java observer pattern (listener)

Java Patterns: observer (listener)

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.

Application.java

public class Application implements ComputerListener {

    private static int counter = 0;
    private int id;

    Application(){ id = ++counter; }

    @Override
    public void onStart() {
        System.out.println("app " + id + " launched...");
    }

    @Override
    public void onStop() {
        System.out.println("app " + id + " saving before comp down...");
    }
}

Computer.java

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

public class Computer {

    List<ComputerListener> listeners = new ArrayList<>();

    void start(){
        System.out.println("started...");
        listeners.forEach(ComputerListener::onStart);
    }

    void stop(){
        System.out.println("comp is going to stop, notifying everyone!");
        listeners.forEach(ComputerListener::onStop);

        System.out.println("computer stopped!");
    }
}

ComputerListener.java

public interface ComputerListener {
    void onStart();
    void onStop();
}

Main.class

public class Main {

    public static void main(String[] args) {
        Computer computer = new Computer();

        computer.listeners.add(new Application());
        computer.listeners.add(new Application());

        computer.start();
        computer.stop();
    }
}