Java Observable addObserver() 方法

描述

Java Observable addObserver(Observer o) 方法将指定的观察者 o 添加到此对象的观察者集合中，前提是该观察者与集合中已有的观察者不同。

声明

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

public void addObserver(Observer o)

参数

o − 要添加的观察者。

返回值

无

异常

NullPointerException − 如果指定的观察者为 null。

为字符串值更改添加观察者示例

以下示例演示了 java.util.Observable.addObserver(Observer) 方法的用法。我们通过扩展 Observable 类创建了一个 ObservedObject 类，然后重写了其 setValue() 方法。在主类中，我们使用 addObserver() 方法添加了观察者，并在输出中打印了 ObservedObject 值的更新情况。

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);
   }
}

输出

让我们编译并运行上述程序，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

为整数值更改添加观察者示例

以下示例演示了 java.util.Observable.addObserver(Observer) 方法的用法。我们通过扩展 Observable 类创建了一个 ObservedObject 类，然后重写了其 setValue() 方法。在主类中，我们使用 addObserver() 方法添加了观察者，并在输出中打印了 ObservedObject 值的更新情况。

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);
   }
}

输出

让我们编译并运行上述程序，这将产生以下结果：

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

为对象值更改添加观察者示例

以下示例演示了 java.util.Observable.addObserver(Observer) 方法的用法。我们通过扩展 Observable 类创建了一个 ObservedObject 类，然后重写了其 setValue() 方法。在主类中，我们使用 addObserver() 方法添加了观察者，并在输出中打印了 ObservedObject 值的更新情况。

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 + " ]";
   }
}

输出

让我们编译并运行上述程序，这将产生以下结果：

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