Merge branch 'master' of github.com:iluwatar/java-design-patterns into java-11

update-website.sh → .github/workflows/maven.yml

-#!/bin/bash
 #
 # The MIT License
 # Copyright © 2014-2019 Ilkka Seppälä
@@ -22,25 +21,40 @@
 # THE SOFTWARE.
 #
 
+# This workflow will build a Java project with Maven
+# For more information see: https://help.github.com/actions/language-and-framework-guides/building-and-testing-java-with-maven
 
-# Clone gh-pages
-git clone https://github.com/iluwatar/java-design-patterns-web.git
-cd java-design-patterns-web
+name: Java CI with Maven
 
-# Init and update submodule to latest
-git submodule update --init --recursive
-cd 30-seconds-of-java
-git pull origin master
-cd ../patterns
-git pull origin master
-cd ../programming-principles
-git pull origin gh-pages
-cd ..
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
 
-# Setup Git
-git config user.name "Travis-CI"
-git config user.email "travis@no.reply"
+jobs:
+  build:
 
-git add .
-git commit -m ":sparkles: :up: Automagic Update via Travis-CI"
-git push --quiet "https://${GH_TOKEN}:x-oauth-basic@github.com/iluwatar/java-design-patterns-web.git" master > /dev/null 2>&1
+    runs-on: ubuntu-18.04
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up JDK 11
+      uses: actions/setup-java@v1
+      with:
+        java-version: 11
+    # Some tests need screen access
+    - name: Install xvfb
+      run: sudo apt-get install xvfb
+    # SonarQube scan does not work for forked repositories
+    # See https://jira.sonarsource.com/browse/MMF-1371
+    - name: Build with Maven
+      if: github.ref != 'refs/heads/master'
+      run: xvfb-run mvn clean verify
+    - name: Build with Maven and run SonarQube analysis
+      if: github.ref == 'refs/heads/master'
+      run: xvfb-run mvn clean verify org.sonarsource.scanner.maven:sonar-maven-plugin:sonar
+      env:
+        # These two env variables are needed for sonar analysis
+        GITHUB_TOKEN: ${{ secrets.REPOSITORY_ACCESS_TOKEN }}
+        SONAR_TOKEN: ${{ secrets.SONAR_TOKEN }}

.travis.yml

-language: java
-dist: bionic
-jdk:
-- openjdk11
-sudo: required
-
-env:
-  global:
-  - secure: "DCpazS3nkLnter3sguXEAS2fC/1ZWNfM+XLyif9MfNFxlZdpni2vCD/jA0Rdpga8puQWHNVLyAec+RPFH/2qSmJ1c1UTV5MaLv8tPqwUX0VFA+1I6XoSv6oX4ldHTBWHEWqQHkRFOLoil0h0edc0tTOWQwXF8U+DLAB+HkRb4gw="
-
-services:
-- xvfb
-
-addons:
-  sonarcloud:
-    organization: "iluwatar"
-    token:
-      secure: "FpHwMYPMkdWU6CeIB7+O3qIeIM4vJMp47UjkKK53f0w0s6tPZofZZkab+gcL2TqKSil7sFVB/AQXU1cUubflRszwcLbNsc8H2yFehD79o0o0Mqd1Dd5ip/q0KQbHkkln+InFlVLfvrLB4Xd4mlQVxbGhqpULBhXjKzFzQlRFcuU="
-script:
-  # Because of Travis security restrictions, SonarCloud analysis cannot be run on pull requests originated from forks
-  # See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
-  - 'if [ "$TRAVIS_PULL_REQUEST" != "false" ]; then mvn clean verify; fi'
-  - 'if [ "$TRAVIS_PULL_REQUEST" = "false" ]; then mvn clean verify sonar:sonar -Dsonar.projectKey=iluwatar_java-design-patterns -Dsonar.host.url=https://sonarcloud.io; fi'
-
-after_success:
-- bash update-website.sh
-
-notifications:
-  email:
-    - iluwatar@gmail.com
-  webhooks:
-    urls:
-      - https://webhooks.gitter.im/e/3319623945358a093a6f
-    on_success: change  # options: [always|never|change] default: always
-    on_failure: always  # options: [always|never|change] default: always
-    on_start: never     # options: [always|never|change] default: always

abstract-document/README.md

@@ -9,21 +9,182 @@ tags:
 ---
 
 ## Intent
-Achieve flexibility of untyped languages and keep the type-safety 
+
+Use dynamic properties and achieve flexibility of untyped languages while keeping type-safety. 
+
+## Explanation
+
+The Abstract Document pattern enables handling additional, non-static properties. This pattern
+uses concept of traits to enable type safety and separate properties of different classes into
+set of interfaces.
+
+Real world example
+
+>  Consider a car that consists of multiple parts. However we don't know if the specific car really has all the parts, or just some of them. Our cars are dynamic and extremely flexible.
+
+In plain words
+
+> Abstract Document pattern allows attaching properties to objects without them knowing about it.
+
+Wikipedia says
+
+> An object-oriented structural design pattern for organizing objects in loosely typed key-value stores and exposing 
+the data using typed views. The purpose of the pattern is to achieve a high degree of flexibility between components 
+in a strongly typed language where new properties can be added to the object-tree on the fly, without losing the 
+support of type-safety. The pattern makes use of traits to separate different properties of a class into different 
+interfaces.
+
+**Programmatic Example**
+
+Let's first define the base classes `Document` and `AbstractDocument`. They basically make the object hold a property
+map and any amount of child objects.
+
+```java
+public interface Document {
+
+  Void put(String key, Object value);
+
+  Object get(String key);
+
+  <T> Stream<T> children(String key, Function<Map<String, Object>, T> constructor);
+}
+
+public abstract class AbstractDocument implements Document {
+
+  private final Map<String, Object> properties;
+
+  protected AbstractDocument(Map<String, Object> properties) {
+    Objects.requireNonNull(properties, "properties map is required");
+    this.properties = properties;
+  }
+
+  @Override
+  public Void put(String key, Object value) {
+    properties.put(key, value);
+    return null;
+  }
+
+  @Override
+  public Object get(String key) {
+    return properties.get(key);
+  }
+
+  @Override
+  public <T> Stream<T> children(String key, Function<Map<String, Object>, T> constructor) {
+    return Stream.ofNullable(get(key))
+        .filter(Objects::nonNull)
+        .map(el -> (List<Map<String, Object>>) el)
+        .findAny()
+        .stream()
+        .flatMap(Collection::stream)
+        .map(constructor);
+  }
+  ...
+}
+```
+Next we define an enum `Property` and a set of interfaces for type, price, model and parts. This allows us to create
+static looking interface to our `Car` class.
+
+```java
+public enum Property {
+
+  PARTS, TYPE, PRICE, MODEL
+}
+
+public interface HasType extends Document {
+
+  default Optional<String> getType() {
+    return Optional.ofNullable((String) get(Property.TYPE.toString()));
+  }
+}
+
+public interface HasPrice extends Document {
+
+  default Optional<Number> getPrice() {
+    return Optional.ofNullable((Number) get(Property.PRICE.toString()));
+  }
+}
+public interface HasModel extends Document {
+
+  default Optional<String> getModel() {
+    return Optional.ofNullable((String) get(Property.MODEL.toString()));
+  }
+}
+
+public interface HasParts extends Document {
+
+  default Stream<Part> getParts() {
+    return children(Property.PARTS.toString(), Part::new);
+  }
+}
+```
+
+Now we are ready to introduce the `Car`.
+
+```java
+public class Car extends AbstractDocument implements HasModel, HasPrice, HasParts {
+
+  public Car(Map<String, Object> properties) {
+    super(properties);
+  }
+}
+```
+
+And finally here's how we construct and use the `Car` in a full example.
+
+```java
+    LOGGER.info("Constructing parts and car");
+
+    var wheelProperties = Map.of(
+        Property.TYPE.toString(), "wheel",
+        Property.MODEL.toString(), "15C",
+        Property.PRICE.toString(), 100L);
+
+    var doorProperties = Map.of(
+        Property.TYPE.toString(), "door",
+        Property.MODEL.toString(), "Lambo",
+        Property.PRICE.toString(), 300L);
+
+    var carProperties = Map.of(
+        Property.MODEL.toString(), "300SL",
+        Property.PRICE.toString(), 10000L,
+        Property.PARTS.toString(), List.of(wheelProperties, doorProperties));
+
+    var car = new Car(carProperties);
+
+    LOGGER.info("Here is our car:");
+    LOGGER.info("-> model: {}", car.getModel().orElseThrow());
+    LOGGER.info("-> price: {}", car.getPrice().orElseThrow());
+    LOGGER.info("-> parts: ");
+    car.getParts().forEach(p -> LOGGER.info("\t{}/{}/{}",
+        p.getType().orElse(null),
+        p.getModel().orElse(null),
+        p.getPrice().orElse(null))
+    );
+
+    // Constructing parts and car
+    // Here is our car:
+    // model: 300SL
+    // price: 10000
+    // parts: 
+    // wheel/15C/100
+    // door/Lambo/300
+```
 
 ## Class diagram
-![alt text](./etc/abstract-document.png "Abstract Document Traits and Domain")
 
+![alt text](./etc/abstract-document.png "Abstract Document Traits and Domain")
 
 ## Applicability
-Use the Abstract Document Pattern when
 
-* there is a need to add new properties on the fly
-* you want a flexible way to organize domain in tree like structure
-* you want more loosely coupled system
+Use the Abstract Document Pattern when
 
+* There is a need to add new properties on the fly
+* You want a flexible way to organize domain in tree like structure
+* You want more loosely coupled system
 
 ## Credits
 
 * [Wikipedia: Abstract Document Pattern](https://en.wikipedia.org/wiki/Abstract_Document_Pattern)
 * [Martin Fowler: Dealing with properties](http://martinfowler.com/apsupp/properties.pdf)
+* [Pattern-Oriented Software Architecture Volume 4: A Pattern Language for Distributed Computing (v. 4)](https://www.amazon.com/gp/product/0470059028/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=0470059028&linkId=e3aacaea7017258acf184f9f3283b492)

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

@@ -43,9 +43,11 @@ public class App {
   private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
 
   /**
-   * Executes the App.
+   * Program entry point.
+   *
+   * @param args command line args
    */
-  public App() {
+  public static void main(String[] args) {
     LOGGER.info("Constructing parts and car");
 
     var wheelProperties = Map.of(
@@ -75,14 +77,4 @@ public class App {
         p.getPrice().orElse(null))
     );
   }
-
-  /**
-   * Program entry point.
-   *
-   * @param args command line args
-   */
-  public static void main(String[] args) {
-    new App();
-  }
-
 }

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

@@ -32,7 +32,6 @@ import java.util.stream.Stream;
  */
 public interface HasParts extends Document {
 
-
   default Stream<Part> getParts() {
     return children(Property.PARTS.toString(), Part::new);
   }

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

@@ -32,7 +32,6 @@ import java.util.Optional;
  */
 public interface HasPrice extends Document {
 
-
   default Optional<Number> getPrice() {
     return Optional.ofNullable((Number) get(Property.PRICE.toString()));
   }

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

@@ -32,7 +32,6 @@ import java.util.Optional;
  */
 public interface HasType extends Document {
 
-
   default Optional<String> getType() {
     return Optional.ofNullable((String) get(Property.TYPE.toString()));
   }

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

@@ -40,14 +40,14 @@ public class AbstractDocumentTest {
   private static final String KEY = "key";
   private static final String VALUE = "value";
 
-  private class DocumentImplementation extends AbstractDocument {
+  private static class DocumentImplementation extends AbstractDocument {
 
     DocumentImplementation(Map<String, Object> properties) {
       super(properties);
     }
   }
 
-  private DocumentImplementation document = new DocumentImplementation(new HashMap<>());
+  private final DocumentImplementation document = new DocumentImplementation(new HashMap<>());
 
   @Test
   public void shouldPutAndGetValue() {

abstract-factory/README.md

@@ -9,13 +9,16 @@ tags:
 ---
 
 ## Also known as
+
 Kit
 
 ## Intent
+
 Provide an interface for creating families of related or dependent
 objects without specifying their concrete classes.
 
 ## Explanation
+
 Real world example
 
 > To create a kingdom we need objects with common theme. Elven kingdom needs an Elven king, Elven castle and Elven army whereas Orcish kingdom needs an Orcish king, Orcish castle and Orcish army. There is a dependency between the objects in the kingdom.
@@ -36,9 +39,11 @@ Translating the kingdom example above. First of all we have some interfaces and
 public interface Castle {
   String getDescription();
 }
+
 public interface King {
   String getDescription();
 }
+
 public interface Army {
   String getDescription();
 }
@@ -66,7 +71,7 @@ public class ElfArmy implements Army {
   }
 }
 
-// Orcish implementations similarly...
+// Orcish implementations similarly -> ...
 
 ```
 
@@ -112,9 +117,17 @@ var castle = factory.createCastle();
 var king = factory.createKing();
 var army = factory.createArmy();
 
-castle.getDescription();  // Output: This is the Elven castle!
-king.getDescription(); // Output: This is the Elven king!
-army.getDescription(); // Output: This is the Elven Army!
+castle.getDescription();
+king.getDescription();
+army.getDescription();
+```
+
+Program output:
+
+```java
+This is the Elven castle!
+This is the Elven king!
+This is the Elven Army!
 ```
 
 Now, we can design a factory for our different kingdom factories. In this example, we created FactoryMaker, responsible for returning an instance of either ElfKingdomFactory or OrcKingdomFactory.  
@@ -156,37 +169,39 @@ public static void main(String[] args) {
 ```
 
 ## Class diagram
+
 ![alt text](./etc/abstract-factory.urm.png "Abstract Factory class diagram")
 
 
 ## Applicability
+
 Use the Abstract Factory pattern when
 
-* a system should be independent of how its products are created, composed and represented
-* a system should be configured with one of multiple families of products
-* a family of related product objects is designed to be used together, and you need to enforce this constraint
-* you want to provide a class library of products, and you want to reveal just their interfaces, not their implementations
-* the lifetime of the dependency is conceptually shorter than the lifetime of the consumer.
-*	you need a run-time value to construct a particular dependency
-*	you want to decide which product to call from a family at runtime.
-*	you need to supply one or more parameters only known at run-time before you can resolve a dependency.
-* when you need consistency among products
-* you don’t want to change existing code when adding new products or families of products to the program.
+* The system should be independent of how its products are created, composed and represented
+* The system should be configured with one of multiple families of products
+* The family of related product objects is designed to be used together, and you need to enforce this constraint
+* You want to provide a class library of products, and you want to reveal just their interfaces, not their implementations
+* The lifetime of the dependency is conceptually shorter than the lifetime of the consumer.
+* You need a run-time value to construct a particular dependency
+* You want to decide which product to call from a family at runtime.
+* You need to supply one or more parameters only known at run-time before you can resolve a dependency.
+* When you need consistency among products
+* You don’t want to change existing code when adding new products or families of products to the program.
 
 ## Use Cases:	
 
-*	Selecting to call the appropriate implementation of FileSystemAcmeService or DatabaseAcmeService or NetworkAcmeService at runtime.
-*	Unit test case writing becomes much easier
+* Selecting to call the appropriate implementation of FileSystemAcmeService or DatabaseAcmeService or NetworkAcmeService at runtime.
+* Unit test case writing becomes much easier
 * UI tools for different OS
 
 ## Consequences:
 
-*	Dependency injection in java hides the service class dependencies that can lead to runtime errors that would have been caught at compile time.
+* Dependency injection in java hides the service class dependencies that can lead to runtime errors that would have been caught at compile time.
 * While the pattern is great when creating predefined objects, adding the new ones might be challenging.
-* The code may become more complicated than it should be, since a lot of new interfaces and classes are introduced along with the pattern.
-
+* The code becomes more complicated than it should be, since a lot of new interfaces and classes are introduced along with the pattern.
 
 ## Tutorial
+
 * [Abstract Factory Pattern Tutorial](https://www.journaldev.com/1418/abstract-factory-design-pattern-in-java) 
 
 
@@ -198,4 +213,5 @@ Use the Abstract Factory pattern when
 
 ## Credits
 
-* [Design Patterns: Elements of Reusable Object-Oriented Software](http://www.amazon.com/Design-Patterns-Elements-Reusable-Object-Oriented/dp/0201633612)
+* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
+* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)

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/README.md

@@ -9,12 +9,126 @@ tags:
 ---
 
 ## Intent
-Allow new functions to be added to existing class hierarchies without affecting those hierarchies, and without creating the troublesome dependency cycles that are inherent to the GOF VISITOR Pattern.
+
+Allow new functions to be added to existing class hierarchies without affecting those hierarchies, and without creating 
+the troublesome dependency cycles that are inherent to the GoF Visitor Pattern.
+
+## Explanation
+
+Real world example
+
+> We have a hierarchy of modem classes. The modems in this hierarchy need to be visited by an external algorithm based 
+> on filtering criteria (is it Unix or DOS compatible modem). 
+
+In plain words
+
+> Acyclic Visitor allows functions to be added to existing class hierarchies without modifying the hierarchies.
+
+[WikiWikiWeb](https://wiki.c2.com/?AcyclicVisitor) says
+
+> The Acyclic Visitor pattern allows new functions to be added to existing class hierarchies without affecting those 
+> hierarchies, and without creating the dependency cycles that are inherent to the GangOfFour VisitorPattern.
+
+**Programmatic Example**
+
+Here's the `Modem` hierarchy.
+
+```java
+public abstract class Modem {
+  public abstract void accept(ModemVisitor modemVisitor);
+}
+
+public class Zoom extends Modem {
+  ...
+  @Override
+  public void accept(ModemVisitor modemVisitor) {
+    if (modemVisitor instanceof ZoomVisitor) {
+      ((ZoomVisitor) modemVisitor).visit(this);
+    } else {
+      LOGGER.info("Only ZoomVisitor is allowed to visit Zoom modem");
+    }
+  }
+}
+
+public class Hayes extends Modem {
+  ...
+  @Override
+  public void accept(ModemVisitor modemVisitor) {
+    if (modemVisitor instanceof HayesVisitor) {
+      ((HayesVisitor) modemVisitor).visit(this);
+    } else {
+      LOGGER.info("Only HayesVisitor is allowed to visit Hayes modem");
+    }
+  }
+}
+```
+
+Next we introduce the `ModemVisitor` hierarchy.
+
+```java
+public interface ModemVisitor {
+}
+
+public interface HayesVisitor extends ModemVisitor {
+  void visit(Hayes hayes);
+}
+
+public interface ZoomVisitor extends ModemVisitor {
+  void visit(Zoom zoom);
+}
+
+public interface AllModemVisitor extends ZoomVisitor, HayesVisitor {
+}
+
+public class ConfigureForDosVisitor implements AllModemVisitor {
+  ...
+  @Override
+  public void visit(Hayes hayes) {
+    LOGGER.info(hayes + " used with Dos configurator.");
+  }
+  @Override
+  public void visit(Zoom zoom) {
+    LOGGER.info(zoom + " used with Dos configurator.");
+  }
+}
+
+public class ConfigureForUnixVisitor implements ZoomVisitor {
+  ...
+  @Override
+  public void visit(Zoom zoom) {
+    LOGGER.info(zoom + " used with Unix configurator.");
+  }
+}
+```
+
+Finally here are the visitors in action.
+
+```java
+    var conUnix = new ConfigureForUnixVisitor();
+    var conDos = new ConfigureForDosVisitor();
+    var zoom = new Zoom();
+    var hayes = new Hayes();
+    hayes.accept(conDos);
+    zoom.accept(conDos);
+    hayes.accept(conUnix);
+    zoom.accept(conUnix);   
+```
+
+Program output:
+
+```
+    // Hayes modem used with Dos configurator.
+    // Zoom modem used with Dos configurator.
+    // Only HayesVisitor is allowed to visit Hayes modem
+    // Zoom modem used with Unix configurator.
+```
 
 ## Class diagram
+
 ![alt text](./etc/acyclic-visitor.png "Acyclic Visitor")
 
 ## Applicability
+
 This pattern can be used:
 
 * When you need to add a new function to an existing hierarchy without the need to alter or affect that hierarchy.
@@ -24,6 +138,7 @@ This pattern can be used:
 * When the recompilation, relinking, retesting or redistribution of the derivatives of Element is very expensive.
 
 ## Consequences
+
 The good:
 
 * No dependency cycles between class hierarchies.
@@ -32,11 +147,14 @@ The good:
 
 The bad:
 
-* Violates the principle of least surprise or Liskov's Substitution principle by showing that it can accept all visitors but actually only being interested in particular visitors.
+* Violates [Liskov's Substitution Principle](https://java-design-patterns.com/principles/#liskov-substitution-principle) by showing that it can accept all visitors but actually only being interested in particular visitors.
 * Parallel hierarchy of visitors has to be created for all members in visitable class hierarchy.
 
 ## Related patterns
-* [Visitor Pattern](../visitor/)
+
+* [Visitor Pattern](https://java-design-patterns.com/patterns/visitor/)
 
 ## Credits
-* [Acyclic Visitor](http://condor.depaul.edu/dmumaugh/OOT/Design-Principles/acv.pdf)
+
+* [Acyclic Visitor by Robert C. Martin](http://condor.depaul.edu/dmumaugh/OOT/Design-Principles/acv.pdf)
+* [Acyclic Visitor in WikiWikiWeb](https://wiki.c2.com/?AcyclicVisitor)

acyclic-visitor/etc/Acyclic Visitor.ucls

-<?xml version="1.0" encoding="UTF-8"?>
-<class-diagram version="1.2.2" icons="true" always-add-relationships="false" generalizations="true" realizations="true" 
-  associations="true" dependencies="false" nesting-relationships="true" router="FAN">  
-  <interface id="1" language="java" name="com.iluwatar.acyclicvisitor.ModemVisitor" project="acyclic-visitor" 
-    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ModemVisitor.java" binary="false" 
-    corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="860" y="67"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </interface>  
-  <class id="2" language="java" name="com.iluwatar.acyclicvisitor.Modem" project="acyclic-visitor" 
-    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/Modem.java" binary="false" corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="327" y="77"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <class id="3" language="java" name="com.iluwatar.acyclicvisitor.ConfigureForUnixVisitor" project="acyclic-visitor" 
-    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ConfigureForUnixVisitor.java" binary="false" 
-    corner="BOTTOM_RIGHT">    
-    <position height="124" width="196" x="647" y="225"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <class id="4" language="java" name="com.iluwatar.acyclicvisitor.Zoom" project="acyclic-visitor" 
-    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/Zoom.java" binary="false" corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="203" y="305"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <interface id="5" language="java" name="com.iluwatar.acyclicvisitor.HayesVisitor" project="acyclic-visitor" 
-    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/HayesVisitor.java" binary="false" 
-    corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="1019" y="468"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </interface>  
-  <interface id="6" language="java" name="com.iluwatar.acyclicvisitor.ZoomVisitor" project="acyclic-visitor" 
-    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ZoomVisitor.java" binary="false" 
-    corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="758" y="467"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </interface>  
-  <class id="7" language="java" name="com.iluwatar.acyclicvisitor.Hayes" project="acyclic-visitor" 
-    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/Hayes.java" binary="false" corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="479" y="307"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <class id="8" language="java" name="com.iluwatar.acyclicvisitor.ConfigureForDosVisitor" project="acyclic-visitor" 
-    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ConfigureForDosVisitor.java" binary="false" 
-    corner="BOTTOM_RIGHT">    
-    <position height="142" width="192" x="883" y="225"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <generalization id="9">    
-    <end type="SOURCE" refId="7"/>    
-    <end type="TARGET" refId="2"/>  
-  </generalization>  
-  <realization id="10">    
-    <end type="SOURCE" refId="8"/>    
-    <end type="TARGET" refId="6"/>  
-  </realization>  
-  <realization id="11">    
-    <end type="SOURCE" refId="8"/>    
-    <end type="TARGET" refId="5"/>  
-  </realization>  
-  <realization id="12">    
-    <end type="SOURCE" refId="3"/>    
-    <end type="TARGET" refId="6"/>  
-  </realization>  
-  <realization id="13">    
-    <end type="SOURCE" refId="3"/>    
-    <end type="TARGET" refId="1"/>  
-  </realization>  
-  <realization id="14">    
-    <end type="SOURCE" refId="8"/>    
-    <end type="TARGET" refId="1"/>  
-  </realization>  
-  <generalization id="15">    
-    <end type="SOURCE" refId="4"/>    
-    <end type="TARGET" refId="2"/>  
-  </generalization>  
-  <classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-    sort-features="false" accessors="true" visibility="true">    
-    <attributes public="true" package="true" protected="true" private="true" static="true"/>    
-    <operations public="true" package="true" protected="true" private="true" static="true"/>  
-  </classifier-display>  
-  <association-display labels="true" multiplicity="true"/>
-</class-diagram>
\ No newline at end of file

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();
@@ -129,5 +129,7 @@ An object adapter
 
 ## Credits
 
-* [Design Patterns: Elements of Reusable Object-Oriented Software](http://www.amazon.com/Design-Patterns-Elements-Reusable-Object-Oriented/dp/0201633612)
-* [J2EE Design Patterns](http://www.amazon.com/J2EE-Design-Patterns-William-Crawford/dp/0596004273/ref=sr_1_2)
+* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
+* [J2EE Design Patterns](https://www.amazon.com/gp/product/0596004273/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596004273&linkCode=as2&tag=javadesignpat-20&linkId=48d37c67fb3d845b802fa9b619ad8f31)
+* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
+* [Refactoring to Patterns](https://www.amazon.com/gp/product/0321213351/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0321213351&linkCode=as2&tag=javadesignpat-20&linkId=2a76fcb387234bc71b1c61150b3cc3a7)

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/README.md

@@ -5,7 +5,8 @@ folder: ambassador
 permalink: /patterns/ambassador/
 categories: Structural
 tags:
- - Decoupling
+  - Decoupling
+  - Cloud distributed
 ---
 
 ## Intent
@@ -22,8 +23,7 @@ In plain words
 
 Microsoft documentation states
 
-> An ambassador service can be thought of as an out-of-process proxy that is co-located with the client.
-  This pattern can be useful for offloading common client connectivity tasks such as monitoring, logging, routing, security (such as TLS), and resiliency patterns in a language agnostic way. It is often used with legacy applications, or other applications that are difficult to modify, in order to extend their networking capabilities. It can also enable a specialized team to implement those features.
+> An ambassador service can be thought of as an out-of-process proxy that is co-located with the client. This pattern can be useful for offloading common client connectivity tasks such as monitoring, logging, routing, security (such as TLS), and resiliency patterns in a language agnostic way. It is often used with legacy applications, or other applications that are difficult to modify, in order to extend their networking capabilities. It can also enable a specialized team to implement those features.
 
 **Programmatic Example**
 

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;

api-gateway/README.md

@@ -5,14 +5,142 @@ folder: api-gateway
 permalink: /patterns/api-gateway/
 categories: Architectural
 tags:
-- Cloud distributed
-- Decoupling
+  - Cloud distributed
+  - Decoupling
+  - Microservices
 ---
 
 ## Intent
 
-Aggregate calls to microservices in a single location: the API Gateway. The user makes a single
-call to the API Gateway, and the API Gateway then calls each relevant microservice.
+Aggregate calls to microservices in a single location: the API Gateway. The user makes a single call to the API Gateway, 
+and the API Gateway then calls each relevant microservice.
+
+## Explanation
+
+With the Microservices pattern, a client may need data from multiple different microservices. If the client called each 
+microservice directly, that could contribute to longer load times, since the client would have to make a network request 
+for each microservice called. Moreover, having the client call each microservice directly ties the client to that 
+microservice - if the internal implementations of the microservices change (for example, if two microservices are 
+combined sometime in the future) or if the location (host and port) of a microservice changes, then every client that 
+makes use of those microservices must be updated.
+
+The intent of the API Gateway pattern is to alleviate some of these issues. In the API Gateway pattern, an additional 
+entity (the API Gateway) is placed between the client and the microservices. The job of the API Gateway is to aggregate 
+the calls to the microservices. Rather than the client calling each microservice individually, the client calls the 
+API Gateway a single time. The API Gateway then calls each of the microservices that the client needs.
+
+Real world example
+
+> We are implementing microservices and API Gateway pattern for an e-commerce site. In this system the API Gateway makes 
+calls to the Image and Price microservices.
+
+In plain words
+
+> For a system implemented using microservices architecture, API Gateway is the single entry point that aggregates the 
+calls to the individual microservices. 
+
+Wikipedia says
+
+> API Gateway is a server that acts as an API front-end, receives API requests, enforces throttling and security 
+policies, passes requests to the back-end service and then passes the response back to the requester. A gateway often 
+includes a transformation engine to orchestrate and modify the requests and responses on the fly. A gateway can also 
+provide functionality such as collecting analytics data and providing caching. The gateway can provide functionality to 
+support authentication, authorization, security, audit and regulatory compliance.
+
+**Programmatic Example**
+
+This implementation shows what the API Gateway pattern could look like for an e-commerce site. The `ApiGateway` makes 
+calls to the Image and Price microservices using the `ImageClientImpl` and `PriceClientImpl` respectively. Customers 
+viewing the site on a desktop device can see both price information and an image of a product, so the `ApiGateway` calls 
+both of the microservices and aggregates the data in the `DesktopProduct` model. However, mobile users only see price 
+information; they do not see a product image. For mobile users, the `ApiGateway` only retrieves price information, which 
+it uses to populate the `MobileProduct`.
+
+Here's the Image microservice implementation.
+
+```java
+public interface ImageClient {
+  String getImagePath();
+}
+
+public class ImageClientImpl implements ImageClient {
+
+  @Override
+  public String getImagePath() {
+    var httpClient = HttpClient.newHttpClient();
+    var httpGet = HttpRequest.newBuilder()
+        .GET()
+        .uri(URI.create("http://localhost:50005/image-path"))
+        .build();
+
+    try {
+      var httpResponse = httpClient.send(httpGet, BodyHandlers.ofString());
+      return httpResponse.body();
+    } catch (IOException | InterruptedException e) {
+      e.printStackTrace();
+    }
+
+    return null;
+  }
+}
+```
+
+Here's the Price microservice implementation.
+
+```java
+public interface PriceClient {
+  String getPrice();
+}
+
+public class PriceClientImpl implements PriceClient {
+
+  @Override
+  public String getPrice() {
+    var httpClient = HttpClient.newHttpClient();
+    var httpGet = HttpRequest.newBuilder()
+        .GET()
+        .uri(URI.create("http://localhost:50006/price"))
+        .build();
+
+    try {
+      var httpResponse = httpClient.send(httpGet, BodyHandlers.ofString());
+      return httpResponse.body();
+    } catch (IOException | InterruptedException e) {
+      e.printStackTrace();
+    }
+
+    return null;
+  }
+}
+```
+
+And here we can see how API Gateway maps the requests to the microservices.
+
+```java
+public class ApiGateway {
+
+  @Resource
+  private ImageClient imageClient;
+
+  @Resource
+  private PriceClient priceClient;
+
+  @RequestMapping(path = "/desktop", method = RequestMethod.GET)
+  public DesktopProduct getProductDesktop() {
+    var desktopProduct = new DesktopProduct();
+    desktopProduct.setImagePath(imageClient.getImagePath());
+    desktopProduct.setPrice(priceClient.getPrice());
+    return desktopProduct;
+  }
+
+  @RequestMapping(path = "/mobile", method = RequestMethod.GET)
+  public MobileProduct getProductMobile() {
+    var mobileProduct = new MobileProduct();
+    mobileProduct.setPrice(priceClient.getPrice());
+    return mobileProduct;
+  }
+}
+```
 
 ## Class diagram
 ![alt text](./etc/api-gateway.png "API Gateway")
@@ -21,10 +149,11 @@ call to the API Gateway, and the API Gateway then calls each relevant microservi
 
 Use the API Gateway pattern when
 
-* you're also using the Microservices pattern and need a single point of aggregation for your
-microservice calls
+* You're using microservices architecture and need a single point of aggregation for your microservice calls.
 
 ## Credits
 
 * [microservices.io - API Gateway](http://microservices.io/patterns/apigateway.html)
 * [NGINX - Building Microservices: Using an API Gateway](https://www.nginx.com/blog/building-microservices-using-an-api-gateway/)
+* [Microservices Patterns: With examples in Java](https://www.amazon.com/gp/product/1617294543/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=1617294543&linkId=ac7b6a57f866ac006a309d9086e8cfbd)
+* [Building Microservices: Designing Fine-Grained Systems](https://www.amazon.com/gp/product/1491950358/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=1491950358&linkId=4c95ca9831e05e3f0dadb08841d77bf1)

arrange-act-assert/README.md

+---
+layout: pattern
+title: Arrange/Act/Assert
+folder: arrange-act-assert
+permalink: /patterns/arrange-act-assert/
+categories: Idiom
+tags:
+ - Testing
+---
+
+## Also known as
+Given/When/Then
+
+## Intent
+The Arrange/Act/Assert (AAA) is a pattern for organizing unit tests.
+It breaks tests down into three clear and distinct steps:
+1. Arrange: Perform the setup and initialization required for the test.
+2. Act: Take action(s) required for the test.
+3. Assert: Verify the outcome(s) of the test.
+
+## Applicability
+Use Arrange/Act/Assert pattern when
+
+* you need to structure your unit tests so they're easier to read, maintain, and enhance. 
+
+## Credits
+
+* [Arrange, Act, Assert: What is AAA Testing?](https://blog.ncrunch.net/post/arrange-act-assert-aaa-testing.aspx)
+* [Bill Wake: 3A – Arrange, Act, Assert](https://xp123.com/articles/3a-arrange-act-assert/)
+* [Martin Fowler: GivenWhenThen](https://martinfowler.com/bliki/GivenWhenThen.html)

arrange-act-assert/etc/arrange-act-assert.urm.puml

+@startuml
+package com.iluwatar.arrangeactassert {
+  class Cash {
+    - amount : int
+    ~ Cash(amount : int)
+    ~ count() : int
+    ~ minus(subtrahend : int) : boolean
+    ~ plus(addend : int)
+  }
+}
+@enduml
\ No newline at end of file

arrange-act-assert/pom.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    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 xmlns="http://maven.apache.org/POM/4.0.0"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <parent>
+    <artifactId>java-design-patterns</artifactId>
+    <groupId>com.iluwatar</groupId>
+    <version>1.23.0-SNAPSHOT</version>
+  </parent>
+  <modelVersion>4.0.0</modelVersion>
+
+  <artifactId>arrange-act-assert</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>
\ No newline at end of file

arrange-act-assert/src/main/java/com/iluwatar/arrangeactassert/Cash.java

+/*
+ * 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.arrangeactassert;
+
+/**
+ * Arrange/Act/Assert (AAA) is a unit test pattern. In this simple example, we have a ({@link Cash})
+ * object for plus, minus and counting amount.
+ */
+public class Cash {
+
+  private int amount;
+
+  Cash(int amount) {
+    this.amount = amount;
+  }
+
+  //plus
+  void plus(int addend) {
+    amount += addend;
+  }
+
+  //minus
+  boolean minus(int subtrahend) {
+    if (amount >= subtrahend) {
+      amount -= subtrahend;
+      return true;
+    } else {
+      return false;
+    }
+  }
+
+  //count
+  int count() {
+    return amount;
+  }
+}

arrange-act-assert/src/test/java/com/iluwatar/arrangeactassert/CashAAATest.java

+/*
+ * 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.arrangeactassert;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+import org.junit.Test;
+
+/**
+ * Arrange/Act/Assert (AAA) is a pattern for organizing unit tests. It is a way to structure your
+ * tests so they're easier to read, maintain and enhance.
+ *
+ * <p>It breaks tests down into three clear and distinct steps:
+ * <p>1. Arrange: Perform the setup and initialization required for the test.
+ * <p>2. Act: Take action(s) required for the test.
+ * <p>3. Assert: Verify the outcome(s) of the test.
+ *
+ * <p>This pattern has several significant benefits. It creates a clear separation between a test's
+ * setup, operations, and results. This structure makes the code easier to read and understand. If
+ * you place the steps in order and format your code to separate them, you can scan a test and
+ * quickly comprehend what it does.
+ *
+ * <p>It also enforces a certain degree of discipline when you write your tests. You have to think
+ * clearly about the three steps your test will perform. But it makes tests more natural to write at
+ * the same time since you already have an outline.
+ *
+ * <p>In ({@link CashAAATest}) we have four test methods. Each of them has only one reason to
+ * change and one reason to fail. In a large and complicated code base, tests that honor the single
+ * responsibility principle are much easier to troubleshoot.
+ */
+public class CashAAATest {
+
+  @Test
+  public void testPlus() {
+    //Arrange
+    var cash = new Cash(3);
+    //Act
+    cash.plus(4);
+    //Assert
+    assertEquals(7, cash.count());
+  }
+
+  @Test
+  public void testMinus() {
+    //Arrange
+    var cash = new Cash(8);
+    //Act
+    var result = cash.minus(5);
+    //Assert
+    assertTrue(result);
+    assertEquals(3, cash.count());
+  }
+
+  @Test
+  public void testInsufficientMinus() {
+    //Arrange
+    var cash = new Cash(1);
+    //Act
+    var result = cash.minus(6);
+    //Assert
+    assertFalse(result);
+    assertEquals(1, cash.count());
+  }
+
+  @Test
+  public void testUpdate() {
+    //Arrange
+    var cash = new Cash(5);
+    //Act
+    cash.plus(6);
+    var result = cash.minus(3);
+    //Assert
+    assertTrue(result);
+    assertEquals(8, cash.count());
+  }
+}

arrange-act-assert/src/test/java/com/iluwatar/arrangeactassert/CashAntiAAATest.java

+/*
+ * 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.arrangeactassert;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+import org.junit.Test;
+
+/**
+ * ({@link CashAAATest}) is an anti-example of AAA pattern. This test is functionally correct, but
+ * with the addition of new feature, it needs refactoring. There are an awful lot of steps in that
+ * test method, but it verifies the class' important behavior in just eleven lines. It violates the
+ * single responsibility principle. If this test method failed after a small code change, it might
+ * take some digging to discover why.
+ */
+public class CashAntiAAATest {
+
+  @Test
+  public void testCash() {
+    //initialize
+    var cash = new Cash(3);
+    //test plus
+    cash.plus(4);
+    assertEquals(7, cash.count());
+    //test minus
+    cash = new Cash(8);
+    assertTrue(cash.minus(5));
+    assertEquals(3, cash.count());
+    assertFalse(cash.minus(6));
+    assertEquals(3, cash.count());
+    //test update
+    cash.plus(5);
+    assertTrue(cash.minus(5));
+    assertEquals(3, cash.count());
+  }
+}

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();
       }

bridge/README.md

@@ -194,4 +194,5 @@ Use the Bridge pattern when
 
 ## Credits
 
-* [Design Patterns: Elements of Reusable Object-Oriented Software](http://www.amazon.com/Design-Patterns-Elements-Reusable-Object-Oriented/dp/0201633612)
+* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
+* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)

builder/README.md

@@ -131,5 +131,7 @@ Use the Builder pattern when
 
 ## Credits
 
-* [Design Patterns: Elements of Reusable Object-Oriented Software](http://www.amazon.com/Design-Patterns-Elements-Reusable-Object-Oriented/dp/0201633612)
-* [Effective Java (2nd Edition)](http://www.amazon.com/Effective-Java-Edition-Joshua-Bloch/dp/0321356683)
+* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
+* [Effective Java](https://www.amazon.com/gp/product/0134685997/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0134685997&linkCode=as2&tag=javadesignpat-20&linkId=4e349f4b3ff8c50123f8147c828e53eb)
+* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
+* [Refactoring to Patterns](https://www.amazon.com/gp/product/0321213351/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0321213351&linkCode=as2&tag=javadesignpat-20&linkId=2a76fcb387234bc71b1c61150b3cc3a7)

business-delegate/README.md

@@ -26,4 +26,4 @@ Use the Business Delegate pattern when
 
 ## Credits
 
-* [J2EE Design Patterns](http://www.amazon.com/J2EE-Design-Patterns-William-Crawford/dp/0596004273/ref=sr_1_2)
+* [J2EE Design Patterns](https://www.amazon.com/gp/product/0596004273/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596004273&linkCode=as2&tag=javadesignpat-20&linkId=48d37c67fb3d845b802fa9b619ad8f31)

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/README.md

@@ -5,7 +5,8 @@ folder: caching
 permalink: /patterns/caching/
 categories: Behavioral
 tags:
- - Performance
+  - Performance
+  - Cloud distributed
 ---
 
 ## Intent
@@ -25,4 +26,4 @@ Use the Caching pattern(s) when
 
 * [Write-through, write-around, write-back: Cache explained](http://www.computerweekly.com/feature/Write-through-write-around-write-back-Cache-explained)
 * [Read-Through, Write-Through, Write-Behind, and Refresh-Ahead Caching](https://docs.oracle.com/cd/E15357_01/coh.360/e15723/cache_rtwtwbra.htm#COHDG5177)
-* [Cache-Aside](https://msdn.microsoft.com/en-us/library/dn589799.aspx)
+* [Cache-Aside pattern](https://docs.microsoft.com/en-us/azure/architecture/patterns/cache-aside)

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;

callback/README.md

@@ -9,9 +9,64 @@ tags:
 ---
 
 ## Intent
-Callback is a piece of executable code that is passed as an
-argument to other code, which is expected to call back (execute) the argument
-at some convenient time.
+Callback is a piece of executable code that is passed as an argument to other code, which is expected to call back 
+(execute) the argument at some convenient time.
+
+## Explanation
+
+Real world example
+
+> We need to be notified after executing task has finished. We pass a callback method for the executor and wait for it to call back on us.     
+
+In plain words
+
+> Callback is a method passed to the executor which will be called at defined moment. 
+
+Wikipedia says
+
+> In computer programming, a callback, also known as a "call-after" function, is any executable code that is passed as an argument to other code; that other code is expected to call back (execute) the argument at a given time.
+
+**Programmatic Example**
+
+Callback is a simple interface with single method.
+
+```java
+public interface Callback {
+
+  void call();
+}
+```
+
+Next we define a task that will execute the callback after the task execution has finished.
+
+```java
+public abstract class Task {
+
+  final void executeWith(Callback callback) {
+    execute();
+    Optional.ofNullable(callback).ifPresent(Callback::call);
+  }
+
+  public abstract void execute();
+}
+
+public final class SimpleTask extends Task {
+
+  private static final Logger LOGGER = getLogger(SimpleTask.class);
+
+  @Override
+  public void execute() {
+    LOGGER.info("Perform some important activity and after call the callback method.");
+  }
+}
+```
+
+Finally here's how we execute a task and receive a callback when it's finished.
+
+```java
+    var task = new SimpleTask();
+    task.executeWith(() -> LOGGER.info("I'm done now."));
+```
 
 ## Class diagram
 ![alt text](./etc/callback.png "Callback")

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

@@ -33,7 +33,7 @@ public abstract class Task {
   /**
    * Execute with callback.
    */
-  final void executeWith(final Callback callback) {
+  final void executeWith(Callback callback) {
     execute();
     Optional.ofNullable(callback).ifPresent(Callback::call);
   }

callback/src/main/java/com/iluwatar/callback/module-info.java

-/*
- * 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.
- */
-
-module com.iluwatar.callback {
-	requires org.slf4j;
-}
\ No newline at end of file

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;
@@ -157,4 +157,5 @@ Use Chain of Responsibility when
 
 ## Credits
 
-* [Design Patterns: Elements of Reusable Object-Oriented Software](http://www.amazon.com/Design-Patterns-Elements-Reusable-Object-Oriented/dp/0201633612)
+* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
+* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)

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;

chain/src/main/java/com/iluwatar/chain/module-info.java

-/*
- * 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.
- */
-
-module com.iluwatar.chain {
-	requires org.slf4j;
-}
\ No newline at end of file

circuit-breaker/README.md

@@ -5,8 +5,9 @@ folder: circuit-breaker
 permalink: /patterns/circuit-breaker/
 categories: Behavioral
 tags:
- - Performance
- - Decoupling
+  - Performance
+  - Decoupling
+  - Cloud distributed
 ---
 
 ## Intent
@@ -187,4 +188,4 @@ Use the Circuit Breaker pattern when
 * [Understanding Circuit Breaker Pattern](https://itnext.io/understand-circuitbreaker-design-pattern-with-simple-practical-example-92a752615b42)
 * [Martin Fowler on Circuit Breaker](https://martinfowler.com/bliki/CircuitBreaker.html)
 * [Fault tolerance in a high volume, distributed system](https://medium.com/netflix-techblog/fault-tolerance-in-a-high-volume-distributed-system-91ab4faae74a)
-* [Microsoft docs](https://docs.microsoft.com/en-us/azure/architecture/patterns/circuit-breaker)
+* [Circuit Breaker pattern](https://docs.microsoft.com/en-us/azure/architecture/patterns/circuit-breaker)

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/main/java/com/iluwatar/collectionpipeline/module-info.java

-/*
- * 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.
- */
-
-module com.iluwatar.collectionpipeline {
-	requires org.slf4j;
-}
\ No newline at end of file

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

@@ -12,9 +12,214 @@ tags:
 Action, Transaction
 
 ## Intent
-Encapsulate a request as an object, thereby letting you
-parameterize clients with different requests, queue or log requests, and
-support undoable operations.
+Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
+
+## Explanation
+Real world example
+
+> There is a wizard casting spells on a goblin. The spells are executed on the goblin one by one. The first spell shrinks the goblin and the second makes him invisible. Then the wizard reverses the spells one by one. Each spell here is a command object that can be undone.
+
+In plain words
+
+> Storing requests as command objects allows performing an action or undoing it at a later time.
+
+Wikipedia says
+
+> In object-oriented programming, the command pattern is a behavioral design pattern in which an object is used to encapsulate all information needed to perform an action or trigger an event at a later time.
+
+**Programmatic Example**
+
+Here's the sample code with wizard and goblin. Let's start from the wizard class.
+
+```java
+public class Wizard {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(Wizard.class);
+
+  private final Deque<Command> undoStack = new LinkedList<>();
+  private final Deque<Command> redoStack = new LinkedList<>();
+
+  public Wizard() {}
+
+  public void castSpell(Command command, Target target) {
+    LOGGER.info("{} casts {} at {}", this, command, target);
+    command.execute(target);
+    undoStack.offerLast(command);
+  }
+
+  public void undoLastSpell() {
+    if (!undoStack.isEmpty()) {
+      var previousSpell = undoStack.pollLast();
+      redoStack.offerLast(previousSpell);
+      LOGGER.info("{} undoes {}", this, previousSpell);
+      previousSpell.undo();
+    }
+  }
+
+  public void redoLastSpell() {
+    if (!redoStack.isEmpty()) {
+      var previousSpell = redoStack.pollLast();
+      undoStack.offerLast(previousSpell);
+      LOGGER.info("{} redoes {}", this, previousSpell);
+      previousSpell.redo();
+    }
+  }
+
+  @Override
+  public String toString() {
+    return "Wizard";
+  }
+}
+```
+
+Next we present the spell hierarchy.
+
+```java
+public abstract class Command {
+
+  public abstract void execute(Target target);
+
+  public abstract void undo();
+
+  public abstract void redo();
+
+  @Override
+  public abstract String toString();
+}
+
+public class InvisibilitySpell extends Command {
+
+  private Target target;
+
+  @Override
+  public void execute(Target target) {
+    target.setVisibility(Visibility.INVISIBLE);
+    this.target = target;
+  }
+
+  @Override
+  public void undo() {
+    if (target != null) {
+      target.setVisibility(Visibility.VISIBLE);
+    }
+  }
+
+  @Override
+  public void redo() {
+    if (target != null) {
+      target.setVisibility(Visibility.INVISIBLE);
+    }
+  }
+
+  @Override
+  public String toString() {
+    return "Invisibility spell";
+  }
+}
+
+public class ShrinkSpell extends Command {
+
+  private Size oldSize;
+  private Target target;
+
+  @Override
+  public void execute(Target target) {
+    oldSize = target.getSize();
+    target.setSize(Size.SMALL);
+    this.target = target;
+  }
+
+  @Override
+  public void undo() {
+    if (oldSize != null && target != null) {
+      var temp = target.getSize();
+      target.setSize(oldSize);
+      oldSize = temp;
+    }
+  }
+
+  @Override
+  public void redo() {
+    undo();
+  }
+
+  @Override
+  public String toString() {
+    return "Shrink spell";
+  }
+}
+```
+
+And last we have the goblin who's the target of the spells.
+
+```java
+public abstract class Target {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(Target.class);
+
+  private Size size;
+
+  private Visibility visibility;
+
+  public Size getSize() {
+    return size;
+  }
+
+  public void setSize(Size size) {
+    this.size = size;
+  }
+
+  public Visibility getVisibility() {
+    return visibility;
+  }
+
+  public void setVisibility(Visibility visibility) {
+    this.visibility = visibility;
+  }
+
+  @Override
+  public abstract String toString();
+
+  public void printStatus() {
+    LOGGER.info("{}, [size={}] [visibility={}]", this, getSize(), getVisibility());
+  }
+}
+
+public class Goblin extends Target {
+
+  public Goblin() {
+    setSize(Size.NORMAL);
+    setVisibility(Visibility.VISIBLE);
+  }
+
+  @Override
+  public String toString() {
+    return "Goblin";
+  }
+
+}
+```
+
+Finally here's the whole example in action.
+
+```java
+var wizard = new Wizard();
+var goblin = new Goblin();
+goblin.printStatus();
+// Goblin, [size=normal] [visibility=visible]
+wizard.castSpell(new ShrinkSpell(), goblin);
+// Wizard casts Shrink spell at Goblin
+goblin.printStatus();
+// Goblin, [size=small] [visibility=visible]
+wizard.castSpell(new InvisibilitySpell(), goblin);
+// Wizard casts Invisibility spell at Goblin
+goblin.printStatus();
+// Goblin, [size=small] [visibility=invisible]
+wizard.undoLastSpell();
+// Wizard undoes Invisibility spell
+goblin.printStatus();
+// Goblin, [size=small] [visibility=visible]
+```
 
 ## Class diagram
 ![alt text](./etc/command.png "Command")
@@ -43,4 +248,7 @@ Use the Command pattern when you want to
 
 ## Credits
 
-* [Design Patterns: Elements of Reusable Object-Oriented Software](http://www.amazon.com/Design-Patterns-Elements-Reusable-Object-Oriented/dp/0201633612)
+* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
+* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
+* [Refactoring to Patterns](https://www.amazon.com/gp/product/0321213351/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0321213351&linkCode=as2&tag=javadesignpat-20&linkId=2a76fcb387234bc71b1c61150b3cc3a7)
+* [J2EE Design Patterns](https://www.amazon.com/gp/product/0596004273/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596004273&linkCode=as2&tag=javadesignpat-20&linkId=f27d2644fbe5026ea448791a8ad09c94)

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

-/*
- * 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

-/*
- * 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

command/src/main/java/com/iluwatar/command/module-info.java

-/*
- * 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.
- */
-
-module com.iluwatar.command {
-	requires org.slf4j;
-}
\ No newline at end of file

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;
@@ -168,4 +168,6 @@ Use the Composite pattern when
 
 ## Credits
 
-* [Design Patterns: Elements of Reusable Object-Oriented Software](http://www.amazon.com/Design-Patterns-Elements-Reusable-Object-Oriented/dp/0201633612)
+* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
+* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
+* [Refactoring to Patterns](https://www.amazon.com/gp/product/0321213351/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0321213351&linkCode=as2&tag=javadesignpat-20&linkId=2a76fcb387234bc71b1c61150b3cc3a7)

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

-/*
- * 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();
+  }
+}

composite/src/main/java/com/iluwatar/composite/module-info.java

-/*
- * 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.
- */
-
-module com.iluwatar.composite {
-	requires org.slf4j;
-}
\ No newline at end of file

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/main/java/com/iluwatar/converter/module-info.java

-/*
- * 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.
- */
-
-module com.iluwatar.converter {
-	requires org.slf4j;
-}
\ No newline at end of file

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/README.md

@@ -5,8 +5,8 @@ folder: cqrs
 permalink: /patterns/cqrs/
 categories: Architectural
 tags:
- - Performance
- - Cloud distributed
+  - Performance
+  - Cloud distributed
 ---
 
 ## Intent
@@ -18,12 +18,13 @@ CQRS Command Query Responsibility Segregation - Separate the query side from the
 ## Applicability
 Use the CQRS pattern when
 
-* you want to scale the queries and commands independently.
-* you want to use different data models for queries and commands. Useful when dealing with complex domains.
-* you want to use architectures like event sourcing or task based UI.
+* You want to scale the queries and commands independently.
+* You want to use different data models for queries and commands. Useful when dealing with complex domains.
+* You want to use architectures like event sourcing or task based UI.
 
 ## Credits
 
 * [Greg Young - CQRS, Task Based UIs, Event Sourcing agh!](http://codebetter.com/gregyoung/2010/02/16/cqrs-task-based-uis-event-sourcing-agh/)
 * [Martin Fowler - CQRS](https://martinfowler.com/bliki/CQRS.html)
 * [Oliver Wolf - CQRS for Great Good](https://www.youtube.com/watch?v=Ge53swja9Dw)
+* [Command and Query Responsibility Segregation (CQRS) pattern](https://docs.microsoft.com/en-us/azure/architecture/patterns/cqrs)

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

@@ -9,8 +9,298 @@ tags:
 ---
 
 ## Intent
-Object provides an abstract interface to some type of database or
-other persistence mechanism.
+Object provides an abstract interface to some type of database or other persistence mechanism.
+
+## Explanation
+
+Real world example
+
+> There's a set of customers that need to be persisted to database. Additionally we need the whole set of CRUD (create/read/update/delete) operations so we can operate on customers easily. 
+
+In plain words
+
+> DAO is an interface we provide over the base persistence mechanism. 
+
+Wikipedia says
+
+> In computer software, a data access object (DAO) is a pattern that provides an abstract interface to some type of database or other persistence mechanism.
+
+**Programmatic Example**
+
+Walking through our customers example, here's the basic Customer entity.
+
+```java
+public class Customer {
+
+  private int id;
+  private String firstName;
+  private String lastName;
+
+  public Customer(int id, String firstName, String lastName) {
+    this.id = id;
+    this.firstName = firstName;
+    this.lastName = lastName;
+  }
+
+  public int getId() {
+    return id;
+  }
+
+  public void setId(final int id) {
+    this.id = id;
+  }
+
+  public String getFirstName() {
+    return firstName;
+  }
+
+  public void setFirstName(final String firstName) {
+    this.firstName = firstName;
+  }
+
+  public String getLastName() {
+    return lastName;
+  }
+
+  public void setLastName(final String lastName) {
+    this.lastName = lastName;
+  }
+
+  @Override
+  public String toString() {
+    return "Customer{" + "id=" + getId() + ", firstName='" + getFirstName() + '\'' + ", lastName='"
+        + getLastName() + '\'' + '}';
+  }
+
+  @Override
+  public boolean equals(final Object that) {
+    var isEqual = false;
+    if (this == that) {
+      isEqual = true;
+    } else if (that != null && getClass() == that.getClass()) {
+      final var customer = (Customer) that;
+      if (getId() == customer.getId()) {
+        isEqual = true;
+      }
+    }
+    return isEqual;
+  }
+
+  @Override
+  public int hashCode() {
+    return getId();
+  }
+}
+```
+
+Here's the DAO interface and two different implementations for it. InMemoryCustomerDao keeps a simple map of customers 
+in memory while DBCustomerDao is the real RDBMS implementation.
+
+```java
+public interface CustomerDao {
+
+  Stream<Customer> getAll() throws Exception;
+
+  Optional<Customer> getById(int id) throws Exception;
+
+  boolean add(Customer customer) throws Exception;
+
+  boolean update(Customer customer) throws Exception;
+
+  boolean delete(Customer customer) throws Exception;
+}
+
+public class InMemoryCustomerDao implements CustomerDao {
+
+  private final Map<Integer, Customer> idToCustomer = new HashMap<>();
+
+  @Override
+  public Stream<Customer> getAll() {
+    return idToCustomer.values().stream();
+  }
+
+  @Override
+  public Optional<Customer> getById(final int id) {
+    return Optional.ofNullable(idToCustomer.get(id));
+  }
+
+  @Override
+  public boolean add(final Customer customer) {
+    if (getById(customer.getId()).isPresent()) {
+      return false;
+    }
+
+    idToCustomer.put(customer.getId(), customer);
+    return true;
+  }
+
+  @Override
+  public boolean update(final Customer customer) {
+    return idToCustomer.replace(customer.getId(), customer) != null;
+  }
+
+  @Override
+  public boolean delete(final Customer customer) {
+    return idToCustomer.remove(customer.getId()) != null;
+  }
+}
+
+public class DbCustomerDao implements CustomerDao {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(DbCustomerDao.class);
+
+  private final DataSource dataSource;
+
+  public DbCustomerDao(DataSource dataSource) {
+    this.dataSource = dataSource;
+  }
+
+  @Override
+  public Stream<Customer> getAll() throws Exception {
+    try {
+      var connection = getConnection();
+      var statement = connection.prepareStatement("SELECT * FROM CUSTOMERS");
+      var resultSet = statement.executeQuery();
+      return StreamSupport.stream(new Spliterators.AbstractSpliterator<Customer>(Long.MAX_VALUE,
+          Spliterator.ORDERED) {
+
+        @Override
+        public boolean tryAdvance(Consumer<? super Customer> action) {
+          try {
+            if (!resultSet.next()) {
+              return false;
+            }
+            action.accept(createCustomer(resultSet));
+            return true;
+          } catch (SQLException e) {
+            throw new RuntimeException(e);
+          }
+        }
+      }, false).onClose(() -> mutedClose(connection, statement, resultSet));
+    } catch (SQLException e) {
+      throw new CustomException(e.getMessage(), e);
+    }
+  }
+
+  private Connection getConnection() throws SQLException {
+    return dataSource.getConnection();
+  }
+
+  private void mutedClose(Connection connection, PreparedStatement statement, ResultSet resultSet) {
+    try {
+      resultSet.close();
+      statement.close();
+      connection.close();
+    } catch (SQLException e) {
+      LOGGER.info("Exception thrown " + e.getMessage());
+    }
+  }
+
+  private Customer createCustomer(ResultSet resultSet) throws SQLException {
+    return new Customer(resultSet.getInt("ID"),
+        resultSet.getString("FNAME"),
+        resultSet.getString("LNAME"));
+  }
+
+  @Override
+  public Optional<Customer> getById(int id) throws Exception {
+
+    ResultSet resultSet = null;
+
+    try (var connection = getConnection();
+         var statement = connection.prepareStatement("SELECT * FROM CUSTOMERS WHERE ID = ?")) {
+
+      statement.setInt(1, id);
+      resultSet = statement.executeQuery();
+      if (resultSet.next()) {
+        return Optional.of(createCustomer(resultSet));
+      } else {
+        return Optional.empty();
+      }
+    } catch (SQLException ex) {
+      throw new CustomException(ex.getMessage(), ex);
+    } finally {
+      if (resultSet != null) {
+        resultSet.close();
+      }
+    }
+  }
+
+  @Override
+  public boolean add(Customer customer) throws Exception {
+    if (getById(customer.getId()).isPresent()) {
+      return false;
+    }
+
+    try (var connection = getConnection();
+         var statement = connection.prepareStatement("INSERT INTO CUSTOMERS VALUES (?,?,?)")) {
+      statement.setInt(1, customer.getId());
+      statement.setString(2, customer.getFirstName());
+      statement.setString(3, customer.getLastName());
+      statement.execute();
+      return true;
+    } catch (SQLException ex) {
+      throw new CustomException(ex.getMessage(), ex);
+    }
+  }
+
+  @Override
+  public boolean update(Customer customer) throws Exception {
+    try (var connection = getConnection();
+         var statement =
+             connection
+                 .prepareStatement("UPDATE CUSTOMERS SET FNAME = ?, LNAME = ? WHERE ID = ?")) {
+      statement.setString(1, customer.getFirstName());
+      statement.setString(2, customer.getLastName());
+      statement.setInt(3, customer.getId());
+      return statement.executeUpdate() > 0;
+    } catch (SQLException ex) {
+      throw new CustomException(ex.getMessage(), ex);
+    }
+  }
+
+  @Override
+  public boolean delete(Customer customer) throws Exception {
+    try (var connection = getConnection();
+         var statement = connection.prepareStatement("DELETE FROM CUSTOMERS WHERE ID = ?")) {
+      statement.setInt(1, customer.getId());
+      return statement.executeUpdate() > 0;
+    } catch (SQLException ex) {
+      throw new CustomException(ex.getMessage(), ex);
+    }
+  }
+}
+```
+
+Finally here's how we use our DAO to manage customers.
+
+```java
+    final var dataSource = createDataSource();
+    createSchema(dataSource);
+    final var customerDao = new DbCustomerDao(dataSource);
+    
+    addCustomers(customerDao);
+    log.info(ALL_CUSTOMERS);
+    try (var customerStream = customerDao.getAll()) {
+      customerStream.forEach((customer) -> log.info(customer.toString()));
+    }
+    log.info("customerDao.getCustomerById(2): " + customerDao.getById(2));
+    final var customer = new Customer(4, "Dan", "Danson");
+    customerDao.add(customer);
+    log.info(ALL_CUSTOMERS + customerDao.getAll());
+    customer.setFirstName("Daniel");
+    customer.setLastName("Danielson");
+    customerDao.update(customer);
+    log.info(ALL_CUSTOMERS);
+    try (var customerStream = customerDao.getAll()) {
+      customerStream.forEach((cust) -> log.info(cust.toString()));
+    }
+    customerDao.delete(customer);
+    log.info(ALL_CUSTOMERS + customerDao.getAll());
+    
+    deleteSchema(dataSource);
+```
+
 
 ## Class diagram
 ![alt text](./etc/dao.png "Data Access Object")
@@ -23,4 +313,4 @@ Use the Data Access Object in any of the following situations
 
 ## Credits
 
-* [J2EE Design Patterns](http://www.amazon.com/J2EE-Design-Patterns-William-Crawford/dp/0596004273/ref=sr_1_2)
+* [J2EE Design Patterns](https://www.amazon.com/gp/product/0596004273/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596004273&linkCode=as2&tag=javadesignpat-20&linkId=48d37c67fb3d845b802fa9b619ad8f31)

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/Customer.java

@@ -35,7 +35,7 @@ public class Customer {
   /**
    * Creates an instance of customer.
    */
-  public Customer(final int id, final String firstName, final String lastName) {
+  public Customer(int id, String firstName, String lastName) {
     this.id = id;
     this.firstName = firstName;
     this.lastName = lastName;

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/main/java/com/iluwatar/dao/module-info.java

-/*
- * 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.
- */
-
-module com.iluwatar.dao {
-	requires org.slf4j;
-	requires java.sql;
-	requires h2;
-	requires java.naming;
-}
\ No newline at end of file

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

-/*
- * 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-mapper/src/main/java/com/iluwatar/datamapper/module-info.java

-/*
- * 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.
- */
-
-module com.iluwatar.datamapper {
-	requires org.slf4j;
-}
\ No newline at end of file

data-transfer-object/README.md

@@ -9,8 +9,89 @@ tags:
 ---
 
 ## Intent
-Pass data with multiple attributes in one shot from client to server,
-to avoid multiple calls to remote server. 
+Pass data with multiple attributes in one shot from client to server, to avoid multiple calls to remote server. 
+
+## Explanation
+
+Real world example
+
+> We need to fetch information about customers from remote database. Instead of querying the attributes one at a time, we use DTOs to transfer all the relevant attributes in a single shot.     
+
+In plain words
+
+> Using DTO relevant information can be fetched with a single backend query. 
+
+Wikipedia says
+
+> In the field of programming a data transfer object (DTO) is an object that carries data between processes. The 
+motivation for its use is that communication between processes is usually done resorting to remote interfaces 
+(e.g., web services), where each call is an expensive operation. Because the majority of the cost of each call is 
+related to the round-trip time between the client and the server, one way of reducing the number of calls is to use an 
+object (the DTO) that aggregates the data that would have been transferred by the several calls, but that is served by 
+one call only.
+
+**Programmatic Example**
+
+Let's first introduce our simple customer DTO class.
+
+```java
+public class CustomerDto {
+  private final String id;
+  private final String firstName;
+  private final String lastName;
+
+  public CustomerDto(String id, String firstName, String lastName) {
+    this.id = id;
+    this.firstName = firstName;
+    this.lastName = lastName;
+  }
+
+  public String getId() {
+    return id;
+  }
+
+  public String getFirstName() {
+    return firstName;
+  }
+
+  public String getLastName() {
+    return lastName;
+  }
+}
+```
+
+Customer resource class acts as the server for customer information.
+
+```java
+public class CustomerResource {
+  private final List<CustomerDto> customers;
+
+  public CustomerResource(List<CustomerDto> customers) {
+    this.customers = customers;
+  }
+
+  public List<CustomerDto> getAllCustomers() {
+    return customers;
+  }
+
+  public void save(CustomerDto customer) {
+    customers.add(customer);
+  }
+
+  public void delete(String customerId) {
+    customers.removeIf(customer -> customer.getId().equals(customerId));
+  }
+}
+```
+
+Now fetching customer information is easy since we have the DTOs.
+
+```java
+    var allCustomers = customerResource.getAllCustomers();
+    allCustomers.forEach(customer -> LOGGER.info(customer.getFirstName()));
+    // Kelly
+    // Alfonso
+```
 
 ## Class diagram
 ![alt text](./etc/data-transfer-object.urm.png "data-transfer-object")
@@ -26,3 +107,5 @@ Use the Data Transfer Object pattern when
 
 * [Design Pattern - Transfer Object Pattern](https://www.tutorialspoint.com/design_pattern/transfer_object_pattern.htm)
 * [Data Transfer Object](https://msdn.microsoft.com/en-us/library/ff649585.aspx)
+* [J2EE Design Patterns](https://www.amazon.com/gp/product/0596004273/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596004273&linkCode=as2&tag=javadesignpat-20&linkId=f27d2644fbe5026ea448791a8ad09c94)
+* [Patterns of Enterprise Application Architecture](https://www.amazon.com/gp/product/0321127420/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0321127420&linkCode=as2&tag=javadesignpat-20&linkId=014237a67c9d46f384b35e10151956bd)

data-transfer-object/etc/data-transfer-object.ucls

 <?xml version="1.0" encoding="UTF-8"?>
 <class-diagram version="1.2.0" icons="true" always-add-relationships="false" generalizations="true" realizations="true" 
   associations="true" dependencies="false" nesting-relationships="true" router="FAN">  
-  <class id="1" language="java" name="com.iluwatar.datatransfer.CustomerClientApp" project="data-transfer-object" 
+  <class id="1" language="java" name="com.iluwatar.datatransfer.App" project="data-transfer-object"
     file="/data-transfer-object/src/main/java/com/iluwatar/datatransfer/CustomerClientApp.java" binary="false" 
     corner="BOTTOM_RIGHT">    
     <position height="-1" width="-1" x="145" y="93"/>    

data-transfer-object/pom.xml

@@ -48,7 +48,7 @@
                         <configuration>
                             <archive>
                                 <manifest>
-                                    <mainClass>com.iluwatar.datatransfer.CustomerClientApp</mainClass>
+                                    <mainClass>com.iluwatar.datatransfer.App</mainClass>
                                 </manifest>
                             </archive>
                         </configuration>

data-transfer-object/src/main/java/com/iluwatar/datatransfer/CustomerClientApp.java → data-transfer-object/src/main/java/com/iluwatar/datatransfer/App.java

@@ -32,15 +32,15 @@ import org.slf4j.LoggerFactory;
  * The Data Transfer Object pattern is a design pattern in which an data transfer object is used to
  * serve related information together to avoid multiple call for each piece of information.
  *
- * <p>In this example, ({@link CustomerClientApp}) as as customer details consumer i.e. client to
+ * <p>In this example, ({@link App}) as as customer details consumer i.e. client to
  * request for customer details to server.
  *
  * <p>CustomerResource ({@link CustomerResource}) act as server to serve customer information. And
  * The CustomerDto ({@link CustomerDto} is data transfer object to share customer information.
  */
-public class CustomerClientApp {
+public class App {
 
-  private static final Logger LOGGER = LoggerFactory.getLogger(CustomerClientApp.class);
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
 
   /**
    * Method as act client and request to server for details.

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.

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

-/*
- * 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.
- */
-
-module com.iluwatar.datatransfer {
-	requires org.slf4j;
-}
\ No newline at end of file

abstract-document/src/main/java/com/iluwatar/abstractdocument/module-info.java → data-transfer-object/src/test/java/com/iluwatar/datatransfer/AppTest.java

@@ -21,6 +21,13 @@
  * THE SOFTWARE.
  */
 
-module com.iluwatar.abstractdocument {
-	requires org.slf4j;
-}
\ No newline at end of file
+package com.iluwatar.datatransfer;
+
+import org.junit.jupiter.api.Test;
+
+public class AppTest {
+    @Test
+    public void test() throws Exception {
+        App.main(new String[]{});
+    }
+}

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;
@@ -131,6 +131,9 @@ Use Decorator
 
 ## Credits
 
-* [Design Patterns: Elements of Reusable Object-Oriented Software](http://www.amazon.com/Design-Patterns-Elements-Reusable-Object-Oriented/dp/0201633612)
-* [Functional Programming in Java: Harnessing the Power of Java 8 Lambda Expressions](http://www.amazon.com/Functional-Programming-Java-Harnessing-Expressions/dp/1937785467/ref=sr_1_1)
-* [J2EE Design Patterns](http://www.amazon.com/J2EE-Design-Patterns-William-Crawford/dp/0596004273/ref=sr_1_2)
+* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
+* [Functional Programming in Java: Harnessing the Power of Java 8 Lambda Expressions](https://www.amazon.com/gp/product/1937785467/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1937785467&linkCode=as2&tag=javadesignpat-20&linkId=7e4e2fb7a141631491534255252fd08b)
+* [J2EE Design Patterns](https://www.amazon.com/gp/product/0596004273/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596004273&linkCode=as2&tag=javadesignpat-20&linkId=48d37c67fb3d845b802fa9b619ad8f31)
+* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
+* [Refactoring to Patterns](https://www.amazon.com/gp/product/0321213351/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0321213351&linkCode=as2&tag=javadesignpat-20&linkId=2a76fcb387234bc71b1c61150b3cc3a7)
+* [J2EE Design Patterns](https://www.amazon.com/gp/product/0596004273/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596004273&linkCode=as2&tag=javadesignpat-20&linkId=f27d2644fbe5026ea448791a8ad09c94)

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/main/java/com/iluwatar/decorator/module-info.java

-/*
- * 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.
- */
-
-module com.iluwatar.decorator {
-	requires org.slf4j;
-}
\ No newline at end of file

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/main/java/com/iluwatar/delegation/module-info.java

-/*
- * 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.
- */
-
-module com.iluwatar.delegation {
-	requires org.slf4j;
-}
\ No newline at end of file

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

@@ -9,12 +9,78 @@ tags:
 ---
 
 ## Intent
-Dependency Injection is a software design pattern in which one or
-more dependencies (or services) are injected, or passed by reference, into a
-dependent object (or client) and are made part of the client's state. The
-pattern separates the creation of a client's dependencies from its own
-behavior, which allows program designs to be loosely coupled and to follow the
-inversion of control and single responsibility principles.
+Dependency Injection is a software design pattern in which one or more dependencies (or services) are injected, or 
+passed by reference, into a dependent object (or client) and are made part of the client's state. The pattern separates 
+the creation of a client's dependencies from its own behavior, which allows program designs to be loosely coupled and 
+to follow the inversion of control and single responsibility principles.
+
+## Explanation
+Real world example
+
+> The old wizard likes to fill his pipe and smoke tobacco once in a while. However, he doesn't want to depend on a single tobacco brand only but likes to be able to enjoy them all interchangeably.    
+
+In plain words
+
+> Dependency Injection separates creation of client's dependencies from its own behavior. 
+
+Wikipedia says
+
+> In software engineering, dependency injection is a technique in which an object receives other objects that it depends on. These other objects are called dependencies.
+
+**Programmatic Example**
+
+Let's first introduce the tobacco brands.
+
+```java
+public abstract class Tobacco {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(Tobacco.class);
+
+  public void smoke(Wizard wizard) {
+    LOGGER.info("{} smoking {}", wizard.getClass().getSimpleName(),
+        this.getClass().getSimpleName());
+  }
+}
+
+public class SecondBreakfastTobacco extends Tobacco {
+}
+
+public class RivendellTobacco extends Tobacco {
+}
+
+public class OldTobyTobacco extends Tobacco {
+}
+```
+
+Next here's the wizard class hierarchy.
+
+```java
+public interface Wizard {
+
+  void smoke();
+}
+
+public class AdvancedWizard implements Wizard {
+
+  private final Tobacco tobacco;
+
+  public AdvancedWizard(Tobacco tobacco) {
+    this.tobacco = tobacco;
+  }
+
+  @Override
+  public void smoke() {
+    tobacco.smoke(this);
+  }
+}
+```
+
+And lastly we can show how easy it is to give the old wizard any brand of tobacco.
+
+```java
+    var advancedWizard = new AdvancedWizard(new SecondBreakfastTobacco());
+    advancedWizard.smoke();
+```
 
 ## Class diagram
 ![alt text](./etc/dependency-injection.png "Dependency Injection")
@@ -22,5 +88,12 @@ inversion of control and single responsibility principles.
 ## Applicability
 Use the Dependency Injection pattern when
 
-* when you need to remove knowledge of concrete implementation from object
-* to enable unit testing of classes in isolation using mock objects or stubs
+* When you need to remove knowledge of concrete implementation from object
+* To enable unit testing of classes in isolation using mock objects or stubs
+
+## Credits
+
+* [Dependency Injection Principles, Practices, and Patterns](https://www.amazon.com/gp/product/161729473X/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=161729473X&linkId=57079257a5c7d33755493802f3b884bd)
+* [Clean Code: A Handbook of Agile Software Craftsmanship](https://www.amazon.com/gp/product/0132350882/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0132350882&linkCode=as2&tag=javadesignpat-20&linkId=2c390d89cc9e61c01b9e7005c7842871)
+* [Java 9 Dependency Injection: Write loosely coupled code with Spring 5 and Guice](https://www.amazon.com/gp/product/1788296257/ref=as_li_tl?ie=UTF8&tag=javadesignpat-20&camp=1789&creative=9325&linkCode=as2&creativeASIN=1788296257&linkId=4e9137a3bf722a8b5b156cce1eec0fc1)
+* [Google Guice Tutorial: Open source Java based dependency injection framework](https://www.amazon.com/gp/product/B083P7DZ8M/ref=as_li_tl?ie=UTF8&tag=javadesignpat-20&camp=1789&creative=9325&linkCode=as2&creativeASIN=B083P7DZ8M&linkId=04f0f902c877921e45215b624a124bfe)

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/README.md

@@ -9,6 +9,9 @@ tags:
  - Performance
 ---
 
+## Also known as
+* IsDirty pattern
+
 ## Intent
 To avoid expensive re-acquisition of resources. The resources retain their identity, are kept in some
 fast-access storage, and are re-used to avoid having to acquire them again.
@@ -24,3 +27,4 @@ Use the Dirty Flag pattern when
 ## Credits
 
 * [Design Patterns: Dirty Flag](https://www.takeupcode.com/podcast/89-design-patterns-dirty-flag/)
+* [J2EE Design Patterns](https://www.amazon.com/gp/product/0596004273/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596004273&linkCode=as2&tag=javadesignpat-20&linkId=48d37c67fb3d845b802fa9b619ad8f31)

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>();

dirty-flag/src/main/java/com/iluwatar/dirtyflag/module-info.java

-/*
- * 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.
- */
-
-module com.iluwatar.dirtyflag {
-	requires org.slf4j;
-}
\ No newline at end of file