java语言对观察者模式的支持

12.1 java语言对观察者模式的支持

  

        在java语言的java.util库里面，提供一个Observable类以及Observer接口，构成java语言对观察者模式的支持。

    

         例子代码：

       

import java.util.Observer;
import java.util.Vector;
//java 库中的类，非用户定义的类，可以直接调用
public class Observable {
    private boolean changed = false;
    private Vector obs;
  
    public Observable() {
	obs = new Vector();
    }
    public synchronized void addObserver(Observer o) {
        if (o == null)
            throw new NullPointerException();
	if (!obs.contains(o)) {
	    obs.addElement(o);
	}
    }
    public synchronized void deleteObserver(Observer o) {
        obs.removeElement(o);
    }

    public void notifyObservers() {
	notifyObservers(null);
    }
    public void notifyObservers(Object arg) {
        Object[] arrLocal;
	synchronized (this) {
	    if (!changed)
                return;
            arrLocal = obs.toArray();
            clearChanged();
        }

        for (int i = arrLocal.length-1; i>=0; i--)
            ((Observer)arrLocal[i]).update(this, arg);
    }
    public synchronized void deleteObservers() {
	obs.removeAllElements();
    }
    protected synchronized void setChanged() {
	changed = true;
    }
    protected synchronized void clearChanged() {
	changed = false;
    }
    public synchronized boolean hasChanged() {
	return changed;
    }
    public synchronized int countObservers() {
	return obs.size();
    }
}

 

public interface Observer {

    void update(Observable o, Object arg);
}

 

import java.util.Observable;

public class Watched extends Observable{
    private String data = "";

    public String retrieveData(){
		  return data;
    }

    public void changeData(String data){
        if ( !this.data.equals( data) ) {
            this.data = data;
            setChanged();
        }

        notifyObservers();
    }
}

 

import java.util.Observable;
import java.util.Observer;

public class Watcher implements Observer{
   public Watcher(Watched w){
        w.addObserver(this);
    }

    public void update( Observable ob, Object arg){
        System.out.println("Data has been changed to:  '" + ((Watched)ob).retrieveData() + "'");
    }
}

 

import java.util.Observer;

public class Tester
{
	static private Watched watched;
	static private Observer watcher;

    public static void main(String[] args) {
		watched = new Watched();

        watcher = new Watcher(watched);

        watched.changeData("In C, we create bugs.");
        watched.changeData("In Java, we inherit bugs.");
        watched.changeData("In Java, we inherit bugs.");
        watched.changeData("In Visual Basic, we visualize bugs.");
    }
}