Java Observable 的 hasChanged() 方法

描述

Java Observable 的 hasChanged() 方法用於判斷此物件是否已更改。

宣告

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

public boolean hasChanged()

引數

無

返回值

無

異常

無

檢查字串值觀察物件的更改示例

以下示例演示了 java.util.Observable.hasChanged() 方法的使用。我們透過擴充套件 Observable 類建立了一個 ObservedObject 類，並覆蓋了它的 setValue() 方法。在主類中，我們使用 addObserver() 方法添加了觀察者。然後使用 hasChanged()，我們列印了 ObservedObject 的值發生的更新。

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)) {
         watchedValue = value;

         // mark as value changed
         setChanged();
      }
   }
}
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
      System.out.println("setValue method called...");
      watched.setValue("New Value");
      
      // check if value has changed
      if(watched.hasChanged())
      System.out.println("Value changed");
      else
      System.out.println("Value not changed");
   }
   
   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：

setValue method called...
Value changed

檢查整數值觀察物件的更改示例

以下示例演示了 java.util.Observable.hasChanged() 方法的使用。我們透過擴充套件 Observable 類建立了一個 ObservedObject 類，並覆蓋了它的 setValue() 方法。在主類中，我們使用 addObserver() 方法添加了觀察者。然後使用 hasChanged()，我們列印了 ObservedObject 的值發生的更新。

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)) {
         watchedValue = value;

         // mark as value changed
         setChanged();
      }
   }
}
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
      System.out.println("setValue method called...");
      watched.setValue(2);
      
      // check if value has changed
      if(watched.hasChanged())
         System.out.println("Value changed");
      else
         System.out.println("Value not changed");
   }
   
   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：

setValue method called...
Value changed

檢查物件值觀察物件的更改示例

以下示例演示了 java.util.Observable.hasChanged() 方法的使用。我們透過擴充套件 Observable 類建立了一個 ObservedObject 類，並覆蓋了它的 setValue() 方法。在主類中，我們使用 addObserver() 方法添加了觀察者。然後使用 hasChanged()，我們列印了 ObservedObject 的值發生的更新。

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)) {
         watchedValue = value;

         // mark as value changed
         setChanged();
      }
   }
}
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
      System.out.println("setValue method called...");
      watched.setValue(new Student(2, "Robert"));
      
      // check if value has changed
      if(watched.hasChanged())
         System.out.println("Value changed");
      else
         System.out.println("Value not changed");
   }
   
   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：

setValue method called...
Value changed