Java.util.Observable.countObservers() Method


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Description

The java.util.Observable.countObservers() returns the number of observers of this Observable object.

Declaration

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

public int countObservers()

Parameters

  • NA

Return Value

This method returns the number of observers of this object.

Exception

  • NA

Example

The following example shows the usage of java.util.Observable.countObservers() method.

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);
   }
}

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

java_util_observable.htm

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