What is the difference between listeners and adapters?

Question

I'm trying to differentiate between listeners and adapters.

Are they pretty much the same but in listeners you have to implement all the methods in the interface, but with adapters you have an option to only implement the methods you need so the code is cleaners and easier to read?

I also got told that adapters enable instantiation with only one implementation and you can't instantiate listeners, I don't fully understand this.

Can someone please explain which one is better to use and things you can do with one but you can't with the other?

Answer 1

WindowListener is interface which force you to override all of the methods, while WindowAdapter is implementation of WindowListener and you only need to override the method(s) that you interest to deal with.

WindowListener is interface which mean you cant instantiation the WindowListener, while WindowAdapter is concrete class that you can use new operator to instantiation.

When you use WindowAdapter, the code is more clean where your class only override the method(s) that you want. For example:

WindowListener

public class CloseListener implements WindowListener {
    // im not interest  on this event, but still need to override it
    @Override
    public void windowOpened(WindowEvent e) {

    }
    // im not interest  on this event, but still need to override it    
    @Override
    public void windowClosing(WindowEvent e) {

    }

    @Override
    public void windowClosed(WindowEvent e) {
        System.exit(0);

    }
    // im not interest  on this event, but still need to override it    
    @Override
    public void windowIconified(WindowEvent e) {

    }
    // im not interest  on this event, but still need to override it
    @Override
    public void windowDeiconified(WindowEvent e) {

    }

}

WindowAdapter

While using adapter the code is cleaner:

// at JFrame class
addWindowListener(new CloseListener());

// reusable Close Listener
public class CloseListener extends WindowAdapter {
    @Override
    public void windowClosed(WindowEvent e) {
        System.exit(0);
    }
}

Or

addWindowListener(new WindowAdapter() {
    @Override
    public void windowClosed(WindowEvent e) {
         System.exit(0);
     }
});

So I would recommend to use WindowAdapter, but not must follow. However, two of the API about the same just that WindowAdapter exists as convenience for creating listener objects.

EDIT:

Since WindowListener is interface, you can implement it at your JFrame subclass.

public class MainWindow extends JFrame implements WindowListener {
    // this is ok
}
public class MainWindow extends JFrame, WindowAdapter {
    // this is not allow
}

But you cant do it with WindowAdapter.