There are many scenarios for observer mode, such as monitoring dom events of js, such as chat programs, such as the implementation of serlet's listener, such as mail subscription;

Its core: when the state of an object changes, it needs to inform a series of objects to make them react;

There are two ways: 1. Push to the client actively
2. Active pull by client

Here is the main way to realize push

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Specific code

Receive object interface

package observer;

public interface Observer {
	void update(Subject s);

}

Receive object implementation class

package observer;

public class Observer1 implements Observer {
	int state; 
	@Override
	public void update(Subject s) {
		// TODO Auto-generated method stub
		this.setState(((ConcreateObserver)s).getState());

	}
	public int getState() {
		return state;
	}
	public void setState(int state) {
		this.state = state;
	}

}

Broadcaster main class

package observer;

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

public class Subject {
	private List<Observer> list = new ArrayList<Observer>();
	public void registerObserver(Observer o) {
		list.add(o);
	}
	public void removeObserver(Observer o) {
		list.remove(o);
	}
	public void noticeAllObservers() {
		for (Observer observer : list) {
			observer.update(this);
		}
		
	}
}

Broadcaster subclass

package observer;

public class ConcreateObserver extends Subject{
	private int state;
	public int getState() {
		return state;
	}
	public void setState(int state) {
		this.state= state;
		this.noticeAllObservers();
	}
	
}

Client call

package observer;

public class Client {
	public static void main(String[] args) throws CloneNotSupportedException {
		ConcreateObserver concreateObserver= new ConcreateObserver();
		Observer1 o1= new Observer1();
		Observer1 o2= new Observer1();
		Observer1 o3= new Observer1();
		concreateObserver.registerObserver(o1);
		concreateObserver.registerObserver(o2);
		concreateObserver.registerObserver(o3);
		
		concreateObserver.setState(1000);
		
		System.out.println(o1.getState());
		System.out.println(o2.getState());
		System.out.println(o3.getState());
		
		concreateObserver.setState(2000);
		
		System.out.println(o1.getState());
		System.out.println(o2.getState());
		System.out.println(o3.getState());
	}
	
	
}

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Observer pattern of java internal implementation

Broadcasting class

package observer;

import java.util.Observable;

public class ConCreateObserver1 extends  Observable{
	private int state;
	
	public void set(int s) {
		state = s;
		
		
		setChanged();
		
		notifyObservers();
	}

	public int getState() {
		return state;
	}

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

}

Receiving class

package observer;

import java.util.Observable;
import java.util.Observer;

public class Observer2 implements Observer {

	private int myState;
	
	public int getMyState() {
		return myState;
	}
	
	public void setMyState(int myState) {
		this.myState = myState;
	}
	
	@Override
	public void update(Observable o, Object arg) {
		// TODO Auto-generated method stub
		this.setMyState( ((ConCreateObserver1)o).getState());
	}
	

}

Code implementation

package observer;

public class Client {
	public static void main(String[] args) throws CloneNotSupportedException {
//		ConcreateObserver concreateObserver= new ConcreateObserver();
//		Observer1 o1= new Observer1();
//		Observer1 o2= new Observer1();
//		Observer1 o3= new Observer1();
//		concreateObserver.registerObserver(o1);
//		concreateObserver.registerObserver(o2);
//		concreateObserver.registerObserver(o3);
//		
//		concreateObserver.setState(1000);
//		
//		System.out.println(o1.getState());
//		System.out.println(o2.getState());
//		System.out.println(o3.getState());
//		
//		concreateObserver.setState(2000);
//		
//		System.out.println(o1.getState());
//		System.out.println(o2.getState());
//		System.out.println(o3.getState());
		
		ConCreateObserver1 concreateObserver= new ConCreateObserver1();
		Observer2 o1= new Observer2();
		Observer2 o2= new Observer2();
		Observer2 o3= new Observer2();
		concreateObserver.addObserver(o1);
		concreateObserver.addObserver(o2);
		concreateObserver.addObserver(o3);
		
		concreateObserver.set(1000);
		
		System.out.println(o1.getMyState());
		System.out.println(o2.getMyState());
		System.out.println(o3.getMyState());
		
		concreateObserver.set(2000);
		
		System.out.println(o1.getMyState());
		System.out.println(o2.getMyState());
		System.out.println(o3.getMyState());
	}
	
	
}