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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);
        }
    }
}

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