标题：Java观察者模式

观察者模式在对象之间存在一对多关系时使用，例如，如果一个对象被修改，它的依赖对象将被自动通知。 观察者模式属于行为模式类别。

实现实例

观察者模式使用三个操作类- Subject, Observer 和 Client。Subject是具有将观察者附加和分离到Client对象的方法的对象。创建了一个抽象类Observer和一个具体类Subject，它们都是扩展了 Observer 类。

ObserverPatternDemo这是一个演示类，将使用 Subject 和具体类对象在操作中显示观察者模式。

观察者模式示例的结构如下图所示 -

第1步

创建一个 Subject 类，如下代码所示 -
Subject.java

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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();
      }
   }     
}
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();
      }
   }     
}
Java

第2步

创建一个 Observer 类，如下代码所示 -
Observer.java

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

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

第3步

创建具体的观察者类，如下代码所示 -
BinaryObserver.java

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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() ) ); 
   }
}
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() ) ); 
   }
}
Java

OctalObserver.java

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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() ) ); 
   }
}
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() ) ); 
   }
}
Java

HexaObserver.java

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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() ); 
   }
}
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() ); 
   }
}
Java

第4步

使用 Subject 和具体的观察者对象。
ObserverPatternDemo.java

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public class ObserverPatternDemo {
   public static void main(String[] args) {
      Subject subject = new Subject();
 
      new HexaObserver(subject);
      new OctalObserver(subject);
      new BinaryObserver(subject);
 
      System.out.println("First state change: 15");    
      subject.setState(15);
      System.out.println("Second state change: 10");    
      subject.setState(10);
   }
}
Java

public class ObserverPatternDemo {
   public static void main(String[] args) {
      Subject subject = new Subject();

      new HexaObserver(subject);
      new OctalObserver(subject);
      new BinaryObserver(subject);

      System.out.println("First state change: 15");    
      subject.setState(15);
      System.out.println("Second state change: 10");    
      subject.setState(10);
   }
}
Java

第5步

验证输出，执行上面的代码得到以下结果 -

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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
Java

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
Java