观察者模式

转载自：http://www.tutorialspoint.com/design_pattern/observer_pattern.htm

Observer pattern is used when there is one-to-many relationship between objects such as if one object is modified, its depenedent objects are to be notified automatically. Observer pattern falls under behavioral pattern category.

Implementation

Observer pattern uses three actor classes. Subject, Observer and Client. Subject is an object having methods to attach and detach observers to a client object. We have created an abstract class Observer and a concrete class Subject that is extending class Observer.

ObserverPatternDemo, our demo class, will use Subject and concrete class object to show observer pattern in action.

Step 1

Create Subject class.

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();      }   } }

Step 2

Create Observer class.

Observer.java

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

Step 3

Create concrete observer classes

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() );    }}

Step 4

Use Subject and concrete observer objects.

ObserverPatternDemo.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);   }}

Step 5

Verify the output.

First state change: 15Hex String: FOctal String: 17Binary String: 1111Second state change: 10Hex String: AOctal String: 12Binary String: 1010