Java Observable addObserver() 方法

描述

Java Observable addObserver(Observer o) 方法將指定的觀察者 o 新增到此物件的觀察者集合中，前提是該觀察者與集合中已有的觀察者不同。

宣告

以下是 Java.util.Observable.addObserver() 方法的宣告：

public void addObserver(Observer o)

引數

o − 要新增的觀察者。

返回值

無

異常

NullPointerException − 如果指定的觀察者為 null。

為字串值更改新增觀察者示例

以下示例演示了 java.util.Observable.addObserver(Observer) 方法的用法。我們透過擴充套件 Observable 類建立了一個 ObservedObject 類，然後重寫了其 setValue() 方法。在主類中，我們使用 addObserver() 方法添加了觀察者，並在輸出中列印了 ObservedObject 值的更新情況。

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);
   }
}

輸出

讓我們編譯並執行上述程式，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

為整數值更改新增觀察者示例

以下示例演示了 java.util.Observable.addObserver(Observer) 方法的用法。我們透過擴充套件 Observable 類建立了一個 ObservedObject 類，然後重寫了其 setValue() 方法。在主類中，我們使用 addObserver() 方法添加了觀察者，並在輸出中列印了 ObservedObject 值的更新情況。

package com.tutorialspoint;

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

class ObservedObject extends Observable {
   private Integer watchedValue;
   
   public ObservedObject(Integer value) {
      watchedValue = value;
   }
      
   public void setValue(Integer 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(1);
      // watcher object listens to object change
      ObservableDemo watcher = new ObservableDemo();

      // trigger value change
      watched.setValue(2);

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

      // trigger value change
      watched.setValue(3);
   }
      
   public void update(Observable obj, Object arg) {
      System.out.println("Update called with Arguments: "+arg);
   }
}

輸出

讓我們編譯並執行上述程式，這將產生以下結果：

Value changed to new value: 2
Value changed to new value: 3
Update called with Arguments: 3

為物件值更改新增觀察者示例

以下示例演示了 java.util.Observable.addObserver(Observer) 方法的用法。我們透過擴充套件 Observable 類建立了一個 ObservedObject 類，然後重寫了其 setValue() 方法。在主類中，我們使用 addObserver() 方法添加了觀察者，並在輸出中列印了 ObservedObject 值的更新情況。

package com.tutorialspoint;

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

class ObservedObject extends Observable {
   private Student watchedValue;
   
   public ObservedObject(Student value) {
      watchedValue = value;
   }
      
   public void setValue(Student 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(new Student(1, "Julie"));
      // watcher object listens to object change
      ObservableDemo watcher = new ObservableDemo();

      // trigger value change
      watched.setValue(new Student(2, "Robert"));

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

      // trigger value change
      watched.setValue(new Student(3, "Adam"));
   }
      
   public void update(Observable obj, Object arg) {
      System.out.println("Update called with Arguments: "+arg);
   }
}

class Student {
   int rollNo;
   String name;

   Student(int rollNo, String name){
      this.rollNo = rollNo;
      this.name = name;
   }

   @Override
   public String toString() {
      return "[ " + this.rollNo + ", " + this.name + " ]";
   }
}

輸出

讓我們編譯並執行上述程式，這將產生以下結果：

Value changed to new value: [ 2, Robert ]
Value changed to new value: [ 3, Adam ]
Update called with Arguments: [ 3, Adam ]