Design Patterns - Observer Pattern

Observer pattern is used when there is one-to-many relationship between objects such as if one object is modified, its depenedent objects are to be notified automatically. Observer pattern falls under behavioral pattern category.

Implementation

Observer pattern uses three actor classes. Subject, Observer and Cpent. Subject is an object having methods to attach and detach observers to a cpent object. We have created an abstract class Observer and a concrete class Subject that is extending class Observer.

ObserverPatternDemo, our demo class, will use Subject and concrete class object to show observer pattern in action.

Step 1

Create Subject class.

Subject.java

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

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

   pubpc int getState() {
      return state;
   }

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

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

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

Step 2

Create Observer class.

Observer.java

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

Step 3

Create concrete observer classes

BinaryObserver.java

pubpc class BinaryObserver extends Observer{

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

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

OctalObserver.java

pubpc class OctalObserver extends Observer{

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

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

HexaObserver.java

pubpc class HexaObserver extends Observer{

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

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

Step 4

Use Subject and concrete observer objects.

ObserverPatternDemo.java

pubpc class ObserverPatternDemo {
   pubpc 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);
   }
}

Step 5

Verify the output.

First state change: 15
Hex String: F
Octal String: 17
Binary String: 1111
Second state change: 10
Hex String: A
Octal String: 12
Binary String: 1010