Java Observable notifyObservers() 方法

描述

Java Observable notifyObservers() 方法通知所有觀察者物件已更改。此方法之後呼叫clearChanged方法，以指示此物件不再更改。此方法等效於notifyObservers(null)。

宣告

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

public void notifyObservers()

引數

無

返回值

無

異常

無

針對字串值更改示例的通知觀察者

以下示例演示了 java.util.Observable.notifyObservers() 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() 方法並使用 notifyObservers() 方法，通知觀察者。在主類中，我們使用 addObserver() 方法添加了觀察者，在輸出中，我們列印了 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();
      }
   }
}
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 watcher = new ObservableDemo("Watcher");

      // add observer to the watched object
      watched.addObserver(watcher);
      
      // trigger value change
      watched.setValue("New Value");
   }
      
   public void update(Observable obj, Object arg) {
      System.out.println("Update called");
   }
}

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
Update called

針對整數值更改示例的通知觀察者

以下示例演示了 java.util.Observable.notifyObservers() 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() 方法並使用 notifyObservers() 方法，通知觀察者。在主類中，我們使用 addObserver() 方法添加了觀察者，在輸出中，我們列印了 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();
      }
   }
}
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(1);
      
      // watcher object listens to object change
      ObservableDemo watcher = new ObservableDemo("Watcher");

      // add observer to the watched object
      watched.addObserver(watcher);
      
      // trigger value change
      watched.setValue(2);
   }
      
   public void update(Observable obj, Object arg) {
      System.out.println("Update called");
   }
}

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
Update called

針對物件值更改示例的通知觀察者

以下示例演示了 java.util.Observable.notifyObservers() 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() 方法並使用 notifyObservers() 方法，通知觀察者。在主類中，我們使用 addObserver() 方法添加了觀察者，在輸出中，我們列印了 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();
      }
   }
}
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(new Student(1, "Julie"));
      
      // watcher object listens to object change
      ObservableDemo watcher = new ObservableDemo("Watcher");

      // add observer to the watched object
      watched.addObserver(watcher);
      
      // trigger value change
      watched.setValue(new Student(2, "Robert"));
   }
      
   public void update(Observable obj, Object arg) {
      System.out.println("Update called");
   }
}
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 ]
Update called