Java Observer and Observable Example
September 17, 2021
On this page we will learn Java Observer and Observable example.
1. The
java.util.Observer is an interface and it must be implemented by the class which needs to be informed about changes in our observable class.
2. The
java.util.Observable class must be extended by the class which is being observed.
3. In
Observer interface, there is a method update() that is called by Observable class.
4. The
Observable class calls its setChanged() method to the change its return value as true.
5. The
notifyObservers() method of Observable class notifies observers about the changes.
Now find the example. In our example news is being observed by two news readers.
Example
ObserverObservableDemo.java
package com.concretepage;
import java.util.Observable;
import java.util.Observer;
// First observer
class FirstNewsReader implements Observer {
public void update(Observable obj, Object arg) {
System.out.println("FirstNewsReader got The news:"+(String)arg);
}
}
//Second Observer
class SecondNewsReader implements Observer {
public void update(Observable obj, Object arg) {
System.out.println("SecondNewsReader got The news:"+(String)arg);
}
}
// This is the class being observed.
class News extends Observable {
void news() {
String[] news = {"News 1", "News 2", "News 3"};
for(String s: news){
//set change
setChanged();
//notify observers for change
notifyObservers(s);
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
System.out.println("Error Occurred.");
}
}
}
}
//Run Observer and Observable
public class ObserverObservableDemo {
public static void main(String args[]) {
News observedNews = new News();
FirstNewsReader reader1 = new FirstNewsReader();
SecondNewsReader reader2 = new SecondNewsReader();
observedNews.addObserver(reader1);
observedNews.addObserver(reader2);
observedNews.news();
}
}
SecondNewsReader got The news:News 1 FirstNewsReader got The news:News 1 SecondNewsReader got The news:News 2 FirstNewsReader got The news:News 2 SecondNewsReader got The news:News 3 FirstNewsReader got The news:News 3
Observer interface and Observable class have been deprecated.
References
Interface ObserverClass Observable
