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package com.haizhitao.observer;public interface Watched{public void addWatcher(Watcher watcher);public void removeWatcher(Watcher watcher);public void notifyWatcher(String str);}package com.haizhitao.observer;public interface Watcher{public void update(String str);}package com.haizhitao.observer;import java.util.ArrayList;import java.util.List;public class ConcreteWatched implements Watched{private List<Watcher> list = new ArrayList<Watcher>();@Overridepublic void addWatcher(Watcher watcher){list.add(watcher);}@Overridepublic void removeWatcher(Watcher watcher){list.remove(watcher);}@Overridepublic void notifyWatcher(String str){for(Watcher watcher : list){watcher.update(str);}}}package com.haizhitao.observer;public class ConcreteWatcher implements Watcher{@Overridepublic void update(String str){System.out.println(str);}}package com.haizhitao.observer;public class Test{public static void main(String[] args){ConcreteWatched girl = new ConcreteWatched();ConcreteWatcher watcher1 = new ConcreteWatcher();ConcreteWatcher watcher2 = new ConcreteWatcher();ConcreteWatcher watcher3 = new ConcreteWatcher();girl.addWatcher(watcher1);girl.addWatcher(watcher2);girl.addWatcher(watcher3);girl.notifyWatcher("hello! How are you!");girl.removeWatcher(watcher2);girl.notifyWatcher("welcome");}}