java.util.Observable.countObservers() 返回此Observable对象的observers数量。

声明

以下是java.util.Observable.countObservers()方法的声明

public int countObservers()

参数

NA

返回值

这个方法返回当前对象的observers数量。

异常

NA

例子

下面的例子显示java.util.Observable.countObservers()方法的用法。

package com.yiibai;

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);
   }
}

让我们来编译和运行上面的程序，这将产生以下结果：

2 observers listening
Value changed to new value: New Value
Watcher2 called with Arguments: New Value
Watcher1 called with Arguments: New Value
1 observers listening
Value changed to new value: Latest Value
Watcher2 called with Arguments: Latest Value