Java Java.util.Observable.addObserver()用法及代碼示例

描述

這個java.util.addObserver(Observer o)方法添加指定的觀察者o到集合observers對於這個對象，如果觀察者與集合中已有的觀察者不同。

聲明

以下是聲明Java.util.Observable.addObserver()方法

public void addObserver(Observer o)

參數

o- 要添加的觀察者。

返回值

NA

異常

NullPointerException- 如果指定的觀察者是null。

示例

下麵的例子展示了 java.util.Observable.addObserver(Observer) 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 static void main(String[] args) {
      
      // create watched and watcher objects
      ObservedObject watched = new ObservedObject("Original Value");
      // watcher object listens to object change
      ObservableDemo watcher = new ObservableDemo();

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

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

      // trigger value change
      watched.setValue("Latest Value");
   }
      
   public void update(Observable obj, Object arg) {
      System.out.println("Update called with Arguments:"+arg);
   }
}

讓我們編譯並運行上麵的程序，這將產生以下結果 -

Value changed to new value:New Value
Value changed to new value:Latest Value
Update called with Arguments:Latest Value