Java.util.Observable.countObservers() 方法

package com.codingdict;

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

class ObservedObject extends Observable {
   private String watchedValue;

   public ObservedObject(String value) {
      watchedValue = value;
   }

   public void setValue(String value) {

      // if value has changed notify observers
      if(!watchedValue.equals(value)) {
         System.out.println("Value changed to new value: "+value);
         watchedValue = value;

         // mark as value changed
         setChanged();

         // trigger notification
         notifyObservers(value);
      }
   }
}
public class ObservableDemo implements Observer {
   public String name;

   public ObservableDemo(String name) {
      this.name = name;
   }

   public static void main(String[] args) {

      // create watched and watcher objects
      ObservedObject watched = new ObservedObject("Original Value");

      // watcher object listens to object change
      ObservableDemo watcher1 = new ObservableDemo("Watcher1");
      ObservableDemo watcher2 = new ObservableDemo("Watcher2");

      // add observer to the watched object
      watched.addObserver(watcher1);
      watched.addObserver(watcher2);

      // count observers
      int count = watched.countObservers();
      System.out.println(count+" observers listening");

      // trigger value change
      watched.setValue("New Value");

      // remove first observer
      watched.deleteObserver(watcher1);
      count = watched.countObservers();
      System.out.println(count+" observers listening");

      // trigger value change
      watched.setValue("Latest Value");
   }

   public void update(Observable obj, Object arg) {
      System.out.println(name+" called with Arguments: "+arg);
   }
}