ISubject

AbstractSubject

SubjectA

IObserver

package Observer;public interface IObserver {    void update(String changeMsg);}

ObserverA

package Observer;public class ObserverA  implements IObserver{    @Override    public void update(String changeMsg) {        System.out.println("observerA recieves the information:" + changeMsg);            }}

ObserverB

package Observer;public class ObserverB implements IObserver{    @Override    public void update(String changeMsg) {        System.out.println("observerB recieves the information:" + changeMsg);    }}

ObserverC

package Observer;public class ObserverC implements IObserver{    @Override    public void update(String changeMsg) {        System.out.println("observerC recieved the information:" + changeMsg);    }}

Test.java

public static void main(String[] args)    {        SubjectA sA = new SubjectA();        sA.addObserver(new ObserverA());        sA.addObserver(new ObserverB());        sA.addObserver(new ObserverC());        sA.inform("change my name to cc!");    }

运行结果

JDK已经实现了一套观察者模式，可以直接复用相关代码,主要业务逻辑会被封装在Observer的update方法中。

subject类：java.util.Observable

observer类：java.util.Observer