Java Design Pattern: Observer

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In brief, Observer Pattern = publisher + subscriber.

Observer pattern has been used in GUI action listener. Swing GUI example shows how action listener works like an observer.

The following is a typical example about head hunter. There are two roles in this diagram – HeadHunter and JobSeeker. Job seekers subscribe to a head hunter, and head hunter notifies job seekers when there is a new job opportunity.

Class Diagram

Java Code

Subject interface.

public interface Subject {
	public void registerObserver(Observer o);
	public void removeObserver(Observer o);
	public void notifyAllObservers();
}

Observer interface.

public interface Observer {
	public void update(Subject s);
}

HeadHunter class implements Subject.

import java.util.ArrayList;
 
public class HeadHunter implements Subject{
 
	//define a list of users, such as Mike, Bill, etc.
	private ArrayList<Observer> userList;
	private ArrayList<String> jobs;
 
	public HeadHunter(){
		userList = new ArrayList<Observer>();
		jobs = new ArrayList<String>();
	}
 
	@Override
	public void registerObserver(Observer o) {
		userList.add(o);
	}
 
	@Override
	public void removeObserver(Observer o) {}
 
	@Override
	public void notifyAllObservers() {
		for(Observer o: userList){
			o.update(this);
		}
	}
 
	public void addJob(String job) {
		this.jobs.add(job);
		notifyAllObservers();
	}
 
	public ArrayList<String> getJobs() {
		return jobs;
	}
 
	public String toString(){
		return jobs.toString();
	}
}

JobSeeker is an observer.

public class JobSeeker implements Observer {
 
	private String name;
 
	public JobSeeker(String name){
		this.name = name;
	}
	@Override
	public void update(Subject s) {
		System.out.println(this.name + " got notified!");
		//print job list
		System.out.println(s);
	}
}

Start Point.

public class Main {
 
	public static void main(String[] args) {
		HeadHunter hh = new HeadHunter();
		hh.registerObserver(new JobSeeker("Mike"));
		hh.registerObserver(new JobSeeker("Chris"));
		hh.registerObserver(new JobSeeker("Jeff"));
 
		//Each time, a new job is added, all registered job seekers will get noticed.
		hh.addJob("Google Job");
		hh.addJob("Yahoo Job");
	}
}

Observer pattern in JDK

java.util.EventListener

Swing GUI example

4 thoughts on “Java Design Pattern: Observer”

  1. Following the example: what if there are multiple head-hunters each adding new jobs. A certain head-hunter is sending too many jobs and Jeff doesn’t like that. You can implement it simply by:
    if (this.name.equals(“Jeff”) && ((HeadHunter)s).getJobs().size() > 10) { s.removeObserver(this); }
    Now, imagine you don’t have access to the subject, only the jobs list. How would you know who sent it to you and tell them to shut up?

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