Java Observable 的 hasChanged() 方法

描述

Java Observable 的 hasChanged() 方法用于判断此对象是否已更改。

声明

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

public boolean hasChanged()

参数

无

返回值

无

异常

无

检查字符串值观察对象的更改示例

以下示例演示了 java.util.Observable.hasChanged() 方法的使用。我们通过扩展 Observable 类创建了一个 ObservedObject 类，并覆盖了它的 setValue() 方法。在主类中，我们使用 addObserver() 方法添加了观察者。然后使用 hasChanged()，我们打印了 ObservedObject 的值发生的更新。

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");
   }
}

输出

让我们编译并运行上述程序，这将产生以下结果：

setValue method called...
Value changed

检查整数值观察对象的更改示例

以下示例演示了 java.util.Observable.hasChanged() 方法的使用。我们通过扩展 Observable 类创建了一个 ObservedObject 类，并覆盖了它的 setValue() 方法。在主类中，我们使用 addObserver() 方法添加了观察者。然后使用 hasChanged()，我们打印了 ObservedObject 的值发生的更新。

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");
   }
}

输出

让我们编译并运行上述程序，这将产生以下结果：

setValue method called...
Value changed

检查对象值观察对象的更改示例

以下示例演示了 java.util.Observable.hasChanged() 方法的使用。我们通过扩展 Observable 类创建了一个 ObservedObject 类，并覆盖了它的 setValue() 方法。在主类中，我们使用 addObserver() 方法添加了观察者。然后使用 hasChanged()，我们打印了 ObservedObject 的值发生的更新。

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 + " ]";
   }
}

输出

让我们编译并运行上述程序，这将产生以下结果：

setValue method called...
Value changed