Java Java.util.Observable.countObservers()用法及代码示例

描述

这个java.util.Observable.countObservers()返回此观察者的数量Observable目的。

声明

以下是声明java.util.Observable.countObservers()方法

public int countObservers()

参数

NA

返回值

此方法返回此对象的观察者数量。

异常

NA

示例

下面的例子展示了 java.util.Observable.countObservers() 方法的用法。

package com.tutorialspoint;

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);
   }
}

让我们编译并运行上面的程序，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