java.util.addObserver(Observer o) 方法将指定observer o 到该observers为此对象，如果observer 不同于已经在集合中一些observer。

声明

以下是Java.util.Observable.addObserver()方法的声明

public void addObserver(Observer o)

参数

• o -- 要添加的observer

返回值

NA

异常

• NullPointerException -- 如果指定的 observer 为 null.

例子

下面的示例演示java.util.Observable.addObserver(Observer)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 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);
   }
}

让我们来编译和运行上面的程序，这将产生以下结果：

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