Observer Pattern Example

ObserverExample.java

import java.util.*;

/*
 * Example of Observer Pattern:
 * 	Student is the model
 * 	View1 and View2 display information of the student
 * 	View3 changes information of the student
 * 	Whenever the student is changed, the views displaying it should be updated
 */

interface Observer {
	public abstract void update();
}

abstract class Subject {
	private List<Observer> observers = new LinkedList<Observer>();

	protected void notifyObserver() {
		for (Observer o : observers)
			o.update();
	}

	public void register(Observer o) {
		observers.add(o);
	}

	public void remove(Observer o) {
		observers.remove(o);
	}
}

class Student extends Subject {
	private int id;
	private String name;
	private String birth;

	public Student(int id, String name, String birth) {
		this.id = id;
		this.name = name;
		this.birth = birth;
	}

	public void setId(int id) {
		this.id = id;
		notifyObserver();
	}

	public int getId() {
		return id;
	}

	public void setName(String name) {
		this.name = name;
		// there's change, notify observers
		notifyObserver();
	}

	public String getName() {
		return name;
	}

	public void setBirth(String birth) {
		this.birth = birth;
		// there's change, notify observers
		notifyObserver();
	}

	public String getBirth() {
		return birth;
	}
}

class View1 implements Observer {
	private Student student;

	public View1(Student s) {
		this.student = s;
	}

	public void update() {
		System.out.println("View1:" + student.getId() + student.getName());
	}
}

class View2 implements Observer {
	private Student student;

	public View2(Student s) {
		this.student = s;
	}

	public void update() {
		System.out.println("View1:" + student.getId() + student.getBirth());
	}
}

class View3 {
	private Student student;

	public View3(Student s) {
		this.student = s;
	}

	public void changeStudent() {
		// change the student
		student.setId(2);
	}
}

public class ObserverExample {
	public static void main(String[] args) {
		// create the subject
		Student s = new Student(1, "A", "1991-01-01");

		// create observers and register them
		Observer o1 = new View1(s);
		Observer o2 = new View2(s);
		s.register(o1);
		s.register(o2);

		// change subject
		View3 v = new View3(s);
		v.changeStudent();
	}
}