculate(double amount) {
+    return amount * TAX_PERCENTAGE / 100.0;
+  }
+
+}
+```
+
+```DomesticTaxCalculator``` which levies 20% tax for international products.
+```java
+public class DomesticTaxCalculator implements TaxCalculator {
+
+  public static final double TAX_PERCENTAGE = 20;
+
+  @Override
+  public double calculate(double amount) {
+    return amount * TAX_PERCENTAGE / 100.0;
+  }
+
+}
+```
+
+These both implementations are instantiated and injected in the client class by the ```App.java``` class
+
+```java
+    var internationalProductInvoice = new InvoiceGenerator(PRODUCT_COST, new ForeignTaxCalculator());
+
+    LOGGER.info("Foreign Tax applied: {}", "" + internationalProductInvoice.getAmountWithTax());
+
+    var domesticProductInvoice = new InvoiceGenerator(PRODUCT_COST, new DomesticTaxCalculator());
+
+    LOGGER.info("Domestic Tax applied: {}", "" + domesticProductInvoice.getAmountWithTax());
+```
+
+## Class diagram
+![alt text](./etc/class_diagram.png "Separated Interface")
+
+## Applicability
+Use the Separated interface pattern when
+
+* You are developing a framework package, and your framework needs to call some application code through interfaces.
+* You have separate packages implementing the functionalities which may be plugged in your client code at runtime or compile-time.
+* Your code resides in a layer that is not allowed to call the interface implementation layer by rule. For example, a domain layer needs to call a data mapper.
+
+## Tutorial 
+
+* [Separated Interface Tutorial](https://www.youtube.com/watch?v=d3k-hOA7k2Y)
+
+## Credits
+
+* [Martin Fowler](https://www.martinfowler.com/eaaCatalog/separatedInterface.html)

separated-interface/etc/separated-interface.urm.puml

@@ -0,0 +1,36 @@
+@startuml
+package com.iluwatar.separatedinterface {
+  class App {
+    - LOGGER : Logger {static}
+    + PRODUCT_COST : double {static}
+    + App()
+    + main(args : String[]) {static}
+  }
+}
+package com.iluwatar.separatedinterface.taxes {
+  class DomesticTaxCalculator {
+    + TAX_PERCENTAGE : double {static}
+    + DomesticTaxCalculator()
+    + calculate(amount : double) : double
+  }
+  class ForeignTaxCalculator {
+    + TAX_PERCENTAGE : double {static}
+    + ForeignTaxCalculator()
+    + calculate(amount : double) : double
+  }
+}
+package com.iluwatar.separatedinterface.invoice {
+  class InvoiceGenerator {
+    - amount : double
+    - taxCalculator : TaxCalculator
+    + InvoiceGenerator(amount : double, taxCalculator : TaxCalculator)
+    + getAmountWithTax() : double
+  }
+  interface TaxCalculator {
+    + calculate(double) : double {abstract}
+  }
+}
+InvoiceGenerator -->  "-taxCalculator" TaxCalculator
+DomesticTaxCalculator ..|> TaxCalculator 
+ForeignTaxCalculator ..|> TaxCalculator 
+@enduml

separated-interface/pom.xml

@@ -0,0 +1,71 @@
+<?xml version="1.0"?>
+<!--
+
+    The MIT License
+    Copyright © 2014-2019 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 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.24.0-SNAPSHOT</version>
+  </parent>
+  <artifactId>separated-interface</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter-engine</artifactId>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter-params</artifactId>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>org.mockito</groupId>
+      <artifactId>mockito-core</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-assembly-plugin</artifactId>
+        <executions>
+          <execution>
+            <configuration>
+              <archive>
+                <manifest>
+                  <mainClass>com.iluwatar.separatedinterface.App</mainClass>
+                </manifest>
+              </archive>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+    </plugins>
+  </build>
+</project>

separated-interface/src/main/java/com/iluwatar/separatedinterface/App.java

@@ -0,0 +1,62 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 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.
+ */
+
+package com.iluwatar.separatedinterface;
+
+import com.iluwatar.separatedinterface.invoice.InvoiceGenerator;
+import com.iluwatar.separatedinterface.taxes.DomesticTaxCalculator;
+import com.iluwatar.separatedinterface.taxes.ForeignTaxCalculator;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * <p>The Separated Interface pattern encourages to separate the interface definition and
+ * implementation in different packages. This allows the client to be completely unaware of the
+ * implementation.</p>
+ *
+ * <p>In this class the {@link InvoiceGenerator} class is injected with different instances of
+ * {@link com.iluwatar.separatedinterface.invoice.TaxCalculator} implementations located in separate
+ * packages, to receive different responses for both of the implementations.</p>
+ */
+public class App {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
+  public static final double PRODUCT_COST = 50.0;
+
+  /**
+   * Program entry point.
+   *
+   * @param args command line args
+   */
+  public static void main(String[] args) {
+    //Create the invoice generator with product cost as 50 and foreign product tax
+    var internationalProductInvoice = new InvoiceGenerator(PRODUCT_COST,
+        new ForeignTaxCalculator());
+    LOGGER.info("Foreign Tax applied: {}", "" + internationalProductInvoice.getAmountWithTax());
+
+    //Create the invoice generator with product cost as 50 and domestic product tax
+    var domesticProductInvoice = new InvoiceGenerator(PRODUCT_COST, new DomesticTaxCalculator());
+    LOGGER.info("Domestic Tax applied: {}", "" + domesticProductInvoice.getAmountWithTax());
+  }
+}

separated-interface/src/main/java/com/iluwatar/separatedinterface/invoice/InvoiceGenerator.java

@@ -0,0 +1,51 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 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.
+ */
+
+package com.iluwatar.separatedinterface.invoice;
+
+/**
+ * InvoiceGenerator class generates an invoice, accepting the product cost and calculating the total
+ * price payable inclusive tax (calculated by {@link TaxCalculator}).
+ */
+public class InvoiceGenerator {
+
+  /**
+   * The TaxCalculator interface to calculate the payable tax.
+   */
+  private final TaxCalculator taxCalculator;
+
+  /**
+   * The base product amount without tax.
+   */
+  private final double amount;
+
+  public InvoiceGenerator(double amount, TaxCalculator taxCalculator) {
+    this.amount = amount;
+    this.taxCalculator = taxCalculator;
+  }
+
+  public double getAmountWithTax() {
+    return amount + taxCalculator.calculate(amount);
+  }
+
+}

separated-interface/src/main/java/com/iluwatar/separatedinterface/invoice/TaxCalculator.java

@@ -0,0 +1,30 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 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.
+ */
+
+package com.iluwatar.separatedinterface.invoice;
+
+public interface TaxCalculator {
+
+  double calculate(double amount);
+
+}

separated-interface/src/main/java/com/iluwatar/separatedinterface/taxes/DomesticTaxCalculator.java

@@ -0,0 +1,40 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 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.
+ */
+
+package com.iluwatar.separatedinterface.taxes;
+
+import com.iluwatar.separatedinterface.invoice.TaxCalculator;
+
+/**
+ * TaxCalculator for Domestic goods with 20% tax.
+ */
+public class DomesticTaxCalculator implements TaxCalculator {
+
+  public static final double TAX_PERCENTAGE = 20;
+
+  @Override
+  public double calculate(double amount) {
+    return amount * TAX_PERCENTAGE / 100.0;
+  }
+
+}

separated-interface/src/main/java/com/iluwatar/separatedinterface/taxes/ForeignTaxCalculator.java

@@ -0,0 +1,40 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 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.
+ */
+
+package com.iluwatar.separatedinterface.taxes;
+
+import com.iluwatar.separatedinterface.invoice.TaxCalculator;
+
+/**
+ * TaxCalculator for foreign goods with 60% tax.
+ */
+public class ForeignTaxCalculator implements TaxCalculator {
+
+  public static final double TAX_PERCENTAGE = 60;
+
+  @Override
+  public double calculate(double amount) {
+    return amount * TAX_PERCENTAGE / 100.0;
+  }
+
+}

separated-interface/src/test/java/com/iluwatar/separatedinterface/AppTest.java

@@ -0,0 +1,41 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 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.
+ */
+
+package com.iluwatar.separatedinterface;
+
+import org.junit.jupiter.api.Test;
+
+import com.iluwatar.separatedinterface.App;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+/**
+ * Application test.
+ */
+class AppTest {
+
+  @Test
+  void shouldExecuteWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
+  }
+}

separated-interface/src/test/java/com/iluwatar/separatedinterface/invoice/InvoiceGeneratorTest.java

@@ -0,0 +1,48 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 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.
+ */
+
+package com.iluwatar.separatedinterface.invoice;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import static org.mockito.Mockito.*;
+
+public class InvoiceGeneratorTest {
+
+  private InvoiceGenerator target;
+
+  @Test
+  public void testGenerateTax() {
+    var productCost = 50.0;
+    var tax = 10.0;
+    TaxCalculator taxCalculatorMock = mock(TaxCalculator.class);
+    doReturn(tax).when(taxCalculatorMock).calculate(productCost);
+
+    target = new InvoiceGenerator(productCost, taxCalculatorMock);
+
+    Assertions.assertEquals(target.getAmountWithTax(), productCost + tax);
+    verify(taxCalculatorMock, times(1)).calculate(productCost);
+  }
+
+}

separated-interface/src/test/java/com/iluwatar/separatedinterface/taxes/DomesticTaxCalculatorTest.java

@@ -0,0 +1,41 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 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.
+ */
+
+package com.iluwatar.separatedinterface.taxes;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class DomesticTaxCalculatorTest {
+
+  private DomesticTaxCalculator target;
+
+  @Test
+  public void testTaxCalculation(){
+    target = new DomesticTaxCalculator();
+
+    var tax=target.calculate(100.0);
+    Assertions.assertEquals(tax,20.0);
+  }
+
+}

separated-interface/src/test/java/com/iluwatar/separatedinterface/taxes/ForeignTaxCalculatorTest.java

@@ -0,0 +1,41 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 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.
+ */
+
+package com.iluwatar.separatedinterface.taxes;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class ForeignTaxCalculatorTest {
+
+  private ForeignTaxCalculator target;
+
+  @Test
+  public void testTaxCalculation(){
+    target = new ForeignTaxCalculator();
+
+    var tax=target.calculate(100.0);
+    Assertions.assertEquals(tax,60.0);
+  }
+
+}

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

@@ -33,7 +33,7 @@ import org.slf4j.LoggerFactory;
  * <p>One of the risks of this pattern is that bugs resulting from setting a singleton up in a
  * distributed environment can be tricky to debug, since it will work fine if you debug with a
  * single classloader. Additionally, these problems can crop up a while after the implementation of
- * a singleton, since they may start out synchronous and only become async with time, so you it may
+ * a singleton, since they may start out synchronous and only become async with time, so it may
  * not be clear why you are seeing certain changes in behaviour.</p>
  *
  * <p>There are many ways to implement the Singleton. The first one is the eagerly initialized