Code cleanup (#1461)

* Code cleanup

* Fix flux tests

* Fix checkstyle errors

* Fix compile error

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

@@ -47,7 +47,7 @@ public class AbstractDocumentTest {
     }
   }
 
-  private DocumentImplementation document = new DocumentImplementation(new HashMap<>());
+  private final DocumentImplementation document = new DocumentImplementation(new HashMap<>());
 
   @Test
   public void shouldPutAndGetValue() {

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

@@ -36,7 +36,7 @@ import org.junit.jupiter.api.Test;
  */
 public class AbstractFactoryTest {
 
-  private App app = new App();
+  private final App app = new App();
   private KingdomFactory elfFactory;
   private KingdomFactory orcFactory;
 

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

@@ -37,7 +37,7 @@ import uk.org.lidalia.slf4jtest.TestLoggerFactory;
  */
 public class ConfigureForDosVisitorTest {
 
-  private TestLogger logger = TestLoggerFactory.getTestLogger(ConfigureForDosVisitor.class);
+  private final TestLogger logger = TestLoggerFactory.getTestLogger(ConfigureForDosVisitor.class);
   
   @Test
   public void testVisitForZoom() {    

adapter/README.md

@@ -56,7 +56,7 @@ And captain expects an implementation of `RowingBoat` interface to be able to mo
 ```java
 public class Captain {
 
-  private RowingBoat rowingBoat;
+  private final RowingBoat rowingBoat;
   // default constructor and setter for rowingBoat
   public Captain(RowingBoat rowingBoat) {
     this.rowingBoat = rowingBoat;
@@ -75,7 +75,7 @@ public class FishingBoatAdapter implements RowingBoat {
 
   private static final Logger LOGGER = LoggerFactory.getLogger(FishingBoatAdapter.class);
 
-  private FishingBoat boat;
+  private final FishingBoat boat;
 
   public FishingBoatAdapter() {
     boat = new FishingBoat();

adapter/src/main/java/com/iluwatar/adapter/FishingBoatAdapter.java

@@ -29,7 +29,7 @@ package com.iluwatar.adapter;
  */
 public class FishingBoatAdapter implements RowingBoat {
 
-  private FishingBoat boat;
+  private final FishingBoat boat;
 
   public FishingBoatAdapter() {
     boat = new FishingBoat();

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

@@ -48,7 +48,7 @@ class RemoteServiceTest {
   }
 
   private static class StaticRandomProvider implements RandomProvider {
-    private double value;
+    private final double value;
 
     StaticRandomProvider(double value) {
       this.value = value;

async-method-invocation/src/main/java/com/iluwatar/async/method/invocation/ThreadAsyncExecutor.java

@@ -100,7 +100,7 @@ public class ThreadAsyncExecutor implements AsyncExecutor {
     void setValue(T value) {
       this.value = value;
       this.state = COMPLETED;
-      this.callback.ifPresent(ac -> ac.onComplete(value, Optional.<Exception>empty()));
+      this.callback.ifPresent(ac -> ac.onComplete(value, Optional.empty()));
       synchronized (lock) {
         lock.notifyAll();
       }

business-delegate/src/main/java/com/iluwatar/business/delegate/Client.java

@@ -28,7 +28,7 @@ package com.iluwatar.business.delegate;
  */
 public class Client {
 
-  private BusinessDelegate businessDelegate;
+  private final BusinessDelegate businessDelegate;
 
   public Client(BusinessDelegate businessDelegate) {
     this.businessDelegate = businessDelegate;

business-delegate/src/main/java/com/iluwatar/business/delegate/ServiceType.java

@@ -28,5 +28,5 @@ package com.iluwatar.business.delegate;
  */
 public enum ServiceType {
 
-  EJB, JMS;
+  EJB, JMS
 }

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

@@ -30,9 +30,9 @@ import java.util.Stack;
  */
 public class VirtualMachine {
 
-  private Stack<Integer> stack = new Stack<>();
+  private final Stack<Integer> stack = new Stack<>();
 
-  private Wizard[] wizards = new Wizard[2];
+  private final Wizard[] wizards = new Wizard[2];
 
   /**
    * Constructor.

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

@@ -104,7 +104,7 @@ public class VirtualMachineTest {
     bytecode[2] = LITERAL.getIntValue();
     bytecode[3] = 50;                        // health amount
     bytecode[4] = SET_HEALTH.getIntValue();
-    bytecode[5] = LITERAL.getIntValue();;
+    bytecode[5] = LITERAL.getIntValue();
     bytecode[6] = wizardNumber;
     bytecode[7] = GET_HEALTH.getIntValue();
 

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

@@ -29,7 +29,7 @@ package com.iluwatar.caching;
 public enum CachingPolicy {
   THROUGH("through"), AROUND("around"), BEHIND("behind"), ASIDE("aside");
 
-  private String policy;
+  private final String policy;
 
   CachingPolicy(String policy) {
     this.policy = policy;

chain/README.md

@@ -65,7 +65,7 @@ Then the request handler hierarchy
 ```java
 public abstract class RequestHandler {
   private static final Logger LOGGER = LoggerFactory.getLogger(RequestHandler.class);
-  private RequestHandler next;
+  private final RequestHandler next;
 
   public RequestHandler(RequestHandler next) {
     this.next = next;

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

@@ -33,7 +33,7 @@ public abstract class RequestHandler {
 
   private static final Logger LOGGER = LoggerFactory.getLogger(RequestHandler.class);
 
-  private RequestHandler next;
+  private final RequestHandler next;
 
   public RequestHandler(RequestHandler next) {
     this.next = next;

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

@@ -87,10 +87,7 @@ public class Car {
     } else if (!model.equals(other.model)) {
       return false;
     }
-    if (year != other.year) {
-      return false;
-    }
-    return true;
+    return year == other.year;
   }
 
   public String getMake() {

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

@@ -29,7 +29,7 @@ import java.util.List;
  * A Person class that has the list of cars that the person owns and use.
  */
 public class Person {
-  private List<Car> cars;
+  private final List<Car> cars;
 
   /**
    * Constructor to create an instance of person.

collection-pipeline/src/test/java/com/iluwatar/collectionpipeline/AppTest.java

@@ -37,7 +37,7 @@ import org.slf4j.LoggerFactory;
 public class AppTest {
   private static final Logger LOGGER = LoggerFactory.getLogger(AppTest.class);
 
-  private List<Car> cars = CarFactory.createCars();
+  private final List<Car> cars = CarFactory.createCars();
 
   @Test
   public void testGetModelsAfter2000UsingFor() {

command/README.md

@@ -36,8 +36,8 @@ public class Wizard {
 
   private static final Logger LOGGER = LoggerFactory.getLogger(Wizard.class);
 
-  private Deque<Command> undoStack = new LinkedList<>();
-  private Deque<Command> redoStack = new LinkedList<>();
+  private final Deque<Command> undoStack = new LinkedList<>();
+  private final Deque<Command> redoStack = new LinkedList<>();
 
   public Wizard() {}
 

command/src/main/java/com/iluwatar/command/Size.java

@@ -1,43 +1,43 @@
-/*
- * 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.command;
-
-/**
- * Enumeration for target size.
- */
-public enum Size {
-
-  SMALL("small"), NORMAL("normal");
-
-  private String title;
-
-  Size(String title) {
-    this.title = title;
-  }
-
-  @Override
-  public String toString() {
-    return title;
-  }
-}
+/*
+ * 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.command;
+
+/**
+ * Enumeration for target size.
+ */
+public enum Size {
+
+  SMALL("small"), NORMAL("normal");
+
+  private final String title;
+
+  Size(String title) {
+    this.title = title;
+  }
+
+  @Override
+  public String toString() {
+    return title;
+  }
+}

command/src/main/java/com/iluwatar/command/Visibility.java

@@ -1,43 +1,43 @@
-/*
- * 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.command;
-
-/**
- * Enumeration for target visibility.
- */
-public enum Visibility {
-
-  VISIBLE("visible"), INVISIBLE("invisible");
-
-  private String title;
-
-  Visibility(String title) {
-    this.title = title;
-  }
-
-  @Override
-  public String toString() {
-    return title;
-  }
-}
+/*
+ * 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.command;
+
+/**
+ * Enumeration for target visibility.
+ */
+public enum Visibility {
+
+  VISIBLE("visible"), INVISIBLE("invisible");
+
+  private final String title;
+
+  Visibility(String title) {
+    this.title = title;
+  }
+
+  @Override
+  public String toString() {
+    return title;
+  }
+}

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

@@ -35,8 +35,8 @@ public class Wizard {
 
   private static final Logger LOGGER = LoggerFactory.getLogger(Wizard.class);
 
-  private Deque<Command> undoStack = new LinkedList<>();
-  private Deque<Command> redoStack = new LinkedList<>();
+  private final Deque<Command> undoStack = new LinkedList<>();
+  private final Deque<Command> redoStack = new LinkedList<>();
 
   public Wizard() {
     // comment to ignore sonar issue: LEVEL critical

commander/src/main/java/com/iluwatar/commander/employeehandle/EmployeeDatabase.java

@@ -33,7 +33,7 @@ import java.util.Hashtable;
  */
 
 public class EmployeeDatabase extends Database<Order> {
-  private Hashtable<String, Order> data;
+  private final Hashtable<String, Order> data;
 
   public EmployeeDatabase() {
     this.data = new Hashtable<>();

commander/src/main/java/com/iluwatar/commander/messagingservice/MessagingDatabase.java

@@ -33,7 +33,7 @@ import java.util.Hashtable;
  */
 
 public class MessagingDatabase extends Database<MessageRequest> {
-  private Hashtable<String, MessageRequest> data;
+  private final Hashtable<String, MessageRequest> data;
 
   public MessagingDatabase() {
     this.data = new Hashtable<>();

commander/src/main/java/com/iluwatar/commander/paymentservice/PaymentDatabase.java

@@ -34,7 +34,7 @@ import java.util.Hashtable;
 
 public class PaymentDatabase extends Database<PaymentRequest> {
 
-  private Hashtable<String, PaymentRequest> data;
+  private final Hashtable<String, PaymentRequest> data;
 
   public PaymentDatabase() {
     this.data = new Hashtable<>();

commander/src/main/java/com/iluwatar/commander/queue/QueueDatabase.java

@@ -35,7 +35,7 @@ import java.util.List;
 
 public class QueueDatabase extends Database<QueueTask> {
 
-  private Queue<QueueTask> data;
+  private final Queue<QueueTask> data;
   public List<Exception> exceptionsList;
 
   public QueueDatabase(Exception... exc) {

commander/src/main/java/com/iluwatar/commander/shippingservice/ShippingDatabase.java

@@ -34,7 +34,7 @@ import java.util.Hashtable;
 
 public class ShippingDatabase extends Database<ShippingRequest> {
 
-  private Hashtable<String, ShippingRequest> data;
+  private final Hashtable<String, ShippingRequest> data;
 
   public ShippingDatabase() {
     this.data = new Hashtable<>();

composite/README.md

@@ -34,7 +34,7 @@ Taking our sentence example from above. Here we have the base class and differen
 ```java
 public abstract class LetterComposite {
 
-  private List<LetterComposite> children = new ArrayList<>();
+  private final List<LetterComposite> children = new ArrayList<>();
 
   public void add(LetterComposite letter) {
     children.add(letter);
@@ -59,7 +59,7 @@ public abstract class LetterComposite {
 
 public class Letter extends LetterComposite {
 
-  private char character;
+  private final char character;
 
   public Letter(char c) {
     this.character = c;

composite/src/main/java/com/iluwatar/composite/Letter.java

@@ -28,7 +28,7 @@ package com.iluwatar.composite;
  */
 public class Letter extends LetterComposite {
 
-  private char character;
+  private final char character;
 
   public Letter(char c) {
     this.character = c;

composite/src/main/java/com/iluwatar/composite/LetterComposite.java

@@ -1,58 +1,58 @@
-/*
- * 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.composite;
-
-import java.util.ArrayList;
-import java.util.List;
-
-/**
- * Composite interface.
- */
-public abstract class LetterComposite {
-
-  private List<LetterComposite> children = new ArrayList<>();
-
-  public void add(LetterComposite letter) {
-    children.add(letter);
-  }
-
-  public int count() {
-    return children.size();
-  }
-
-  protected void printThisBefore() {
-  }
-
-  protected void printThisAfter() {
-  }
-
-  /**
-   * Print.
-   */
-  public void print() {
-    printThisBefore();
-    children.forEach(LetterComposite::print);
-    printThisAfter();
-  }
-}
+/*
+ * 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.composite;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Composite interface.
+ */
+public abstract class LetterComposite {
+
+  private final List<LetterComposite> children = new ArrayList<>();
+
+  public void add(LetterComposite letter) {
+    children.add(letter);
+  }
+
+  public int count() {
+    return children.size();
+  }
+
+  protected void printThisBefore() {
+  }
+
+  protected void printThisAfter() {
+  }
+
+  /**
+   * Print.
+   */
+  public void print() {
+    printThisBefore();
+    children.forEach(LetterComposite::print);
+    printThisAfter();
+  }
+}

converter/src/main/java/com/iluwatar/converter/User.java

@@ -29,10 +29,10 @@ import java.util.Objects;
  * User class.
  */
 public class User {
-  private String firstName;
-  private String lastName;
-  private boolean isActive;
-  private String userId;
+  private final String firstName;
+  private final String lastName;
+  private final boolean isActive;
+  private final String userId;
 
   /**
    * Constructor.

converter/src/main/java/com/iluwatar/converter/UserDto.java

@@ -30,10 +30,10 @@ import java.util.Objects;
  */
 public class UserDto {
 
-  private String firstName;
-  private String lastName;
-  private boolean isActive;
-  private String email;
+  private final String firstName;
+  private final String lastName;
+  private final boolean isActive;
+  private final String email;
 
   /**
    * Constructor.

converter/src/test/java/com/iluwatar/converter/ConverterTest.java

@@ -34,7 +34,7 @@ import org.junit.jupiter.api.Test;
  */
 public class ConverterTest {
 
-  private UserConverter userConverter = new UserConverter();
+  private final UserConverter userConverter = new UserConverter();
 
   /**
    * Tests whether a converter created of opposite functions holds equality as a bijection.

cqrs/src/main/java/com/iluwatar/cqrs/commandes/CommandServiceImpl.java

@@ -34,7 +34,7 @@ import org.hibernate.SessionFactory;
  */
 public class CommandServiceImpl implements ICommandService {
 
-  private SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
+  private final SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
 
   private Author getAuthorByUsername(String username) {
     Author author;

cqrs/src/main/java/com/iluwatar/cqrs/queries/QueryServiceImpl.java

@@ -38,7 +38,7 @@ import org.hibernate.transform.Transformers;
  */
 public class QueryServiceImpl implements IQueryService {
 
-  private SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
+  private final SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
 
   @Override
   public Author getAuthorByUsername(String username) {

dao/README.md

@@ -112,7 +112,7 @@ public interface CustomerDao {
 
 public class InMemoryCustomerDao implements CustomerDao {
 
-  private Map<Integer, Customer> idToCustomer = new HashMap<>();
+  private final Map<Integer, Customer> idToCustomer = new HashMap<>();
 
   @Override
   public Stream<Customer> getAll() {

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

@@ -44,7 +44,7 @@ import org.slf4j.LoggerFactory;
  */
 public class App {
   private static final String DB_URL = "jdbc:h2:~/dao";
-  private static Logger log = LoggerFactory.getLogger(App.class);
+  private static final Logger log = LoggerFactory.getLogger(App.class);
   private static final String ALL_CUSTOMERS = "customerDao.getAllCustomers(): ";
 
   /**

dao/src/main/java/com/iluwatar/dao/InMemoryCustomerDao.java

@@ -36,7 +36,7 @@ import java.util.stream.Stream;
  */
 public class InMemoryCustomerDao implements CustomerDao {
 
-  private Map<Integer, Customer> idToCustomer = new HashMap<>();
+  private final Map<Integer, Customer> idToCustomer = new HashMap<>();
 
   /**
    * An eagerly evaluated stream of customers stored in memory.

dao/src/test/java/com/iluwatar/dao/DbCustomerDaoTest.java

@@ -50,7 +50,7 @@ public class DbCustomerDaoTest {
 
   private static final String DB_URL = "jdbc:h2:~/dao";
   private DbCustomerDao dao;
-  private Customer existingCustomer = new Customer(1, "Freddy", "Krueger");
+  private final Customer existingCustomer = new Customer(1, "Freddy", "Krueger");
 
   /**
    * Creates customers schema.

data-bus/src/main/java/com/iluwatar/databus/members/MessageCollectorMember.java

@@ -41,7 +41,7 @@ public class MessageCollectorMember implements Member {
 
   private final String name;
 
-  private List<String> messages = new ArrayList<>();
+  private final List<String> messages = new ArrayList<>();
 
   public MessageCollectorMember(String name) {
     this.name = name;

data-mapper/src/main/java/com/iluwatar/datamapper/App.java

@@ -1,83 +1,83 @@
-/*
- * 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.datamapper;
-
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
-
-/**
- * The Data Mapper (DM) is a layer of software that separates the in-memory objects from the
- * database. Its responsibility is to transfer data between the two and also to isolate them from
- * each other. With Data Mapper the in-memory objects needn't know even that there's a database
- * present; they need no SQL interface code, and certainly no knowledge of the database schema. (The
- * database schema is always ignorant of the objects that use it.) Since it's a form of Mapper ,
- * Data Mapper itself is even unknown to the domain layer.
- *
- * <p>The below example demonstrates basic CRUD operations: Create, Read, Update, and Delete.
- */
-public final class App {
-
-  private static Logger log = LoggerFactory.getLogger(App.class);
-  private static final String STUDENT_STRING = "App.main(), student : ";
-
-
-  /**
-   * Program entry point.
-   *
-   * @param args command line args.
-   */
-  public static void main(final String... args) {
-
-    /* Create new data mapper for type 'first' */
-    final var mapper = new StudentDataMapperImpl();
-
-    /* Create new student */
-    var student = new Student(1, "Adam", 'A');
-
-    /* Add student in respectibe store */
-    mapper.insert(student);
-
-    log.debug(STUDENT_STRING + student + ", is inserted");
-
-    /* Find this student */
-    final var studentToBeFound = mapper.find(student.getStudentId());
-
-    log.debug(STUDENT_STRING + studentToBeFound + ", is searched");
-
-    /* Update existing student object */
-    student = new Student(student.getStudentId(), "AdamUpdated", 'A');
-
-    /* Update student in respectibe db */
-    mapper.update(student);
-
-    log.debug(STUDENT_STRING + student + ", is updated");
-    log.debug(STUDENT_STRING + student + ", is going to be deleted");
-
-    /* Delete student in db */
-    mapper.delete(student);
-  }
-
-  private App() {
-  }
-}
+/*
+ * 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.datamapper;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * The Data Mapper (DM) is a layer of software that separates the in-memory objects from the
+ * database. Its responsibility is to transfer data between the two and also to isolate them from
+ * each other. With Data Mapper the in-memory objects needn't know even that there's a database
+ * present; they need no SQL interface code, and certainly no knowledge of the database schema. (The
+ * database schema is always ignorant of the objects that use it.) Since it's a form of Mapper ,
+ * Data Mapper itself is even unknown to the domain layer.
+ *
+ * <p>The below example demonstrates basic CRUD operations: Create, Read, Update, and Delete.
+ */
+public final class App {
+
+  private static final Logger log = LoggerFactory.getLogger(App.class);
+  private static final String STUDENT_STRING = "App.main(), student : ";
+
+
+  /**
+   * Program entry point.
+   *
+   * @param args command line args.
+   */
+  public static void main(final String... args) {
+
+    /* Create new data mapper for type 'first' */
+    final var mapper = new StudentDataMapperImpl();
+
+    /* Create new student */
+    var student = new Student(1, "Adam", 'A');
+
+    /* Add student in respectibe store */
+    mapper.insert(student);
+
+    log.debug(STUDENT_STRING + student + ", is inserted");
+
+    /* Find this student */
+    final var studentToBeFound = mapper.find(student.getStudentId());
+
+    log.debug(STUDENT_STRING + studentToBeFound + ", is searched");
+
+    /* Update existing student object */
+    student = new Student(student.getStudentId(), "AdamUpdated", 'A');
+
+    /* Update student in respectibe db */
+    mapper.update(student);
+
+    log.debug(STUDENT_STRING + student + ", is updated");
+    log.debug(STUDENT_STRING + student + ", is going to be deleted");
+
+    /* Delete student in db */
+    mapper.delete(student);
+  }
+
+  private App() {
+  }
+}

data-mapper/src/main/java/com/iluwatar/datamapper/StudentDataMapperImpl.java

@@ -33,7 +33,7 @@ import java.util.Optional;
 public final class StudentDataMapperImpl implements StudentDataMapper {
 
   /* Note: Normally this would be in the form of an actual database */
-  private List<Student> students = new ArrayList<>();
+  private final List<Student> students = new ArrayList<>();
 
   @Override
   public Optional<Student> find(int studentId) {

data-transfer-object/README.md

@@ -64,7 +64,7 @@ Customer resource class acts as the server for customer information.
 
 ```java
 public class CustomerResource {
-  private List<CustomerDto> customers;
+  private final List<CustomerDto> customers;
 
   public CustomerResource(List<CustomerDto> customers) {
     this.customers = customers;

data-transfer-object/src/main/java/com/iluwatar/datatransfer/CustomerResource.java

@@ -30,7 +30,7 @@ import java.util.List;
  * has all customer details.
  */
 public class CustomerResource {
-  private List<CustomerDto> customers;
+  private final List<CustomerDto> customers;
 
   /**
    * Initialise resource with existing customers.

decorator/README.md

@@ -70,7 +70,7 @@ public class ClubbedTroll implements Troll {
 
   private static final Logger LOGGER = LoggerFactory.getLogger(ClubbedTroll.class);
 
-  private Troll decorated;
+  private final Troll decorated;
 
   public ClubbedTroll(Troll decorated) {
     this.decorated = decorated;

decorator/src/main/java/com/iluwatar/decorator/ClubbedTroll.java

@@ -33,7 +33,7 @@ public class ClubbedTroll implements Troll {
 
   private static final Logger LOGGER = LoggerFactory.getLogger(ClubbedTroll.class);
 
-  private Troll decorated;
+  private final Troll decorated;
 
   public ClubbedTroll(Troll decorated) {
     this.decorated = decorated;

decorator/src/test/java/com/iluwatar/decorator/SimpleTrollTest.java

@@ -68,7 +68,7 @@ public class SimpleTrollTest {
 
   private class InMemoryAppender extends AppenderBase<ILoggingEvent> {
 
-    private List<ILoggingEvent> log = new LinkedList<>();
+    private final List<ILoggingEvent> log = new LinkedList<>();
 
     public InMemoryAppender(Class clazz) {
       ((Logger) LoggerFactory.getLogger(clazz)).addAppender(this);

delegation/src/test/java/com/iluwatar/delegation/simple/DelegateTest.java

@@ -86,7 +86,7 @@ public class DelegateTest {
    */
   private class InMemoryAppender extends AppenderBase<ILoggingEvent> {
 
-    private List<ILoggingEvent> log = new LinkedList<>();
+    private final List<ILoggingEvent> log = new LinkedList<>();
 
     public InMemoryAppender() {
       ((Logger) LoggerFactory.getLogger("root")).addAppender(this);

dependency-injection/README.md

@@ -62,7 +62,7 @@ public interface Wizard {
 
 public class AdvancedWizard implements Wizard {
 
-  private Tobacco tobacco;
+  private final Tobacco tobacco;
 
   public AdvancedWizard(Tobacco tobacco) {
     this.tobacco = tobacco;

dependency-injection/src/main/java/com/iluwatar/dependency/injection/AdvancedWizard.java

@@ -29,7 +29,7 @@ package com.iluwatar.dependency.injection;
  */
 public class AdvancedWizard implements Wizard {
 
-  private Tobacco tobacco;
+  private final Tobacco tobacco;
 
   public AdvancedWizard(Tobacco tobacco) {
     this.tobacco = tobacco;

dependency-injection/src/main/java/com/iluwatar/dependency/injection/GuiceWizard.java

@@ -31,7 +31,7 @@ import javax.inject.Inject;
  */
 public class GuiceWizard implements Wizard {
 
-  private Tobacco tobacco;
+  private final Tobacco tobacco;
 
   @Inject
   public GuiceWizard(Tobacco tobacco) {

dependency-injection/src/main/java/com/iluwatar/dependency/injection/SimpleWizard.java

@@ -29,7 +29,7 @@ package com.iluwatar.dependency.injection;
  */
 public class SimpleWizard implements Wizard {
 
-  private OldTobyTobacco tobacco = new OldTobyTobacco();
+  private final OldTobyTobacco tobacco = new OldTobyTobacco();
 
   public void smoke() {
     tobacco.smoke(this);

dependency-injection/src/test/java/com/iluwatar/dependency/injection/utils/InMemoryAppender.java

@@ -37,7 +37,7 @@ import java.util.List;
  */
 public class InMemoryAppender extends AppenderBase<ILoggingEvent> {
 
-  private List<ILoggingEvent> log = new LinkedList<>();
+  private final List<ILoggingEvent> log = new LinkedList<>();
 
   public InMemoryAppender(Class clazz) {
     ((Logger) LoggerFactory.getLogger(clazz)).addAppender(this);

dirty-flag/src/main/java/com/iluwatar/dirtyflag/World.java

@@ -34,7 +34,7 @@ import java.util.List;
 public class World {
 
   private List<String> countries;
-  private DataFetcher df;
+  private final DataFetcher df;
 
   public World() {
     this.countries = new ArrayList<String>();

double-buffer/src/main/java/com/iluwatar/doublebuffer/FrameBuffer.java

@@ -33,7 +33,7 @@ public class FrameBuffer implements Buffer {
   public static final int WIDTH = 10;
   public static final int HEIGHT = 8;
 
-  private Pixel[] pixels = new Pixel[WIDTH * HEIGHT];
+  private final Pixel[] pixels = new Pixel[WIDTH * HEIGHT];
 
   public FrameBuffer() {
     clearAll();

double-buffer/src/main/java/com/iluwatar/doublebuffer/Pixel.java

@@ -31,7 +31,7 @@ public enum Pixel {
   WHITE(0),
   BLACK(1);
 
-  private int color;
+  private final int color;
 
   Pixel(int color) {
     this.color = color;

double-buffer/src/main/java/com/iluwatar/doublebuffer/Scene.java

@@ -35,7 +35,7 @@ public class Scene {
 
   private static final Logger LOGGER = LoggerFactory.getLogger(Scene.class);
 
-  private Buffer[] frameBuffers;
+  private final Buffer[] frameBuffers;
 
   private int current;
 

double-checked-locking/src/test/java/com/iluwatar/doublechecked/locking/InventoryTest.java

@@ -109,7 +109,7 @@ public class InventoryTest {
 
 
   private class InMemoryAppender extends AppenderBase<ILoggingEvent> {
-    private List<ILoggingEvent> log = new LinkedList<>();
+    private final List<ILoggingEvent> log = new LinkedList<>();
 
     public InMemoryAppender(Class clazz) {
       ((Logger) LoggerFactory.getLogger(clazz)).addAppender(this);

double-dispatch/src/main/java/com/iluwatar/doubledispatch/Rectangle.java

@@ -28,10 +28,10 @@ package com.iluwatar.doubledispatch;
  */
 public class Rectangle {
 
-  private int left;
-  private int top;
-  private int right;
-  private int bottom;
+  private final int left;
+  private final int top;
+  private final int right;
+  private final int bottom;
 
   /**
    * Constructor.

event-aggregator/src/main/java/com/iluwatar/event/aggregator/Event.java

@@ -31,7 +31,7 @@ public enum Event {
   STARK_SIGHTED("Stark sighted"), WARSHIPS_APPROACHING("Warships approaching"), TRAITOR_DETECTED(
       "Traitor detected");
 
-  private String description;
+  private final String description;
 
   Event(String description) {
     this.description = description;

event-aggregator/src/main/java/com/iluwatar/event/aggregator/EventEmitter.java

@@ -31,7 +31,7 @@ import java.util.List;
  */
 public abstract class EventEmitter {
 
-  private List<EventObserver> observers;
+  private final List<EventObserver> observers;
 
   public EventEmitter() {
     observers = new LinkedList<>();

event-aggregator/src/main/java/com/iluwatar/event/aggregator/Weekday.java

@@ -36,7 +36,7 @@ public enum Weekday {
   SATURDAY("Saturday"),
   SUNDAY("Sunday");
 
-  private String description;
+  private final String description;
 
   Weekday(String description) {
     this.description = description;

event-aggregator/src/test/java/com/iluwatar/event/aggregator/KingJoffreyTest.java

@@ -74,7 +74,7 @@ public class KingJoffreyTest {
   }
 
   private class InMemoryAppender extends AppenderBase<ILoggingEvent> {
-    private List<ILoggingEvent> log = new LinkedList<>();
+    private final List<ILoggingEvent> log = new LinkedList<>();
 
     public InMemoryAppender(Class<?> clazz) {
       ((Logger) LoggerFactory.getLogger(clazz)).addAppender(this);

event-asynchronous/src/main/java/com/iluwatar/event/asynchronous/Event.java

@@ -33,9 +33,9 @@ public class Event implements IEvent, Runnable {
 
   private static final Logger LOGGER = LoggerFactory.getLogger(Event.class);
 
-  private int eventId;
-  private int eventTime;
-  private boolean isSynchronous;
+  private final int eventId;
+  private final int eventTime;
+  private final boolean isSynchronous;
   private Thread thread;
   private boolean isComplete = false;
   private ThreadCompleteListener eventListener;

event-asynchronous/src/main/java/com/iluwatar/event/asynchronous/EventManager.java

@@ -43,8 +43,8 @@ public class EventManager implements ThreadCompleteListener {
   public static final int MAX_ID = MAX_RUNNING_EVENTS;
   public static final int MAX_EVENT_TIME = 1800; // in seconds / 30 minutes.
   private int currentlyRunningSyncEvent = -1;
-  private Random rand;
-  private Map<Integer, Event> eventPool;
+  private final Random rand;
+  private final Map<Integer, Event> eventPool;
 
   private static final String DOES_NOT_EXIST = " does not exist.";
 

event-driven-architecture/src/main/java/com/iluwatar/eda/event/UserCreatedEvent.java

@@ -32,7 +32,7 @@ import com.iluwatar.eda.model.User;
  */
 public class UserCreatedEvent extends AbstractEvent {
 
-  private User user;
+  private final User user;
 
   public UserCreatedEvent(User user) {
     this.user = user;

event-driven-architecture/src/main/java/com/iluwatar/eda/event/UserUpdatedEvent.java

@@ -32,7 +32,7 @@ import com.iluwatar.eda.model.User;
  */
 public class UserUpdatedEvent extends AbstractEvent {
 
-  private User user;
+  private final User user;
 
   public UserUpdatedEvent(User user) {
     this.user = user;

event-driven-architecture/src/main/java/com/iluwatar/eda/framework/EventDispatcher.java

@@ -32,7 +32,7 @@ import java.util.Map;
  */
 public class EventDispatcher {
 
-  private Map<Class<? extends Event>, Handler<? extends Event>> handlers;
+  private final Map<Class<? extends Event>, Handler<? extends Event>> handlers;
 
   public EventDispatcher() {
     handlers = new HashMap<>();

event-driven-architecture/src/main/java/com/iluwatar/eda/model/User.java

@@ -32,7 +32,7 @@ import com.iluwatar.eda.event.UserUpdatedEvent;
  */
 public class User {
 
-  private String username;
+  private final String username;
 
   public User(String username) {
     this.username = username;

event-queue/src/main/java/com/iluwatar/event/queue/Audio.java

@@ -49,7 +49,7 @@ public class Audio {
 
   private volatile Thread updateThread = null;
 
-  private PlayMessage[] pendingAudio = new PlayMessage[MAX_PENDING];
+  private final PlayMessage[] pendingAudio = new PlayMessage[MAX_PENDING];
 
   // Visible only for testing purposes
   Audio() {

extension-objects/src/main/java/concreteextensions/Commander.java

@@ -35,7 +35,7 @@ public class Commander implements CommanderExtension {
 
   private static final Logger LOGGER = LoggerFactory.getLogger(Commander.class);
 
-  private CommanderUnit unit;
+  private final CommanderUnit unit;
 
   public Commander(CommanderUnit commanderUnit) {
     this.unit = commanderUnit;

extension-objects/src/main/java/concreteextensions/Sergeant.java

@@ -35,7 +35,7 @@ public class Sergeant implements SergeantExtension {
 
   private static final Logger LOGGER = LoggerFactory.getLogger(Sergeant.class);
 
-  private SergeantUnit unit;
+  private final SergeantUnit unit;
 
   public Sergeant(SergeantUnit sergeantUnit) {
     this.unit = sergeantUnit;

extension-objects/src/main/java/concreteextensions/Soldier.java

@@ -34,7 +34,7 @@ import units.SoldierUnit;
 public class Soldier implements SoldierExtension {
   private static final Logger LOGGER = LoggerFactory.getLogger(Soldier.class);
 
-  private SoldierUnit unit;
+  private final SoldierUnit unit;
 
   public Soldier(SoldierUnit soldierUnit) {
     this.unit = soldierUnit;

facade/README.md

@@ -83,7 +83,7 @@ public abstract class DwarvenMineWorker {
 
   public abstract String name();
 
-  static enum Action {
+  enum Action {
     GO_TO_SLEEP, WAKE_UP, GO_HOME, GO_TO_MINE, WORK
   }
 }

facade/src/test/java/com/iluwatar/facade/DwarvenGoldmineFacadeTest.java

@@ -110,7 +110,7 @@ public class DwarvenGoldmineFacadeTest {
 
   private class InMemoryAppender extends AppenderBase<ILoggingEvent> {
 
-    private List<ILoggingEvent> log = new LinkedList<>();
+    private final List<ILoggingEvent> log = new LinkedList<>();
 
     public InMemoryAppender() {
       ((Logger) LoggerFactory.getLogger("root")).addAppender(this);

factory-method/src/main/java/com/iluwatar/factory/method/ElfBlacksmith.java

@@ -32,7 +32,7 @@ import java.util.Map;
  */
 public class ElfBlacksmith implements Blacksmith {
 
-  private static Map<WeaponType, ElfWeapon> ELFARSENAL;
+  private static final Map<WeaponType, ElfWeapon> ELFARSENAL;
 
   static {
     ELFARSENAL = new HashMap<>(WeaponType.values().length);

factory-method/src/main/java/com/iluwatar/factory/method/ElfWeapon.java

@@ -28,7 +28,7 @@ package com.iluwatar.factory.method;
  */
 public class ElfWeapon implements Weapon {
 
-  private WeaponType weaponType;
+  private final WeaponType weaponType;
 
   public ElfWeapon(WeaponType weaponType) {
     this.weaponType = weaponType;

factory-method/src/main/java/com/iluwatar/factory/method/OrcBlacksmith.java

@@ -32,7 +32,7 @@ import java.util.Map;
  */
 public class OrcBlacksmith implements Blacksmith {
 
-  private static Map<WeaponType, OrcWeapon> ORCARSENAL;
+  private static final Map<WeaponType, OrcWeapon> ORCARSENAL;
 
   static {
     ORCARSENAL = new HashMap<>(WeaponType.values().length);

factory-method/src/main/java/com/iluwatar/factory/method/OrcWeapon.java

@@ -28,7 +28,7 @@ package com.iluwatar.factory.method;
  */
 public class OrcWeapon implements Weapon {
 
-  private WeaponType weaponType;
+  private final WeaponType weaponType;
 
   public OrcWeapon(WeaponType weaponType) {
     this.weaponType = weaponType;

factory-method/src/main/java/com/iluwatar/factory/method/WeaponType.java

@@ -30,7 +30,7 @@ public enum WeaponType {
 
   SHORT_SWORD("short sword"), SPEAR("spear"), AXE("axe"), UNDEFINED("");
 
-  private String title;
+  private final String title;
 
   WeaponType(String title) {
     this.title = title;

feature-toggle/src/main/java/com/iluwatar/featuretoggle/pattern/propertiesversion/PropertiesFeatureToggleVersion.java

@@ -42,7 +42,7 @@ import java.util.Properties;
  */
 public class PropertiesFeatureToggleVersion implements Service {
 
-  private boolean isEnhanced;
+  private final boolean isEnhanced;
 
   /**
    * Creates an instance of {@link PropertiesFeatureToggleVersion} using the passed {@link

feature-toggle/src/main/java/com/iluwatar/featuretoggle/user/User.java

@@ -29,7 +29,7 @@ package com.iluwatar.featuretoggle.user;
  */
 public class User {
 
-  private String name;
+  private final String name;
 
   /**
    * Default Constructor setting the username.

feature-toggle/src/main/java/com/iluwatar/featuretoggle/user/UserGroup.java

@@ -35,8 +35,8 @@ import java.util.List;
  */
 public class UserGroup {
 
-  private static List<User> freeGroup = new ArrayList<>();
-  private static List<User> paidGroup = new ArrayList<>();
+  private static final List<User> freeGroup = new ArrayList<>();
+  private static final List<User> paidGroup = new ArrayList<>();
 
 
   /**

fluentinterface/src/main/java/com/iluwatar/fluentinterface/app/App.java

@@ -94,7 +94,7 @@ public class App {
         .filter(positives())
         .first(4)
         .last(2)
-        .map(number -> "String[" + valueOf(number) + "]")
+        .map(number -> "String[" + number + "]")
         .asList();
     prettyPrint("The lazy list contains the last two of the first four positive numbers "
         + "mapped to Strings: ", lastTwoOfFirstFourStringMapped);

fluentinterface/src/main/java/com/iluwatar/fluentinterface/fluentiterable/lazy/LazyFluentIterable.java

@@ -198,7 +198,7 @@ public class LazyFluentIterable<E> implements FluentIterable<E> {
       @Override
       public Iterator<T> iterator() {
         return new DecoratingIterator<T>(null) {
-          Iterator<E> oldTypeIterator = iterable.iterator();
+          final Iterator<E> oldTypeIterator = iterable.iterator();
 
           @Override
           public T computeNext() {

flux/src/main/java/com/iluwatar/flux/action/Action.java

@@ -28,7 +28,7 @@ package com.iluwatar.flux.action;
  */
 public abstract class Action {
 
-  private ActionType type;
+  private final ActionType type;
 
   public Action(ActionType type) {
     this.type = type;

flux/src/main/java/com/iluwatar/flux/action/ActionType.java

@@ -28,6 +28,6 @@ package com.iluwatar.flux.action;
  */
 public enum ActionType {
 
-  MENU_ITEM_SELECTED, CONTENT_CHANGED;
+  MENU_ITEM_SELECTED, CONTENT_CHANGED
 
 }

flux/src/main/java/com/iluwatar/flux/action/Content.java

@@ -31,7 +31,7 @@ public enum Content {
   PRODUCTS("Products - This page lists the company's products."), COMPANY(
       "Company - This page displays information about the company.");
 
-  private String title;
+  private final String title;
 
   Content(String title) {
     this.title = title;

flux/src/main/java/com/iluwatar/flux/action/ContentAction.java

@@ -28,7 +28,7 @@ package com.iluwatar.flux.action;
  */
 public class ContentAction extends Action {
 
-  private Content content;
+  private final Content content;
 
   public ContentAction(Content content) {
     super(ActionType.CONTENT_CHANGED);

flux/src/main/java/com/iluwatar/flux/action/MenuAction.java

@@ -29,7 +29,7 @@ package com.iluwatar.flux.action;
  */
 public class MenuAction extends Action {
 
-  private MenuItem menuItem;
+  private final MenuItem menuItem;
 
   public MenuAction(MenuItem menuItem) {
     super(ActionType.MENU_ITEM_SELECTED);

flux/src/main/java/com/iluwatar/flux/action/MenuItem.java

@@ -30,7 +30,7 @@ public enum MenuItem {
 
   HOME("Home"), PRODUCTS("Products"), COMPANY("Company");
 
-  private String title;
+  private final String title;
 
   MenuItem(String title) {
     this.title = title;

flux/src/main/java/com/iluwatar/flux/dispatcher/Dispatcher.java

@@ -39,7 +39,7 @@ public final class Dispatcher {
 
   private static Dispatcher instance = new Dispatcher();
 
-  private List<Store> stores = new LinkedList<>();
+  private final List<Store> stores = new LinkedList<>();
 
   private Dispatcher() {
   }

flux/src/main/java/com/iluwatar/flux/store/Store.java

@@ -33,7 +33,7 @@ import java.util.List;
  */
 public abstract class Store {
 
-  private List<View> views = new LinkedList<>();
+  private final List<View> views = new LinkedList<>();
 
   public abstract void onAction(Action action);
 

flyweight/src/main/java/com/iluwatar/flyweight/AlchemistShop.java

@@ -34,8 +34,8 @@ public class AlchemistShop {
 
   private static final Logger LOGGER = LoggerFactory.getLogger(AlchemistShop.class);
 
-  private List<Potion> topShelf;
-  private List<Potion> bottomShelf;
+  private final List<Potion> topShelf;
+  private final List<Potion> bottomShelf;
 
   /**
    * Constructor.

front-controller/src/test/java/com/iluwatar/front/controller/utils/InMemoryAppender.java

@@ -36,7 +36,7 @@ import java.util.List;
  */
 public class InMemoryAppender extends AppenderBase<ILoggingEvent> {
 
-  private List<ILoggingEvent> log = new LinkedList<>();
+  private final List<ILoggingEvent> log = new LinkedList<>();
 
   public InMemoryAppender() {
     ((Logger) LoggerFactory.getLogger("root")).addAppender(this);

half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/App.java

@@ -95,7 +95,7 @@ public class App {
    * ArithmeticSumTask.
    */
   static class ArithmeticSumTask implements AsyncTask<Long> {
-    private long numberOfElements;
+    private final long numberOfElements;
 
     public ArithmeticSumTask(long numberOfElements) {
       this.numberOfElements = numberOfElements;

half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/AsynchronousService.java

@@ -48,7 +48,7 @@ public class AsynchronousService {
    * tasks should be performed in the background which does not affect the performance of main
    * thread.
    */
-  private ExecutorService service;
+  private final ExecutorService service;
 
   /**
    * Creates an asynchronous service using {@code workQueue} as communication channel between

hexagonal/src/main/java/com/iluwatar/hexagonal/banking/InMemoryBank.java

@@ -32,7 +32,7 @@ import java.util.Map;
  */
 public class InMemoryBank implements WireTransfers {
 
-  private static Map<String, Integer> accounts = new HashMap<>();
+  private static final Map<String, Integer> accounts = new HashMap<>();
 
   static {
     accounts

hexagonal/src/main/java/com/iluwatar/hexagonal/database/InMemoryTicketRepository.java

@@ -34,7 +34,7 @@ import java.util.Optional;
  */
 public class InMemoryTicketRepository implements LotteryTicketRepository {
 
-  private static Map<LotteryTicketId, LotteryTicket> tickets = new HashMap<>();
+  private static final Map<LotteryTicketId, LotteryTicket> tickets = new HashMap<>();
 
   @Override
   public Optional<LotteryTicket> findById(LotteryTicketId id) {

hexagonal/src/main/java/com/iluwatar/hexagonal/domain/LotteryNumbers.java

@@ -116,7 +116,7 @@ public class LotteryNumbers {
    */
   private static class RandomNumberGenerator {
 
-    private PrimitiveIterator.OfInt randomIterator;
+    private final PrimitiveIterator.OfInt randomIterator;
 
     /**
      * Initialize a new random number generator that generates random numbers in the range [min,

hexagonal/src/main/java/com/iluwatar/hexagonal/domain/LotteryTicketId.java

@@ -30,7 +30,7 @@ import java.util.concurrent.atomic.AtomicInteger;
  */
 public class LotteryTicketId {
 
-  private static AtomicInteger numAllocated = new AtomicInteger(0);
+  private static final AtomicInteger numAllocated = new AtomicInteger(0);
   private final int id;
 
   public LotteryTicketId() {