interface Observer {
    void update(int data);
}

class NewsPaper {
    private LinkedList<Observer> observers;

    public NewsPaper() {
        observers = new LinkedList<>();
    }

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

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

    public void changeData(int data) {
        for (Observer o : observers) {
            o.update(data);
        }
    }
}

class Reader implements Observer {
    private String name;

    public Reader(NewsPaper paper, String name) {
        this.name = name;
        paper.subscribe(this);
    }

    @Override
    public void update(int data) {
        System.out.println(name + " update data:" + data);
    }
}

public class TestObserve {
    public static void main(String[] args) {
        NewsPaper newsPaper = new NewsPaper();
        Reader tom = new Reader(newsPaper, "tom");
        Reader jack = new Reader(newsPaper, "jack");
        newsPaper.changeData(10);
        System.out.println("--");
        newsPaper.unsubscribe(tom);
        newsPaper.changeData(5);
    }
}