Question

How to create a subclass of JButton that makes itself the window’s default button?

I understand that designating a default button is set on the JRootPane rather than the button itself. Instead of adding such code to each window, I would like to specify the default button by instantiating a subclass of JButton, “JButton_Default”. The subclass should locate the JRootPane and set itself to be the default button.

I tried doing this in the subclass’ constructor. Unfortunately, this approach works. I suppose that makes sense, as the button under construction is not yet an a form, so it cannot locate the JRootPane.

Is there some other way to program this JButton subclass?

Here’s my subclass that fails:

import javax.swing.*;

public class JButton_Default extends JButton {
    public JButton_Default() {
        super();
        JRootPane pane = this.getRootPane();
        pane.setDefaultButton(this);
    }
}

SOLVED —————-

Here’s the code for a JButton subsclass that makes itself the default button of the window to which it has been added.

import javax.swing.*;

public class JButton_Default extends JButton {

    @Override
    public void addNotify() {  // Upon being added to a window, make this JButton the default button of the window.
        super.addNotify();
        SwingUtilities.getRootPane( this ).setDefaultButton( this );
    }
}

Answer 1

Override the addNotify() method of your JButton class. I believe this method gets invoked when the component is added to a frame. Or if this doesn’t work then add an AncestorListener to the button and listen for the ancestorAdded event.

Now you know the component is added to a top level container so you can get the root pane and set the button as the default.