Design Pattern: Observer Pattern

Chapters

• Observer Pattern

Observer Pattern

Observer pattern is a design pattern that consists of observers and a subject. Subject is an entity that is being observed by observers. Observer is an entity that observes the Subject. Most event handling systems implement observer pattern.

This example demonstrates observer pattern.

import java.util.ArrayList;

public class ClientCode{

  public static void main(String[] args){
    Subject playerOne = new Controller("Player One");
    Subject playerTwo = new Controller("Player Two");
    Subject.Observer input = new InputManager();
    
    playerOne.addObserver(input);
    playerTwo.addObserver(input);
    
    playerOne.moveUp();
    playerTwo.moveDown();
  }
}

abstract class Subject{
  private String name;
  private ArrayList<Observer> obsList = 
  new ArrayList<>();
  
  Subject(String name){
    this.name = name;
  }
  
  public void addObserver(Observer obs){
    obsList.add(obs);
  }
  
  public void removeObserver(Observer obs){
    obsList.remove(obs);
  }
  
  private void notifyObservers(String name, String event){
    obsList.
    stream().
    forEach(obs -> obs.update(name, event));
  }
  
  public static abstract class Observer{

    protected abstract void update(String name, String event);
  }
  
  public void moveUp(){
    notifyObservers(name, "up");
  }
  
  public void moveDown(){
    notifyObservers(name, "down");
  }
  
}

class Controller extends Subject{
  
  Controller(String name){
    super(name);
  }
}

class InputManager extends Subject.Observer{
  
  @Override
  protected void update(String name, String event){
    System.out.println
    (name + " pressed " + event + " button.");
    System.out.println
    ("InputManager updates the monitor screen and"+ 
    " moves its character " + event + ".");
  }
}

Result
Player One pressed up button.
InputManager updates the monitor screen and moves its character up.
Player Two pressed down button.
InputManager updates the monitor screen and moves its character down.

In the example above, observer and subject are still tightly coupled. The observer design above is alright but if you want to decouple the two further, you may wanna use publish-subscribe pattern where publishers are subjects and subscribers are observers.

Unlike in the example above, publish-subscribe pattern doesn't send a message directly to observers. It instead sends the message to an entity and that entity sends the message to observers. In java, Flow API implements publish-subscribe pattern. I discussed the API in this article.

If your observers are being notified in quick succession, you may want to implement a timer in order to improve the performance of your application. One example that this scenario may happen is redrawing a GUI window. Everytime we redraw the window on our screen, observers that observe the redrawing process will be notified.

Design Pattern: Memento Pattern

Chapters

• Memento Pattern

Memento Pattern

Memento pattern is a design pattern that exposes the private internal state of an object. One example of how this can be used is to restore an object to its previous state (undo via rollback), another is versioning, another is custom serialization.

This pattern consists of three parts: Originator, Caretaker and Memento. Originator is an object with an internal state. Caretaker is an object that retrieves states from the originator and handles them. Memento is an object that contains a state of Originator. Take note that when implementing this pattern, the Originator must be the only one that can retrieve the state in a Memento.

This diagram shows how to implement a memento pattern

Courtesy of Wikipedia

This example demonstrates memento pattern.

import java.util.ArrayDeque;

public class ClientCode{

  public static void main(String[] args){
    Caretaker stateManager = 
    new Caretaker("A");
    
    System.out.println("State: " + 
    stateManager.getValue());
    stateManager.saveState();
    
    stateManager.append("B");
    stateManager.saveState();
    System.out.println("State: " + 
    stateManager.getValue());
    
    stateManager.append("C");
    stateManager.saveState();
    System.out.println("State: " + 
    stateManager.getValue());
    System.out.println();
    
    System.out.println("Undo...");
    stateManager.loadState();
    System.out.println("State: " + 
    stateManager.getValue());
    
    stateManager.loadState();
    System.out.println("State: " + 
    stateManager.getValue());
    
    stateManager.loadState();
    System.out.println("State: " + 
    stateManager.getValue());
  }
}

//Originator can have other members
//that is not part of the state of
//this class
class Originator{
  private StringBuilder state;
  
  Originator(String value){
    state = new StringBuilder(value);
  }
  
  void append(String value){
    state.append(value);
  }
  
  String getValue(){
    return state.toString();
  }
  
  Memento saveState(){
    return new Memento(
    new StringBuilder(state.toString()));
  }
  
  void loadState(Memento memento){
    state = memento.getSavedState();
  }
  
  public static class Memento{
  private final StringBuilder savedState;
    
    Memento(StringBuilder state){
      savedState = state;
    }
    
    //Making this private ensures that
    //only the Originater can retrieve
    //the state in a memento
    private StringBuilder getSavedState(){
      return savedState;
    }
  }
  
}

class Caretaker{
  private Originator originator;
  private ArrayDeque<Originator.Memento> states;
  
  public Caretaker(String value){
    originator = new Originator(value);
    states = new ArrayDeque<>();
  }
  
  public void append(String value){
    originator.append(value);
  }
  
  public String getValue(){
    return originator.getValue();
  }
  
  public void saveState(){
    states.addFirst(originator.saveState());
  }
  
  public void loadState(){
    if(states.isEmpty()){
      System.out.println("No states!");
      return;
    }
    originator.loadState(states.removeFirst());
  }
  
}

Result
State: A
State: AB
State: ABC

Undo...
State: ABC
State: AB
State: A

Design Pattern: Mediator Pattern

Chapters

• Mediator Pattern

Mediator Pattern

Mediator Pattern is a design pattern that encapsulates interactions between objects. This pattern promotes loose coupling between objects and their interactions. Thus, making our code more flexible and maintainable.

This example demonstrates mediator pattern.

public class ClientCode{

  public static void main(String[] args){
    BookShelf bookShelf1 = 
    new RoomBookShelf(new String[]{"1", "2", "3"});
    BookShelf bookShelf2 = 
    new RoomBookShelf(new String[]{"A", "B", "C"});
    
    Mediator mediator = 
    new BookShelfInteractions(bookShelf1, bookShelf2);
    
    System.out.println("Shelf1: " + bookShelf1.getBook(0));
    System.out.println("Shelf2: " + bookShelf2.getBook(2));
    mediator.swapBooks("1", "C");
    System.out.println("After Swap...");
    System.out.println("Shelf1: " + bookShelf1.getBook(0));
    System.out.println("Shelf2: " + bookShelf2.getBook(2));
  }
}

/*
Assume classes below are in different package
*/

interface Mediator{
  void swapBooks(String bookInShelf, 
                 String bookInAnotherShelf);
}

class BookShelfInteractions implements Mediator{
  private BookShelf bookShelf1, bookShelf2;
  
  BookShelfInteractions(BookShelf bookShelf1,
                        BookShelf bookShelf2){
    this.bookShelf1 = bookShelf1;
    this.bookShelf2 = bookShelf2;
  }
  
  @Override
  public void swapBooks(String bookInShelf1, 
                        String bookInShelf2){
    boolean bookIsInShelf1 = false;
    boolean bookIsInShelf2 = false;
    
    String[] shelf1 = 
    bookShelf1.getBookShelf();
    String[] shelf2 = 
    bookShelf2.getBookShelf();
    
    int bookShelf1Index = 0;
    int bookShelf2Index = 0;
    for(int i = 0; i < shelf1.length; i++)
      if(shelf1[i].equals(bookInShelf1)){
        bookIsInShelf1 = true;
        bookShelf1Index = i;
        break;
      }
    if(!bookIsInShelf1){
      System.out.println
      ("Book " + bookInShelf1 + 
       "Doesn't exist!");
       return;
    }
    
    for(int i = 0; i < shelf2.length; i++)
      if(shelf2[i].equals(bookInShelf2)){
        bookIsInShelf2 = true;
        bookShelf2Index = i;
        break;
      }
    if(!bookIsInShelf2){
      System.out.println
      ("Book " + bookInShelf2 + 
       "Doesn't exist!");
       return;
    }
    
    String tempShelf = shelf2[bookShelf2Index];
    shelf2[bookShelf2Index] = 
    shelf1[bookShelf1Index];
    shelf1[bookShelf1Index] = tempShelf;
    System.out.println("Books have been swapped!");
  }
  
}

abstract class BookShelf{
  private String[] books;
  
  BookShelf(String[] books){
    this.books = books;
  }
  
  public String getBook(int index){
    if(index < 0 || index >= books.length)
      throw new ArrayIndexOutOfBoundsException();
    
    return books[index];
  }
  
  String[] getBookShelf(){
    return books;
  }
}

class RoomBookShelf extends BookShelf{
  
  RoomBookShelf(String[] books){
    super(books);
  }
  
}

Result
Shelf1: 1
Shelf2: C
Books have been swapped!
After Swap...
Shelf1: C
Shelf2: 1

Design Pattern: Iterator Pattern

Chapters

• Iterator Pattern

Iterator Pattern

Iterator pattern is a design pattern used to access and traverse a collection such as a list. This pattern decouples algorithms from containers.

Some programming languages have built-in iterator in them. Those built-in and general-purpose iterators are can solve most problems and I recommend using them. For example, java provides Iterator interface that is used to traverse collections such as ArrayList and others.

This diagram shows a structure of an iterator pattern

Courtesy of Wikipedia

This example demonstrates iterator pattern. Take note that this example is just a mere demonstration and not recommended to be reproduced in production.

import java.util.List;
import java.util.ArrayList;

public class ClientCode{

  public static void main(String[] args){
    Aggregate collection = 
    new ConcreteAggregate();
    
    collection.add("A");
    collection.add("B");
    collection.add("C");
    collection.add("D");
    collection.add("E");
    
    SampleIterator iterator =
    collection.createIterator();
    
    while(iterator.hasNext())
      System.out.println(iterator.next());
  }
}

/*
Assume classes below are in different package
and they're all public except for 
ConcreteIterator class
*/
interface Aggregate{

  void add(String element);
  SampleIterator createIterator();
}

class ConcreteAggregate implements Aggregate{
  private List<String> list;
  private SampleIterator iterator;
  
  ConcreteAggregate(){
    list = new ArrayList<>();
  }
  
  @Override
  public void add(String element){
    list.add(element);
  }
  
  @Override
  public SampleIterator createIterator(){
    return new ConcreteIterator(list);
  }
}

interface SampleIterator{

  String next();
  boolean hasNext();
}

class ConcreteIterator implements SampleIterator{
  private List<String> list;
  private int pointer;
  
  ConcreteIterator(List<String> list){
    this.list = list;
  }
  
  @Override
  public String next(){
    if(pointer >= list.size())
      throw new ArrayIndexOutOfBoundsException();
    String result = list.get(pointer);
    pointer++;
    return result;
  }
  
  @Override
  public boolean hasNext(){
    if(pointer >= list.size())
      return false;
    else
      return true;
  }
  
}

Result
A
B
C
D
E

Design Pattern: Command Pattern

Chapters

• Command Pattern

Command Pattern

Command pattern is a design pattern that wraps an object (receiver) to another object (command) with necessary information that can be processed by a handler (invoker). Command pattern consists of four entities: Client, Command, Receiver and Invoker.

Client is the one that uses our code. Could be a programmer or class. Command are classes that instantiate command objects. Command objects are objects that contain a receiver object and necessary information, sucn as function (instruction) to be called and variables, that is needed by an ivoker in order to perform requests that clients want.

Receiver are classes that instantiate receiver objects. Receiver objects are objects that are receiving commands. Invoker are classes that instantiate invoker objects. These objects contain commands that are executed by them.

This pattern promotes loose coupling between commands and handlers or executors. It means that commands and handlers don't need to be tighly coupled in order to function properly. Thus, increasing the flexibility of our code. Moreover, command pattern is often used in conjunction with chain-of-responsibility pattern.

This diagram shows how to implement a command pattern

Courtesy of Wikipedia

This example demonstrates command pattern.

//Client
public class ClientCode{
  
  public static void main(String[] args){
    
    //Receiver instance
    Controller computerController = 
    new ComputerController();
    Controller consoleController = 
    new ConsoleController();
   
    //command instance
    Command moveUp = 
    new MoveCommand(computerController,
                    Controller.DirectMove.UP);
    Command moveTopLeft = 
    new MoveDiagonalCommand(
    computerController,
    Controller.DiagonalMove.TOP_LEFT);
    
    //Invoker instance
    MoveInput controllerInput = 
    new MoveInput(moveUp, moveTopLeft);  
    controllerInput.move();
    controllerInput.moveDiagonally();
    System.out.println();  
    
    //Command instance
    Command moveDown = 
    new MoveCommand(consoleController,
                    Controller.DirectMove.DOWN);
    Command moveBotRight = 
    new MoveDiagonalCommand(
    consoleController,
    Controller.DiagonalMove.BOTTOM_RIGHT);
    
    MoveInput consoleInput = 
    new MoveInput(moveDown, moveBotRight);
    consoleInput.move();
    consoleInput.moveDiagonally();
  
  }
}

//Receiver interface
interface Controller{
  public enum DirectMove{
    UP, RIGHT, DOWN, LEFT
  }
  
  public enum DiagonalMove{
    TOP_LEFT, TOP_RIGHT, 
    BOTTOM_LEFT, BOTTOM_RIGHT
  }
  
  void move(DirectMove direction);
  void moveDiagonally(DiagonalMove direction);
}
  
//Receiver
class ConsoleController implements Controller{
  
  @Override
  public void move(DirectMove direction){
    System.out.println
    ("Console controller moves " + direction);
  }
  
  @Override
  public void moveDiagonally(DiagonalMove direction){
    System.out.println
    ("Console controller diagonally moves " + direction);
  }
}
  
//Receiver
class ComputerController implements Controller{
  
  @Override
  public void move(DirectMove direction){
    System.out.println
    ("Computer controller moves " + direction);
  }
  
  @Override
  public void moveDiagonally(DiagonalMove direction){
    System.out.println
    ("Console controller diagonally moves " + direction);
  }
}

//command interface
interface Command{
  
  void execute();
}
  
//command
class MoveCommand implements Command{
  
  private Controller controller;
  private Controller.DirectMove movement;
  
  public MoveCommand(Controller controller,
                     Controller.DirectMove movement){
    this.controller = controller;
    this.movement = movement;
  }
  
  @Override
  public void execute(){
    controller.move(movement);
  }
}
  
//command
class MoveDiagonalCommand implements Command{
  
  private Controller controller;
  private Controller.DiagonalMove movement;
  
  public MoveDiagonalCommand(Controller controller,
                             Controller.DiagonalMove movement){
    this.controller = controller;
    this.movement = movement;
  }
  
  @Override
  public void execute(){
    controller.moveDiagonally(movement);
  }
}
  
//invoker
class MoveInput{
  private Command directMovement;
  private Command diagonalMovement;
  
  public MoveInput(Command directMovement,
                   Command diagonalMovement){
    this.directMovement = directMovement;
    this.diagonalMovement = diagonalMovement;
  }
  
  public void move(){
    directMovement.execute();
  }
  
  public void moveDiagonally(){
    diagonalMovement.execute();
  }
}
  
Result
Computer controller move UP
Computer controller diagonally moves TOP_LEFT
  
Console controller moves DOWN
Console controller diagonally moves BOTTOM_RIGHT

Design Pattern: Chain-of-responsibility pattern

Chapters

• Chain-of-responsibility pattern

Chain-of-responsibility pattern

Chain-of-responsibility pattern is a behavioral design pattern that consists of command objects and processing objects. Command objects are objects that are being processed by processing objects.

Typically, every class in the chain has different responsibilities from one another. However, many implementations(such as UI event handling, servlet filters in Java and the example below) breaks this concept and allow several classes in the chain to take the same responsibility. This pattern promotes loose coupling as its processing objects are not closely tied up to command objects.

This example demonstrates chain-of-responsibility pattern.

import java.util.List;
import java.util.Arrays;

public class ClientCode{
  
  public static void main(String[] args){
  
    Handler handler = 
    new Adult(Arrays.asList(Handler.Fruits.all()), "Timothy").
    addHandler(
     new YoungAdult(
     Arrays.asList(Handler.Fruits.APPLE, Handler.Fruits.GUAVA),
                   "Samantha")).
    addHandler(
     new Child(
     Arrays.asList(Handler.Fruits.APPLE, Handler.Fruits.ORANGE),
                   "Louis"));
                   
     handler.offer(Handler.Fruits.APPLE);
     System.out.println();
     handler.offer(Handler.Fruits.GUAVA);
     System.out.println();
     handler.offer(Handler.Fruits.ORANGE);
     System.out.println();
     handler.offer(Handler.Fruits.MELON);
  }
}

//functional interface
interface Handler{
  public enum Fruits{
    AVOCADO, ORANGE, APPLE, GUAVA, MELON;
    
    public static Fruits[] all(){
      return values();
    }
  }
  
  //No need to add Handler reference after
  //Fruits reference. This method is in the
  //scope of Handler already
  //
  //classes that implement this method also
  //don't need to add Handler reference after
  //Fruits reference
  void offer(Fruits fruit);
  
  default Handler addHandler(Handler nextHandler){
    return (fruit) -> {
      offer(fruit);
      nextHandler.offer(fruit);
    };
  }
  
}

abstract class Patron{
  protected List<Handler.Fruits> preferredFruit;
  protected String name;
  
  Patron(List<Handler.Fruits> preferredFruit, 
         String name){
    this.preferredFruit = preferredFruit;
    this.name = name;
  }
  
  protected boolean checkPreferredFruit(Handler.Fruits fruit){
    boolean result = false;
    
    for(Handler.Fruits f : preferredFruit)
      if(f == fruit)
        result = true;
    return result;
  }
  
}

class Child extends Patron implements Handler{
  
  Child(List<Handler.Fruits> preferredFruit, 
         String name){
    super(preferredFruit, name);
  }
  
  @Override
  public void offer(Fruits fruit){
    if(!checkPreferredFruit(fruit))
      return;
    
    System.out.println
    (name + ", a child, took " + fruit);
  }
}

class YoungAdult extends Patron implements Handler{
  
  YoungAdult(List<Handler.Fruits> preferredFruit, 
         String name){
    super(preferredFruit, name);
  }
  
  @Override
  public void offer(Fruits fruit){
    if(!checkPreferredFruit(fruit))
      return;
  
    System.out.println
    (name + ", a young adult, took " + fruit);
  }
}

class Adult extends Patron implements Handler{
  
  Adult(List<Handler.Fruits> preferredFruit, 
         String name){
    super(preferredFruit, name);
  }
  
  @Override
  public void offer(Fruits fruit){
    if(!checkPreferredFruit(fruit))
      return;
    
    System.out.println
    (name + ", an adult, took " + fruit);
  }
}

Result
Timothy, an adult, took APPLE
Samantha, a young adult, took APPLE
Louis, a child, took APPLE

Timothy, an adult, took GUAVA
Samantha, a young adult, took GUAVA

Timothy, an adult, took ORANGE
Louis, a child, took ORANGE

Timothy, an adult, took MELON

In the example above, fruits in the Fruits enum are command objects whereas Adult, Child and YoungAdult instances are processing objects.

Design Pattern: Proxy Pattern

Chapters

• Proxy Pattern

Proxy Pattern

proxy pattern is a software design pattern. A proxy, in its most general form, is a class functioning as an interface to something else.

The proxy could interface to anything: a network connection, a large object in memory, a file, or some other resource that is expensive or impossible to duplicate. In short, a proxy is a wrapper or agent object that is being called by the client to access the real serving object behind the scenes.

Use of the proxy can simply be forwarding to the real object, or can provide additional logic. In the proxy, extra functionality can be provided, for example caching when operations on the real object are resource intensive, or checking preconditions before operations on the real object are invoked. For the client, usage of a proxy object is similar to using the real object, because both implement the same interface.

This diagram shows how to implement a proxy pattern

Courtesy of Wikipedia

This example demonstrates proxy pattern.

public class ClientCode{

  public static void main(String[] args){
    StringConcatInterface sci = 
    new StringConcatProxy(new StringConcat("My "));
    
    sci.concat("String!");
    System.out.println(sci.getText());
    sci.concat("String!String!");
    System.out.println(sci.getText());
    sci.concat("String!String!String!");
    System.out.println(sci.getText());
  }
}

interface StringConcatInterface{

  void concat(String str);
  String getText();
}

class StringConcat implements StringConcatInterface{
  private StringBuilder builder;
  
  StringConcat(String text){
    builder = new StringBuilder(text);
  }
  
  @Override
  public void concat(String str){
    builder.append(str);
  }
  
  @Override
  public String getText(){
    return builder.toString();
  }
}

class StringConcatProxy implements StringConcatInterface{
  private StringConcat sc;
  
  StringConcatProxy(StringConcat sc){
    this.sc = sc;
  }
  
  @Override
  public void concat(String str){
    if(sc.getText().length() > 10){
      System.out.println
      ("Max characters has been reached!");
    }
    else
      sc.concat(str);
  }
  
  @Override
  public String getText(){
    return sc.getText();
  }
  
}

Result
My String!
My String!String!String!
Max characters has been reached!
My String!String!String!

You might have noticed that proxy pattern is similar to decorator pattern. Their structure is similar but their purpose are not. We use decorator pattern if we want to add functionalities to a class while not affecting other related classes. We use proxy pattern if we want some kind of mirror that mirrors our class.

Design Pattern: Flyweight Pattern

Chapters

• Flyweight Pattern

Flyweight Pattern

Flyweight pattern refers to an object that minimizes memory usage by sharing some of its data with other similar objects. The flyweight pattern is useful when dealing with large numbers of objects with simple repeated elements that would use a large amount of memory if individually stored.

This example demonstrates flyweight pattern.

import java.util.Queue;
import java.util.ArrayDeque;

public class ClientCode{

  public static void main(String[] args){
    int x = 0;
    int y = 0;
    
    TileFlyWeightFactory tileFactory =
    TileFlyWeightFactory.getInstance();
    
    //4x4 grid
    for(int i = 0; i < 4; i++){
      for(int j = 0; j < 4; j++){
        tileFactory.
        useThenAdd().
        draw(x, y);
        x += 64;
      }
      x = 0;
      y += 64;
      System.out.println();
    }
    
  }
}

interface TileInterface{

  void draw(int posx, int posy);
}

class Tile implements TileInterface{
  //intrinsic state
  private final String tileSource;
  
  Tile(String tileSource){
    this.tileSource = tileSource;
  }
  
  @Override
  public void draw(int posx, int posy){
    System.out.println
    (tileSource + " is drawn at " +
     posx + ", " + posy);
  }
  
}

//singleton
class TileFlyWeightFactory{
  private static TileFlyWeightFactory instance;
  private Queue<TileInterface> cache;
  
  private TileFlyWeightFactory(){
    cache = new ArrayDeque<>();
    cache.add(new Tile("Tile1.jpg"));
    cache.add(new Tile("Tile2.jpg"));
    cache.add(new Tile("Tile3.jpg"));
    cache.add(new Tile("Tile4.jpg"));
  }
  
  public static TileFlyWeightFactory getInstance(){
    if(instance == null)
      instance = new TileFlyWeightFactory();
    return instance;
  }
  
  public TileInterface useThenAdd(){
    TileInterface target = null;
    
    if(!cache.isEmpty()){
      target = cache.poll();
      cache.add(target);
    }
    return target;
  }
  
}

Result
Tile1.jpg is drawn at 0, 0
Tile2.jpg is drawn at 64, 0
Tile3.jpg is drawn at 128, 0
Tile4.jpg is drawn at 192, 0

Tile1.jpg is drawn at 0, 64
Tile2.jpg is drawn at 64, 64
Tile3.jpg is drawn at 128, 64
Tile4.jpg is drawn at 192, 64

Tile1.jpg is drawn at 0, 128
Tile2.jpg is drawn at 64, 128
Tile3.jpg is drawn at 128, 128
Tile4.jpg is drawn at 192, 128

Tile1.jpg is drawn at 0, 192
Tile2.jpg is drawn at 64, 192
Tile3.jpg is drawn at 128, 192
Tile4.jpg is drawn at 192, 192

First off, we store two states to our flyweight class (Tile class in this case). One of them is the intrinsic state. This state is constant. The next one is the extrinsic state. This state is not constant and likely to change overtime. In the example above, tileSource variable is the intristic state; posx and posy are extrinsic states.

In the example above, I reuse instantiated Tile objects in order to reuse their tileSource constants (intrinsic) and draw them anywhere on the screen using posx and posy variables (extrinsic). This technique saves a lot of system resources because I don't need to create a new instance of Tile class in order to reuse a tile image that's been already used.

Other information such as caching, retrieval, concurrency can be found in this wiki.

Design Pattern: Facade Pattern

Chapters

• Facade Pattern

Facade Pattern

facade pattern (also spelled façade) is a software-design pattern commonly used in object-oriented programming. Analogous to a facade in architecture, a facade is an object that serves as a front-facing interface masking more complex underlying or structural code in order to simplify the complexity of the system behind the facade.

To implement this pattern, we need a class that implements an interface and the implementations of the methods of the interface are delegated to the system behind the facade. Although, the interface may perform additional functionality before/after forwarding a request. Take a look at this example.

/*
Client
*/

public class ClientCode{

  public static void main(String[] args){
    RoomInterface ri = 
    new RoomController(new RoomControl());
    
    ri.openThenMark();
    System.out.println();
    ri.unmarkThenClose();
  }
}

/*
Facade
*/

interface RoomInterface{

  void openMarkThenClose();
  void openUnmarkThenClose();
  void openThenMark();
  void openThenUnmark();
  void markThenClose();
  void unmarkThenClose();
}

class RoomController implements RoomInterface{
  RoomControlInterface rci;
  
  RoomController(RoomControlInterface rci){
    this.rci = rci;
  }
  
  @Override
  public void openMarkThenClose(){
    rci.openRoom();
    rci.markRoom();
    rci.closeRoom();
  }
  
  @Override
  public void openUnmarkThenClose(){
    rci.openRoom();
    rci.unmarkRoom();
    rci.closeRoom();
  }
  
  @Override
  public void openThenMark(){
    rci.openRoom();
    rci.markRoom();
  }
  
  @Override
  public void openThenUnmark(){
    rci.openRoom();
    rci.unmarkRoom();
  }
  
  @Override
  public void markThenClose(){
    if(!rci.openRoom()){
      System.out.println("Room is not open.");
      System.out.println
      ("Therefore, it can't be closed.");
      return;
    }
    rci.markRoom();
    rci.closeRoom();
  }
  
  @Override
  public void unmarkThenClose(){
    if(!rci.openRoom()){
      System.out.println("Room is not open.");
      System.out.println
      ("Therefore, it can't be closed.");
      return;
    }
    rci.unmarkRoom();
    rci.closeRoom();
  }
  
}

/*
Complex System
*/

interface RoomControlInterface{
  
  boolean openRoom();
  void markRoom();
  void unmarkRoom();
  void closeRoom();
}

class RoomControl implements RoomControlInterface{
  private boolean isMarked = false;
  private boolean isOpen = false;
  
  @Override
  public boolean openRoom(){

    if(isOpen){
      System.out.println
      ("Room is already open!");
    }
    else{
      System.out.println
      ("Room has been opened!");
      isOpen = true;
    }
    return isOpen;
  }
  
  @Override
  public void markRoom(){
     if(isMarked){
      System.out.println
      ("Room is already marked. "+
       "no need to mark again.");
    }
    else{
      isMarked = true;
      System.out.println
      ("Room has been marked!");
    }
  }
  
  @Override
  public void unmarkRoom(){
    if(!isMarked){
      System.out.println
      ("Room is already unmarked. "+
       "no need to unmark again.");
    }
    else{
      isMarked = false;
      System.out.println
      ("Room has been unmarked!");
    }
  }
  
  @Override
  public void closeRoom(){
    
    if(isOpen){
      System.out.println
      ("Room has been closed!");
      isOpen = false;
    }
  }
}

Result
Room has been opened!
Room has been marked!

Room is already open!
Room has been unmarked!
Room has been closed!

Design Pattern: Decorator Pattern

Chapters

• Decorator Pattern

Decorator Pattern

Decorator pattern is a design pattern that allows behavior to be added to an individual object, dynamically, without affecting the behavior of other objects from the same class.

The decorator pattern is often useful for adhering to the Single Responsibility Principle, as it allows functionality to be divided between classes with unique areas of concern.

Decorator use can be more efficient than subclassing, because an object's behavior can be augmented without defining an entirely new object. This example demonstrates decorator pattern.

public class ClientCode{

  public static void main(String[] args){
    Package standard = 
    new StandardPackage("My Package");
    Package business = 
    new BusinessPackage("My Company's Package");
    
    standard.pack();
    business.pack();
    System.out.println();
    
    //with decorator
    new PackageGiftWrap(business).pack();
    System.out.println();
    new PackagePlasticWrap(standard).pack();
  }
}

interface Package{

  void pack();
}

abstract class AbstractPackage implements Package{
  protected String packageName;
  
  AbstractPackage(String packageName){
    this.packageName = packageName;
  }
  
  public String getPackageName(){
    return packageName;
  }
  
}

class StandardPackage extends AbstractPackage{
  
  StandardPackage(String packageName){
    super(packageName);
  }
  
  @Override
  public void pack(){
    System.out.println
    (packageName + " with standard packaging " +
    "has been packed.");
  }
}

class BusinessPackage extends AbstractPackage{
  
  BusinessPackage(String packageName){
    super(packageName);
  }
  
  @Override
  public void pack(){
    System.out.println
    (packageName + " with business packaging " +
    "has been packed.");
  }
}

//abstract decorator
abstract class PackageDecorator implements Package{
  
  protected void setAddon
  (String packageName, String addon){
    System.out.println
    (addon + " add-on has been added to " +
     packageName);
  }
}

//concrete decorator
class PackageGiftWrap extends PackageDecorator{
  private Package itemPackage;
  
  public PackageGiftWrap(Package itemPackage){
    this.itemPackage = itemPackage;
  }
  
  @Override
  public void pack(){
    itemPackage.pack(); //delegation
    giftWrap();
  }
  
  private void giftWrap(){
    if(itemPackage instanceof AbstractPackage){
      AbstractPackage ap = 
      (AbstractPackage) itemPackage;
      System.out.println
      (ap.getPackageName() + 
       " has been gift wrapped.");
    }
    else
      System.out.println("Can't find package name!");
  }
}

//concrete decorator
class PackagePlasticWrap extends PackageDecorator{
  private Package itemPackage;
  
  public PackagePlasticWrap(Package itemPackage){
    this.itemPackage = itemPackage;
  }
  
  @Override
  public void pack(){
    itemPackage.pack(); //delegation
    plasticWrap();
  }
  
  private void plasticWrap(){
    if(itemPackage instanceof AbstractPackage){
      AbstractPackage ap = 
      (AbstractPackage) itemPackage;
      System.out.println
      (ap.getPackageName() + 
       " has been plastic wrapped.");
    }
    else
      System.out.println("Can't find package name!");
  }
}

Result
My Package with standard packaging has been packed.
My Company's Package with business packaging has been packed.

My Company's Package with business packaging has been packed.
My Company's Package has been gift wrapped.

My Package with standard packaging has been packed.
My Package has been plastic wrapped.

In the example above, we can independently add new functionalities to StandardPackage and BusinessPackage instances during run-time by using decorator classes. Decorator and bridge patterns have similar structure.

However, they have different purpose. Bridge pattern is used to separate two related hierarchies so that they can be expanded independently whereas decorator pattern is used to statically or dynamically add new functionalities to an instance related to decorator class instance.

Design Pattern: Composite Pattern

Chapters

• Composite Pattern

Composite Pattern

Composite pattern is a partitioning design pattern. The composite pattern describes a group of objects that are treated the same way as a single instance of the same type of object.

The intent of a composite is to "compose" objects into tree structures to represent part-whole hierarchies. Implementing the composite pattern lets clients treat individual objects and compositions uniformly.

These diagrams demonstrates composite pattern.

Courtesy of Wikipedia

We can think of this method as a tree structure. A branch holds leaves and leaves are in a branch. In the diagram above, Component is a root branch and Composite is a child branch. Take a look at this example.

import java.util.List;
import java.util.ArrayList;

public class ClientCode{
  
  public static void main(String[] args){
    Package myRedPackage = 
    new RedPackage("Delivery Package", "packages");
    
    myRedPackage.addPackage(
    new RedPackage("My Colleague's Package", 
                   "Ballpens"));
    myRedPackage.addPackage(
    new RedPackage("My Package", 
                   "Pens"));
    myRedPackage.addPackage(
    new BluePackage("My Friend's Package", 
                    "Documents", true));
    
    myRedPackage.packageInfo();
    System.out.println();
    
    myRedPackage.getPackage(2).packageInfo();
    
    System.out.println();
    myRedPackage.removePackage(2);
    System.out.println();
    
    Package subPackage = 
    myRedPackage.getPackage(0);
    subPackage.packageInfo();
    System.out.println();
    
    subPackage.addPackage(
    new RedPackage("Optional Package", "Stamps"));
    subPackage.getPackage(0).packageInfo();
  }
}

//root 
abstract class Package{
  private List<Package> packageList;
  private String content;
  private String packageName;
  
  Package(String packageName, String content){
    packageList = new ArrayList<>();
    this.packageName = packageName;
    this.content = content;
  }
  
  public String getPackageName(){
    return packageName;
  }
  
  public String getContent(){
    return content;
  }
  
  public void addPackage(Package element){
    packageList.add(element);
  }
  
  public Package getPackage(int index){
    Package target = null;
    
    try{
      target = packageList.get(index);
    }
    catch(ArrayIndexOutOfBoundsException e){
      System.out.println("Invalid Index!");
    }
    return target;
  }
  
  public void removePackage(int index){
    String target = null;
  
    try{
      target = packageList.get(index).
      getPackageName();
      packageList.remove(index);
    }
    catch(ArrayIndexOutOfBoundsException e){
      System.out.println("Invalid Index!");
      return;
    }
    System.out.println
    ("Package "+ target +" is removed!");
  }
  
  abstract public void packageInfo();
}

//leaf
class RedPackage extends Package{
  
  RedPackage(String packageName, String content){
    super(packageName, content);
  }
  
  @Override
  public void packageInfo(){
    System.out.println("Name: " + 
    getPackageName());
    System.out.println("Color: Red");
    System.out.println("Parent: Package");
    System.out.println("Content: " + getContent());
  }
}

//composite
abstract class CompositeStandardPackage extends Package{
  private boolean isSpecial;
  
  CompositeStandardPackage(String packageName, 
                           String content, 
                           boolean isSpecial){
    super(packageName, content);
    this.isSpecial = isSpecial;
  }
  
  public boolean isSpecial(){
    return isSpecial;
  }
}

//leaf
class BluePackage extends CompositeStandardPackage{
  
  BluePackage(String packageName, String content,
              boolean isSpecial){
    super(packageName, content, isSpecial);
  }
  
  @Override
  public void packageInfo(){
    System.out.println("Name: " + 
    getPackageName());
    System.out.println("Color: Blue");
    System.out.println
    ("Parent: CompositeStandardPackage");
    System.out.println("Content: " + getContent());
    System.out.println("Is Special? " + isSpecial());
  }
}

Result
Name: Delivery Package
Color: Red
Parent: Package
Content: Packages

Name: My Friend's Package
Color: Blue
Parent: CompositeStandardPackage
Content: Documents
Is Special? true

Package My Friend's Package is removed!

Name: My Colleague's Package
Color: Red
Parent: Package
Content: Ballpens

Name: Optional Package
Color: Red
Parent: Package
Content: Stamps

In the example above, leaf and Composite classes can add, get and remove children(packages in this case). There are two types of child-related designs that we can use to design our composite tree structures. Take a look at this diagram.

Courtesy of Wikipedia

In the example above, I used the "Design for Uniformity" design because the composite design pattern emphasizes uniformity over type safety. In "Design for type safety" design, only composite class and its subclasses can have children. This example in wikipedia demonstrates "Design for type safety" design.

Java Tutorial - Bridge Pattern

Chapters

• Bridge Pattern

Bridge Pattern

Bridge Pattern is a design pattern that is meant to "decouple an abstraction from its implementation so that the two can vary independently". For example, we have shape and we want to implement a color feature. To do this, we create a class for shape and color; combine them in one hierarchy.

The problem with this is that these two classes are going to be tightly coupled and we, programmers, avoid tight coupling if possible. To avoid tight coupling in this case, we will decouple a class of color from class of shape.

Class of shape will stand as an abstraction for class of shape color. Then, an implementation for coloring shapes of shape class will be delegated to class of shape color. We will use this diagram as reference:

Courtesy of Wikipedia

This example demonstrates bridge pattern.

/*
Client
*/

public class ClientCode{
  public static void main(String[] args){
    Shape circ = 
    new Circle(new StandardShapeColorStyle());
    
    circ.setColorStyle("STYLE1");
    System.out.println
    ("Color style of this shape:");
    System.out.println
    (circ.getShapeColorStyle());
  }
}

/*
Abstraction
*/
abstract class Shape{
  private ShapeColorStyle scs;
  private String colorStyle;
  
  Shape(ShapeColorStyle scs){
    this.scs = scs;
  }
  
  //implementation of setColorStyle of
  //Shape class delegated to setColorStyle
  //of ShapeColorStyle
  public void setColorStyle(String style){
    if(scs == null){
      System.err.println
      ("ColorStyle instance not defined!");
      return;
    }
    colorStyle = scs.setColorStyle(style);
  }
  
  String getShapeColorStyle(){
    return colorStyle;
  }
}

class Circle extends Shape{

  Circle(ShapeColorStyle scs){
    super(scs);
  }
  
  //refined version of setColorStyle
  //of Shape class. I find this one
  //optional
  @Override
  public void setColorStyle(String style){
    System.out.println
    ("This shape is circle.");
    System.out.println
    ("Circle has infinite vertices.");
    System.out.println
    ("Setting color style...");
    super.setColorStyle(style);
  }
  
}

/*
Implementor
*/

interface ShapeColorStyle{
  String setColorStyle(String style);
}

class StandardShapeColorStyle implements 
                              ShapeColorStyle{
  private final String STYLE1 = "STYLE1";
  private final String STYLE2 = "STYLE2";
  
  @Override
  public String setColorStyle(String style){
    String colorStyle = null;
    
    switch(style){
      case STYLE1:
      colorStyle = "Color Style One";
      break;
      
      case STYLE2:
      colorStyle = "Color Style Two";
      break;
    }
    return colorStyle;
  }
}

Result
This shape is circle.
Circle has infinite vertices
Setting color style...
Color style of this shape:
Color Style One

One of the advantages of bridge pattern is that we can independently expand two related hierarchies. In the example above, Shape and ShapeColorStyle classes can be expanded without affecting each other.

Design Pattern: Adapter Pattern

Chapters

• Adapter Pattern

Adapter Pattern

Adapter Pattern is a design pattern that makes two incompatible interfaces collaborate. We use adapter if a new codebase that we wanna add to our application is not compatible with our existing interface.

For example, we have an application that lists data in plain text format. Then, we have a third library that process analytics. This library is verified compatible with our application but the problem is that this library returns data in XML format. To resolve this problem, we can look for another library. However, even we find another library we need to verify if it's compatible with our application, which takes time.

Another solution is to modify the library itself. However, a lot of problems may arise. One of them is licensing issue, some libraries are protected by license and thus we can't just modify libraries. Another solution is to create an adapter which is a good solution to this problem.

There are two types of adapter pattern: Object adapter pattern and Class adapter pattern. Object adapter pattern implements the target(interface where another interface is gonna be converted) interface by delegating to an adaptee(interface that is gonna be converted to) object at run-time.

Class adapter pattern implements the target interface by inheriting from an adaptee class at compile-time. This example demonstrates object adapter pattern.

//client
public class ClientCode{

  public static void main(String[] args){
    PlainTextInterface plainText = 
    new XMLToPlainTextAdapter(new XMLAnalytics());
    
    plainText.displayData();
  }
}

//Assume our third party library has an
//interface
interface XMLInterface{

  String getAnalytics();
}

//Assume this is a third party library
class XMLAnalytics implements XMLInterface{
  private String content;
  
  XMLAnalytics(){
    content= "<data>Testing</data>";
  }
  
  @Override
  public String getAnalytics(){
    return content;
  }
  
}

//Our app's interface
interface PlainTextInterface{

  void displayData();
}

//Our app's class
class PlainTextDisplay implements PlainTextInterface{
  private String data;
  
  PlainTextDisplay(String data){
    this.data = data;
  }
  
  @Override
  public void displayData(){
    System.out.println("Data in plain text: " + data);
  }
}

//Adapter
//In object adapter pattern, we wrap adaptee interface and
//implements target interface
class XMLToPlainTextAdapter implements PlainTextInterface{
  private XMLInterface xml_analytics;
  
  XMLToPlainTextAdapter(XMLInterface xml_analytics){
    this.xml_analytics = xml_analytics;
  }
  
  @Override
  public void displayData(){
    String noTags = 
    xml_analytics.getAnalytics().
    replaceAll("[<][/]??.+?[>]","");
    
    System.out.println("XML converted to plain text");
    System.out.println("Data in plain text: " + noTags);
  }
}

Result
XML converted to plain text
Data in plain text: Testing

Next, let's implement class adapter pattern. First off, change XMLToPlainTextAdapter class to this:

class XMLToPlainTextAdapter 
      extends XMLAnalytics
      implements PlainTextInterface{
  
  @Override
  public void displayData(){
    String noTags = getAnalytics().
    replaceAll("[<][/]??.+?[>]","");
    
    System.out.println("XML converted to plain text");
    System.out.println("Data in plain text: " + noTags);
  }
}

Then, change ClientCode class to this:

public class ClientCode{

  public static void main(String[] args){
    PlainTextInterface plainText = 
    new XMLToPlainTextAdapter();
    plainText.displayData();
  }
}

The problem with class adapter pattern is that it doesn't work with all OOP languages. For example, java doesn't support multiple inheritance of classes; this pattern won't work with java if we have multiple adaptees in one adapter.

In the example above, one-way conversion has been implemented to our adapter. However, we can implement two-way conversion which converts XML to plain text and vice-versa.

To do this, we can implement both interfaces in one adapter or create another adapter for conversion from plain text to XML. In my opinion, the latter is more preferrable.

Design Pattern: Prototype Pattern

Chapters

• Prototype Pattern

Prototype Pattern

Prototype pattern is a creational design pattern that copies(clone) an object rather than re-creating it. In Object Oriented Programming (OOP), objects are created by instantiating a class.

In java, there's a clone() method that we can use to clone an object rather than re-creating it via instantiation. In this pattern, we create an interface (or abstract class). Then, the client can use the interface to clone its concrete implementations. Take a look at this example.

import java.util.List;
import java.util.Random;

public class ClientCode{
  
  public static void main(String[] args)
                throws CloneNotSupportedException{
    Prototype protoA = 
    new ConcretePrototypeA("protoA");
    
    System.out.println("Original");
    protoA.displayList();
    System.out.println("\n");
    
    Prototype cloneObj = protoA.clone();
    System.out.println("Clone");
    cloneObj.displayList();
    System.out.println("\n");
    
    System.out.println("Compare Instance");
    System.out.println(
    protoA.compareListInstance(cloneObj.getList()));
  }
}

//factory class
interface Prototype extends Cloneable{
  
  Prototype clone() throws CloneNotSupportedException;
  void displayList();
  boolean compareListInstance(List<Integer> list);
  List<Integer> getList();
}

class ConcretePrototypeA implements Prototype{
  private List<Integer> numbers;
  
  ConcretePrototypeA(String name){
    Random rand = new Random();
    Integer[] values = new Integer[5];
    for(int i = 0; i < values.length; i++)
      values[i] = rand.nextInt(1000);
    numbers = java.util.Arrays.asList(values);
  }
  
  @Override
  public Prototype clone() throws CloneNotSupportedException{
    return (ConcretePrototypeA) super.clone();
  }
  
  public void displayList(){
    numbers.stream().forEach(v -> System.out.print(v + " "));
  }
  
  public boolean compareListInstance(List<Integer> list){
    return list == numbers;
  }
  
  public List<Integer> getList(){
    return numbers;
  }
  
}

Result(may vary)
Original
588 68 607 122 106

Clone
588 68 607 122 106

Compare Instance
true

You can use this pattern when you want to create another Object at runtime that is a true copy of the Object you are cloning. True copy means all the attributes of the newly created Object should be the same as the Object you are cloning.

Design Pattern: Builder Pattern

Chapters

• Builder Pattern

Builder Pattern

Builder pattern is a design pattern that separates object creation from objects. It means that we dedicate a class to assemble every parts of an object that we wanna create instead of assembling parts directly in the class of the object. This pattern reduces the complexity of a class. Take a look at this example.

public class ClientCode{

  public static void main(String[] args){
    House myHouse =
    House.newBuilder("My House").
    installRoof("Gable").
    installWallpaper("Green").
    installTiles("Blue").
    installRooms(3).
    build();
    
    myHouse.houseInfo();
    System.out.println();
    
    myHouse = new 
    Director(
    House.newBuilder("My Another House")).
    construct();
    
    myHouse.houseInfo();
  }
}

class House{
  
  private String houseName;
  private String roof;
  private String wallpaper;
  private String tiles;
  private int rooms;
  private boolean garage;
  
  private House(String houseName){
    this.houseName = houseName;
  }
  
  public static House.HouseBuilder newBuilder(String houseName){
    return new House(houseName).new HouseBuilder();
  }
  
  public void houseInfo(){
    System.out.println("House Name: " + houseName);
    System.out.println("Roof: " + roof);
    System.out.println("Wallpaper: " + wallpaper);
    System.out.println("Tile Color: " + tiles);
    System.out.println("# of Rooms: " + rooms);
    System.out.println("Is there a garage? " + garage);
  }
  
  class HouseBuilder implements Builder{
    
    private HouseBuilder(){
      House.this.roof = "None";
      House.this.wallpaper = "None";
      House.this.tiles = "None";
      House.this.rooms = 0;
      House.this.garage = false;
    }
    
    @Override
    public House.HouseBuilder installRoof(String roof){
      House.this.roof = roof;
      return this;
    }
    
    @Override
    public House.HouseBuilder installWallpaper(String wallpaper){
      House.this.wallpaper = wallpaper;
      return this;
    }
    
    @Override
    public House.HouseBuilder installTiles(String tiles){
      House.this.tiles = tiles;
      return this;
    }
    
    @Override
    public House.HouseBuilder installRooms(int rooms){
      House.this.rooms = rooms;
      return this;
    }
    
    @Override
    public House.HouseBuilder installGarage(boolean install){
      House.this.garage = install;
      return this;
    }
    
    @Override
    public House build(){
      return House.this;
    }
    
  }
}

interface Builder{

  House.HouseBuilder installRoof(String roof);
  House.HouseBuilder installWallpaper(String wallpaper);
  House.HouseBuilder installTiles(String tiles);
  House.HouseBuilder installRooms(int rooms);
  House.HouseBuilder installGarage(boolean install);
  House build();
  
}

class Director{
  private Builder builder; 
  
  public Director(Builder builder){
    this.builder = builder;
  }
  
  public House construct(){
    House houseInstance = null;
    
    if(builder instanceof House.HouseBuilder){
      houseInstance = builder.
      installRoof("Dutch").
      installWallpaper("Blue").
      installRooms(4).
      installGarage(true).
      build();
    }
    return houseInstance;
  }
}

Result
House Name: My House
Roof: Gable
Wallpaper: Green
Tile color: Blue
# of Rooms: 3
Is there a garage? false

House Name: My Another House
Roof: Dutch
Wallpaper: Blue
# of Rooms: 4
Is there a garage? true

You may suggest that there are alternatives to this pattern. Let's assume you're suggesting to use a constructor:

... House(String roof, String wallpaper, String tiles, int rooms, boolean garage)
...

This alternative does make sense in the example above. However, what if want two types of builds of our house? also what if each part of a house in every build has different calculations. In this case, builder pattern is more preferrable. For example, we wanna calculate the size of a roof and each build has different ways of calculating roofs.

In this case, we can just add the calculations to their installRoof methods. Imagine coding two or more types of builds in your House. Your code can become harder to read. Using build pattern, we can encapsulate those builds and their functionalities. Making our House class more clear.

Another advantage of this pattern is that clients can build an object in step-by-step manner. Also, I didn't put any getter or setter methods in the example above but we can put those methods in the House class if we want to.

You might have noticed the Director class. This class contains pre-configured builds of our builders. If you have a build that you often use, you can add that builds in this class, so that you don't need to write your build code everytime you wanna use it. We can also use this class to save client's builds.

In java, some classes implement this pattern. HttpRequest.Builder and DateTimeFormatterBuilder are some examples of classes that implement builder pattern.

Degin Pattern: Abstract Factory Pattern

Chapters

• Abstract Factory Pattern

Abstract Factory Pattern

Abstract factory pattern is just like factory pattern, but this pattern deals with a family of objects and its variants. For example, we are creating classes for two GUI component providers. Both create GUI components but they have different design.

We can use abstract factory method to create a class design in this scenario. Take a look at this example.

/*
Client
*/
import factories;

public class ClientCode{

  public static void main(String[] args){
    GUIComponentsFactory factory = 
    GUIComponentsFactory.
    selectFactory("Unix");
    Button createdButton = null;
    RadioButton createdRadioButton = null;
    
    if(factory != null){
      createdButton = 
      factory.createButton("MyButton");
      System.out.println
      ("Created Button: " + 
       createdButton.getName());
      System.out.println();
      
      createdRadioButton = 
      factory.createRadioButton("MyRadioButton");
      System.out.println
      ("Created Button: " + 
       createdRadioButton.getName());
    }
  }
}

/*
Factories
Assume classes below are part of factories package
*/

public interface GUIComponentsFactory{
  
  Button createButton(String name);
  RadioButton createRadioButton(String name);
  
  public static GUIComponentsFactory selectFactory(String provider){
    GUIComponentsFactory factory = null;
    
    switch(provider){
    
      case "Windows":
      factory = new CreateWinComponents();
      break;
      
      case "Unix":
      factory = new CreateUnixComponents();
      break;
    }
    return factory;
  }
}

class CreateWinComponents implements GUIComponentsFactory{
  
  CreateWinComponents(){}
  
  @Override
  public Button createButton(String name){
    return new WinButton(name);
  }
  
  @Override
  public RadioButton createRadioButton(String name){
    return new WinRadio(name);
  }
  
}

class CreateUnixComponents implements GUIComponentsFactory{
  
  CreateUnixComponents(){}
  
  @Override
  public Button createButton(String name){
    return new UnixButton(name);
  }
  
  @Override
  public RadioButton createRadioButton(String name){
    return new UnixRadio(name);
  }
}

/*
Objects
*/

pubic abstract class Button{
  private String name;
  
  Button(String name){
    this.name = name;
  }
  
  public String getName(){
    return name;
  }
  
}

class WinButton extends Button{

  WinButton(String name){
    super(name);
    System.out.println(name + 
    " Windows Button has been created!");
  }
}

class UnixButton extends Button{

  UnixButton(String name){
    super(name);
    System.out.println(name + 
    " Unix Button has been created!");
  }
}

public abstract class RadioButton{
  private String name;
  
  RadioButton(String name){
    this.name = name;
  }
  
  public String getName(){
    return name;
  }
  
}

class WinRadio extends RadioButton{

  WinRadio(String name){
    super(name);
    System.out.println(name + 
    " Windows Radio Button has been created!");
  }
}

class UnixRadio extends RadioButton{

  UnixRadio(String name){
    super(name);
    System.out.println(name + 
    " Unix Radio Button has been created!");
  }
}

Result
MyButton Unix Button has been created!
Created Button: MyButton

MyRadioButton Unix Radio Button has been created!
Created Button: MyRadioButton

Just like in factory pattern, factory superclass and objects superclass should be non-instantiable. In short, they can't be instantiated.

One of the advantages of abstract factory pattern over factory pattern is that this pattern implements greater abstraction than factory pattern. However, if you're not producing a family of objects, it's better to use factory pattern.

This pattern inherits the advantages and disadvantages of factory pattern. For more information, you may visit this website