Java Observable countObservers() Method

Description

The Java Observable countObservers() method returns the number of observers of this Observable object.

Declaration

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

public int countObservers()

Parameters

Return Value

This method returns the number of observers of this object.

Exception

Counting Observers for String Value Change Example

The following example shows the usage of java.util.Observable.countObservers() 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 countObservers() method, we're counting the Observers added 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");

      // add observer to the watched object
      watched.addObserver(watcher1);
      watched.addObserver(watcher2);
      
      // count observers
      int count = watched.countObservers();
      System.out.println(count+" observers listening");
      
      // trigger value change
      watched.setValue("New Value");
      
      // remove first observer
      watched.deleteObserver(watcher1);
      count = watched.countObservers();
      System.out.println(count+" observers listening");
      
      // trigger value change
      watched.setValue("Latest Value");
   }
   
   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 −

2 observers listening
Value changed to new value: New Value
Watcher2 called with Arguments: New Value
Watcher1 called with Arguments: New Value
1 observers listening
Value changed to new value: Latest Value
Watcher2 called with Arguments: Latest Value

Counting Observers for an 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");

      // add observer to the watched object
      watched.addObserver(watcher1);
      watched.addObserver(watcher2);
      
      // count observers
      int count = watched.countObservers();
      System.out.println(count+" observers listening");
      
      // trigger value change
      watched.setValue(2);
      
      // remove first observer
      watched.deleteObserver(watcher1);
      count = watched.countObservers();
      System.out.println(count+" observers listening");
      
      // trigger value change
      watched.setValue(3);
   }
   
   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 −

2 observers listening
Value changed to new value: 2
Watcher2 called with Arguments: 2
Watcher1 called with Arguments: 2
1 observers listening
Value changed to new value: 3
Watcher2 called with Arguments: 3

Counting Observers for an 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");

      // add observer to the watched object
      watched.addObserver(watcher1);
      watched.addObserver(watcher2);
      
      // count observers
      int count = watched.countObservers();
      System.out.println(count+" observers listening");
      
      // trigger value change
      watched.setValue(new Student(2, "Robert"));
      
      // remove first observer
      watched.deleteObserver(watcher1);
      count = watched.countObservers();
      System.out.println(count+" observers listening");
      
      // trigger value change
      watched.setValue(new Student(3, "Adam"));
   }
   
   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 −

2 observers listening
Value changed to new value: [ 2, Robert ]
Watcher2 called with Arguments: [ 2, Robert ]
Watcher1 called with Arguments: [ 2, Robert ]
1 observers listening
Value changed to new value: [ 3, Adam ]
Watcher2 called with Arguments: [ 3, Adam ]

java_util_observable.htm