无涯教程-Java 观察者模式观察者(Observer)模式指多个对象间存在一对多的依赖关系，当一个对象的状态发生改变时

观察者(Observer)模式指多个对象间存在一对多的依赖关系，当一个对象的状态发生改变时，所有依赖于它的对象都得到通知并被自动更新。这种模式有时又称作发布-订阅模式、模型-视图模式，它是对象行为型模式，属于行为模式类别。

观察者模式

观察者模式使用三个actor类。Subject，Observer和Client。Subject是一个对象，该对象具有将观察者附加到客户端对象或将其分离的方法。无涯教程创建了一个抽象类 Observer 和一个具体的类 Subject ，该类扩展了 Observer 类。

演示类 ObserverPatternDemo 将使用 Subject 和具体的类对象显示实际的观察者模式。

第1步 - 创建Subject类。

Subject.java

import java.util.ArrayList;
import java.util.List;
public class Subject {
private List<Observer> observers = new ArrayList<Observer>();
private int state;
public int getState() {
return state;
}
public void setState(int state) {
this.state = state;
notifyAllObservers();
}
public void attach(Observer observer){
observers.add(observer);
}
public void notifyAllObservers(){
for (Observer observer : observers) {
observer.update();
}
}
}

第2步 - 创建观察者类。

Observer.java

public abstract class Observer {
   protected Subject subject;
   public abstract void update();
}

第3步 - 创建具体的观察者类

BinaryObserver.java

public class BinaryObserver extends Observer{
public BinaryObserver(Subject subject){
this.subject = subject;
this.subject.attach(this);
}
@Override
public void update() {
System.out.println( "Binary String: " + Integer.toBinaryString( subject.getState() ) );
}
}

OctalObserver.java

public class OctalObserver extends Observer{
public OctalObserver(Subject subject){
this.subject = subject;
this.subject.attach(this);
}
@Override
public void update() {
System.out.println( "Octal String: " + Integer.toOctalString( subject.getState() ) );
}
}

HexaObserver.java

public class HexaObserver extends Observer{
public HexaObserver(Subject subject){
this.subject = subject;
this.subject.attach(this);
}
@Override
public void update() {
System.out.println( "Hex String: " + Integer.toHexString( subject.getState() ).toUpperCase() );
}
}

第4步 - 使用主题和具体的观察者对象。

ObserverPatternDemo.java

public class ObserverPatternDemo {
   public static void main(String[] args) {
      Subject subject = new Subject();
  </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">HexaObserver</span><span class="pun">(</span><span class="pln">subject</span><span class="pun">);</span><span class="pln">
  </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">OctalObserver</span><span class="pun">(</span><span class="pln">subject</span><span class="pun">);</span><span class="pln">
  </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">BinaryObserver</span><span class="pun">(</span><span class="pln">subject</span><span class="pun">);</span><span class="pln">

  </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"First state change: 15"</span><span class="pun">);</span><span class="pln">	
  subject</span><span class="pun">.</span><span class="pln">setState</span><span class="pun">(</span><span class="lit">15</span><span class="pun">);</span><span class="pln">
  </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"Second state change: 10"</span><span class="pun">);</span><span class="pln">	
  subject</span><span class="pun">.</span><span class="pln">setState</span><span class="pun">(</span><span class="lit">10</span><span class="pun">);</span><span class="pln">

}
}

第5步 - 验证输出。

First state change: 15
Hex String: F
Octal String: 17
Binary String: 1111
Second state change: 10
Hex String: A
Octal String: 12
Binary String: 1010

无涯教程-Java 观察者模式

观察者模式

参考链接