Observer Design Pattern

 

 

--referenced from the book "Head First Design Pattern."

Publishers + Subscribers = Observer Pattern

Observer pattern is used when there is one-to-many relationship between objects such as if one object is modified, 

Its dependent objects are to be notified automatically. It is also a behavioural pattern. 

There some key components in Observer pattern:

 Subject object: Subject is an object that maintains a list of its dependents, called observers. and notifies them automatically of any state changes, usally by calling one of their 　　　　                           methods.

 Observer object:

 

Below is a logical flow of Observer pattern example:

 

Step 1. Create Observer abstract class:

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

 

Step 2. Subject Class 

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();
      }
   } 	
}

 

Step 3. Create Concrete Observer class 

1. BinaryObserver 

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() ) ); 
   }
}

　2. OctalObserver

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() ) ); 
   }
}

　3. HexaObserver

public class HexaObserver extends Observer{

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

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

　

Step 4. ObserverPatternDemo 

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);
   }
}

　　

 There is Java built-in support for Observer patter.

"Head first Java"