Swing's KeyListener and multiple keys pressed at the same time.

Question

Hi everyone,

is there any conventional way in swing of tracking down the events, when two keyboard keys are pressed at the same time? I have a couple of ideas e.g. remembering the key and event generation time so that we could in a consecutive event handler invocation check the time difference between these two events and decide, whether it's a two-button event or not. Phew. But it looks like a kludge.

Thanks in advance.

Answer 1

The KeyListener interface allows detecting key pressing and releasing separately. Therefore, you can maintain a set of "active keys", i.e. keys which have been pressed but not released yet.

Answer 2

Use a collection to remember which keys are currently pressed and check to see if more than one key is pressed every time a key is pressed.

class MultiKeyPressListener implements KeyListener {

    // Set of currently pressed keys
    private final Set<Character> pressed = new HashSet<Character>();

    @Override
    public synchronized void keyPressed(KeyEvent e) {
        pressed.add(e.getKeyChar());
        if (pressed.size() > 1) {
            // More than one key is currently pressed.
            // Iterate over pressed to get the keys.
        }
    }

    @Override
    public synchronized void keyReleased(KeyEvent e) {
        pressed.remove(e.getKeyChar());
    }

    @Override
    public void keyTyped(KeyEvent e) {/* Not used */ }
}