Behavioral Patterns : Observer

Properties:

-          When object state changes, lets interested parties know about the changes.

-          As the name implies observers, observe the object in question for state changes.

-          Observers need to register to the object for receiving notifications when state change occurs.

Implementation:

-          Observer interface declares methods to get updates .

-          Observable interface declares methods to register, de-register for state changes.

-          Concrete Observable class implements the Observable interface and responsible for notifying oberservers about stage changes.

Java Standard Library Implementations:

-          java.util.EventListener

-          javax.util.Observable

Example Usage:

public class ObserverDemo implements ActionListener {

    private JButton b1, b2;

    public ObserverDemo() {

        JPanel panel = new JPanel();

        b1 = new JButton("Button1");//Observable object

        b2 = new JButton("Button2");//Observable object

    

        //Register for state changes

        b1.addActionListener(this);

        b2.addActionListener(this);

        panel.add(b1);

        panel.add(b2);

    }

   

    // observer method

    public void actionPerformed(ActionEvent e) {

        if (b1.equals(e.getSource())) {

            b2.setEnabled(true);

            b1.setEnabled(false);

        } else {

            b1.setEnabled(true);

            b2.setEnabled(false);

        }

    }

}