Question

I’m making a calculator using Swing. So far I have created a GUI that consists of a JFrame with BorderLayout, and in its center I put a JPanel that has a JLabel (representing the screen of the calculator) and some JButtons (representing the keys).

I want the calculator to be able to receive input directly from the keyboard, so I included the method addKeyListener in a class that extends JFrame and I put as an argument a reference to an object of a class that implements KeyListener.

When I run the application it accepts keyboard input until I click on one of the JButtons. After that using the keyboard doesn’t work anymore.

I suspect the problem is about focus, but it is not a problem that I could fix by clicking anywhere on the application. I added the following code:

setFocusable(true);

to the JFrame but it did not help. I have read that using Key Bindings maybe a better option than using a KeyListener, but I am not really sure about this approach.

Answer 1

Swing components are lightweight and use KeyBindings, where KeyListeners are for AWT components which are heavyweight. and known to have focus issues when mixed with Swing components. Thus I suggest changing to KeyBindings (but I see you have heard of them). You would use them something like:

final JButton b=..;

//method to add keybindings to a JComponent ie JButton,JPanel,JTextField etc
private void addKeyBindings(JComponent jc) {

    //not the getInputMap(..) call it could also be JComponent.WHEN_FOCUSED etc
    jc.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke("D"), "D pressed");

    jc.getActionMap().put("D pressed", new AbstractAction() {
        @Override
        public void actionPerformed(ActionEvent ae) {
            //do something when d is pressed
            b.doClick();
        }
    });
}

For general knowledge a very hacky solution would be calling requestFocusInWindow() on the component to which the listeners are attached whenever focus is lost (like after button click etc)