import java.util.ArrayList;

abstract class Subject {
    private ArrayList<Observer>observers;
    public Subject() {
        observers = new ArrayList<>();
    }
    public void attach(Observer observer){
        observers.add(observer);
    }
    public void detach(Observer observer){
        observers.remove(observer);
    }
    public void notifyObserver(int state){
        for (Observer o: observers){
            o.update(state);
        }
    }
}

class ConcreteSubject extends Subject{
    private int subjectState;
    public int getState(){
        return subjectState;
    }
    public void setState(int newState){
        subjectState = newState;
        System.out.println("subject' state change to "+newState);
        this.notifyObserver(subjectState);
    }
}

interface Observer {
    public void update(int state);
}

class ConcreteObserver1 implements Observer {
    public void update(int state){
        System.out.println("ConcreteObserver1 know state had changed to"+state);
        doSomething();
    }
    private void doSomething() {
        System.out.println("ConcreteObserver1 do something!!!");
    }
}

class ConcreteObserver2 implements Observer {
    public void update(int state){
        System.out.println("ConcreteObserver2 know state had changed to"+state);
        doSomething();
    }
    private void doSomething() {
        System.out.println("ConcreteObserver2 do something!!!");
    }
}

class ConcreteObserver3 implements Observer {
    public void update(int state){
        System.out.println("ConcreteObserver3 know state had changed to"+state);
        doSomething();
    }
    private void doSomething() {
        System.out.println("ConcreteObserver3 do something!!!");
    }
}

public class Client {
    public static void main(String[] args){
        ConcreteSubject subject = new ConcreteSubject();
        Observer o1 = new ConcreteObserver1();
        Observer o2 = new ConcreteObserver2();
        Observer o3 = new ConcreteObserver3();

        subject.attach(o1);
        subject.attach(o2);
        subject.attach(o3);

        subject.setState(11);
        subject.detach(o2);

        subject.setState(12);
    }
}