Java 观察者模式，也可以叫做Publish/Subscribe模式。
简单理解，多个Observer可以订阅一个Subject，如果Subject有什么更新，通知Observer。

package cn.true123.observer;

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

public class TestObserver {
    interface IObserver {
        void update(String text);
    }

    interface ISubject {
        public void register(IObserver iObserver);

        public void unRegister(IObserver iObserver);

        public void notifyObservers();

        public void setValue(String value);
    }

    class Observer1 implements IObserver {

        @Override
        public void update(String text) {
            System.out.println("Observer1 receive the message: " + text);
        }

    }

    class Observer2 implements IObserver {

        @Override
        public void update(String text) {
            System.out.println("Observer2 receive the message: " + text);
        }

    }

    class Subject implements ISubject {
        private List<IObserver> observers = new ArrayList<>();
        private String value;

        @Override
        public void register(IObserver iObserver) {
            observers.add(iObserver);
        }

        @Override
        public void unRegister(IObserver iObserver) {
            if (observers.contains(iObserver)) {
                observers.remove(iObserver);
            }
        }

        @Override
        public void notifyObservers() {
            for (IObserver iObserver : observers) {
                iObserver.update(value);
            }
        }

        @Override
        public void setValue(String value) {
            this.value = value;
        }

    }

    public static void main(String[] args) {
        TestObserver testObserver = new TestObserver();
        ISubject subject = testObserver.new Subject();
        IObserver iObserver1 = testObserver.new Observer1();
        IObserver iObserver2 = testObserver.new Observer2();
        subject.register(iObserver1);
        subject.register(iObserver2);
        subject.setValue("news updated");
        subject.notifyObservers();
    }

}

结果：

Observer1 receive the message: news updated
Observer2 receive the message: news updated