Design Pattern

Factory Pattern

How to implement?

Strategy Pattern

How to implement?

Abstract Factory Pattern

Singleton Pattern

Template Pattern

Step 3

Use the Game's template method play() to demonstrate a defined way of playing game.

Observer Pattern

Situation

We have observers listen to a subject. When the subject has changed its state, all the observers will get updated.

Before

public class BinaryObserver extends Observer{

   @Override
   public void update(int state) {
      System.out.println( "Binary String: " + Integer.toBinaryString( state ) ); 
   }
}

public class OctalObserver extends Observer{

   @Override
   public void update(int state) {
     System.out.println( "Octal String: " + Integer.toOctalString( state ) ); 
   }
}

BinaryObserver.java

OctalObserver.java

public abstract class Observer {
   public abstract void update(int state);
}

Observer.java

Before

public class Subject {
	
   private Observer binaryObserver = BinaryObserver();
   private Observer octalObserver = OctalObserver();
   private int state;

   public void setState(int state) {
      this.state = state;
      notifyAllObservers();
   }

   public void notifyAllObservers(){
      binaryObserver.update(this.state);
      octalObserver.update(this.state);
   } 	
}

Subject.java

Before

public class ObserverPatternDemo {
   public static void main(String[] args) {
      Subject subject = new Subject();

      System.out.println("First state change: 15");	
      subject.setState(15);
      System.out.println("Second state change: 10");	
      subject.setState(10);
   }
}

ObserverPatternDemo.java

After

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

public class Subject {
	
   private List<Observer> observers = new ArrayList<Observer>();
   private int state;

   public int getState() {
      return state;
   }

   public void setState(int state) {
      this.state = state;
      notifyAllObservers();
   }

   public void attach(Observer observer){
      observers.add(observer);		
   }

   public void notifyAllObservers(){
      for (Observer observer : observers) {
         observer.update();
      }
   } 	
}

Subject.java

After

public class BinaryObserver extends Observer{

   public BinaryObserver(Subject subject){
      this.subject = subject;
      this.subject.attach(this);
   }

   @Override
   public void update() {
      System.out.println( "Binary String: " + Integer.toBinaryString( subject.getState() ) ); 
   }
}

public class OctalObserver extends Observer{

   public OctalObserver(Subject subject){
      this.subject = subject;
      this.subject.attach(this);
   }

   @Override
   public void update() {
     System.out.println( "Octal String: " + Integer.toOctalString( subject.getState() ) ); 
   }
}

BinaryObserver.java

public abstract class Observer {
   protected Subject subject;
   public abstract void update();
}

OctalObserver.java

Observer.java

After

public class ObserverPatternDemo {
   public static void main(String[] args) {
      Subject subject = new Subject();

      new HexaObserver(subject);
      new OctalObserver(subject);
      new BinaryObserver(subject);

      System.out.println("First state change: 15");	
      subject.setState(15);
      System.out.println("Second state change: 10");	
      subject.setState(10);
   }
}

ObserverPatternDemo.java