Question

How do you use an adapter class in Java?

Answer 1

To use an adapter class in Java, you need to do the following:

1. Create an adapter class that extends the interface you want to implement. An adapter class provides default implementations for all the methods in the interface, which allows you to selectively override only the methods that you need.

2. Override only the methods in the adapter class that you need to implement. If you override all the methods in the adapter class, you're essentially implementing the interface directly, and the adapter class becomes unnecessary.

3. Use an instance of the adapter class in your code and register it as a listener or implementer of the interface.

Here's an example of using an adapter class to handle window events in a Java Swing application:

Suppose you have a class called MyFrame that extends the JFrame class. You want to handle window events on this frame, so you decide to implement the WindowListener interface. However, you're only interested in the windowClosing method of the interface, and don't need to implement the other methods.

To simplify the implementation of the WindowListener interface, you can use the WindowAdapter class, which provides default implementations for all the methods in the WindowListener interface. Here's how you can create an adapter class for the WindowListener interface:

import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;

public class WindowClosingAdapter extends WindowAdapter {
    public void windowClosing(WindowEvent e) {
        // Handle window closing event here
    }
}
 

In this example, we created a class called WindowClosingAdapter that extends the WindowAdapter class and overrides only the windowClosing method of the WindowListener interface. The windowClosing method contains the code to handle the window closing event.

To use the WindowClosingAdapter adapter class in our MyFrame class, we can add an instance of the adapter class to the frame and register it as a window listener using the addWindowListener method. 

Here's how to do this:

public class MyFrame extends JFrame {
    public MyFrame() {
        // Create instance of adapter class
        WindowClosingAdapter windowClosingAdapter = new WindowClosingAdapter();

        // Add window listener to frame
        addWindowListener(windowClosingAdapter);
    }
}
 

In this example, we created an instance of the WindowClosingAdapter adapter class and added it as a window listener to the MyFrame instance using the addWindowListener method. Now, whenever the user closes the frame, the windowClosing method in the WindowClosingAdapter adapter class will be called to handle the event.

Overall, using an adapter class in Java can simplify the implementation of interfaces, by providing default implementations for all the methods in the interface and allowing you to selectively override only the methods that you need. This can help reduce code complexity and save development time.