Java Observable deleteObservers() Method

Description

The Java Observable deleteObservers() method clears the observer list. This object will no longer have any observers.

Declaration

Following is the declaration for Java.util.Observable.deleteObservers() method

public void deleteObservers(Observer o)

Parameters

Return Value

Exception

Deleting Observers for String Value Change Example

The following example shows the usage of java.util.Observable.deleteObservers() method. We've created an ObservedObject class by extending Observable class and then overridden its method setValue(). In main class, We've added the observers using addObserver() method. Then using deleteObserver() method, we're deleting Observers added using deleteObservers() method and in output, we've printed the updates happening to the value of ObservedObject.

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 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 watcher1 = new ObservableDemo("Watcher1");
      ObservableDemo watcher2 = new ObservableDemo("Watcher2");

      // trigger value change
      watched.setValue("Value before addObserver");
      
      // add observer to the watched object
      watched.addObserver(watcher1);
      watched.addObserver(watcher2);
      
      // trigger value change
      watched.setValue("Value after addObserver");
      
      // delete all observers
      watched.deleteObservers();
      
      // trigger value change
      watched.setValue("Value after deleteObservers");
   }
   
   public void update(Observable obj, Object arg) {
      System.out.println(name+" called with Arguments: "+arg);
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Value changed to new value: Value before addObserver
Value changed to new value: Value after addObserver
Watcher2 called with Arguments: Value after addObserver
Watcher1 called with Arguments: Value after addObserver
Value changed to new value: Value after deleteObservers

Deleting Observers for Integer Value Change Example

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 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 watcher1 = new ObservableDemo("Watcher1");
      ObservableDemo watcher2 = new ObservableDemo("Watcher2");

      // trigger value change
      watched.setValue(2);
      
      // add observer to the watched object
      watched.addObserver(watcher1);
      watched.addObserver(watcher2);
      
      // trigger value change
      watched.setValue(3);
      
      // delete all observers
      watched.deleteObservers();
      
      // trigger value change
      watched.setValue(4);
   }
   
   public void update(Observable obj, Object arg) {
      System.out.println(name+" called with Arguments: "+arg);
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Value changed to new value: 2
Value changed to new value: 3
Watcher2 called with Arguments: 3
Watcher1 called with Arguments: 3
Value changed to new value: 4

Deleting Observers for Object Value Change Example

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 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 watcher1 = new ObservableDemo("Watcher1");
      ObservableDemo watcher2 = new ObservableDemo("Watcher2");

      // trigger value change
      watched.setValue(new Student(2, "Robert"));
      
      // add observer to the watched object
      watched.addObserver(watcher1);
      watched.addObserver(watcher2);
      
      // trigger value change
      watched.setValue(new Student(3, "Adam"));
      
      // delete all observers
      watched.deleteObservers();
      
      // trigger value change
      watched.setValue(new Student(4, "Michael"));
   }
   
   public void update(Observable obj, Object arg) {
      System.out.println(name+" 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 + " ]";
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Value changed to new value: [ 2, Robert ]
Value changed to new value: [ 3, Adam ]
Watcher2 called with Arguments: [ 3, Adam ]
Watcher1 called with Arguments: [ 3, Adam ]
Value changed to new value: [ 4, Michael ]

java_util_observable.htm