Observer Pattern

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Observer Pattern

An Observer Pattern says that “just define a one-to-one dependency so that when one object changes state, all its dependents are notified and updated automatically”.

The Memento pattern is also known as Dependents or Publish-Subscribe.

Benefits:

  • It describes the coupling between the objects and the observer.
  • It provides the support for broadcast-type communication.

Usage:

  • When the change of a state in one object must be reflected in another object without keeping the objects tight coupled.
  • When the framework we writes and needs to be enhanced in future with new observers with minimal chamges.

UML for Observer Pattern:

Observer Pattern

 

Implementation of Observer Pattern

Step 1:

Create a ResponseHandler1 class the will implement the java.util.Observer interface.

import java.util.Observable;

import java.util.Observer;

public class ResponseHandler1 implements Observer {

privateString resp;

publicvoidupdate(Observable obj, Object arg) {

if(arg instanceof String) {

resp = (String) arg;

System.out.println(“\nReceived Response: ” + resp);

}

}

}// End of the ResponseHandler1 interface.

Step 2:

Create a ResponseHandler2 class the will implement the java.util.Observer interface.

import java.util.Observable;

import java.util.Observer;

public class ResponseHandler2 implements Observer {

privateString resp;

publicvoidupdate(Observable obj, Object arg) {

if(arg instanceof String) {

resp = (String) arg;

System.out.println(“\nReceived Response: ” + resp);

}

}

}// End of the ResponseHandler2 interface.

Step 3:

Create an EventSource class that will extend the java.util.Observable class .

import java.io.BufferedReader;

import java.io.IOException;

import java.io.InputStreamReader;

import java.util.Observable;

public class EventSource extends Observable implements Runnable {

@Override

publicvoidrun() {

try{

final InputStreamReader isr = new InputStreamReader(System.in);

final BufferedReader br = new BufferedReader(isr);

while(true) {

String response = br.readLine();

setChanged();

notifyObservers(response);

}

} catch (IOException e) {

e.printStackTrace();

}

}

}// End of the Eventsource class.

Output:

Observer Pattern

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