Java tracking keystrokes with inputmap

Question

I have this problem with my code, I'm trying to learn how to use keystrokes in Java, and i want to be able to track which keystrokes I'm pressing. I'm trying to use KeyEvent.VK_UP to track what I'm pressing.

import java.awt.event.*;
import javax.swing.*;

public class TrackArrows
{
    protected static InputMap inputMap;

    public static void main(String[] arg){
        JPanel panel = new JPanel();

        inputMap = panel.getInputMap();

        panel.getActionMap().put("keys", new AbstractAction() {
            public void actionPerformed(ActionEvent e){
                if(inputMap.get(KeyStroke.getKeyStroke(KeyEvent.VK_UP, 0), true)){//My problem lies here
                    System.out.println("Key pressed up");
                }
                if(inputMap.get(KeyStroke.getKeyStroke(KeyEvent.VK_UP, 0), true)){//And here
                    System.out.println("Key pressed down");
                }
            }
        });

        inputMap.put(KeyStroke.getKeyStroke(KeyEvent.VK_UP, 0), "keys");
        inputMap.put(KeyStroke.getKeyStroke(KeyEvent.VK_DOWN, 0), "keys");

        JFrame frame = new JFrame();
        frame.getContentPane().add(panel);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(20,20);
        frame.setVisible(true);
    }
}

Am i wrong in doing this, or is there another way to do this?

Answer 1

The Action does not have access to the KeyStroke. You need to create a separate Action for each key binding. Something like this:

class SimpleAction extends AbstractAction
{
    public SimpleAction(String name)
    {
            putValue( Action.NAME, "Action " + name );
    }

    public void actionPerformed(ActionEvent e)
    {
        System.out.println( getValue( Action.NAME ) );
    }
}

Then you create the Actions like:

Action up = new SimpleAction("Up");

However you will still have problems because the default InputMap only receives the key events when it has focus and by default a JPanel is not focusable. So you have two options:

a) make the panel focusable:

panel.setFocusable( true );

b) use a different InputMap:

inputMap = panel.getInputMap(JPanel.WHEN_ANCESTOR_OF_FOCUSED_COMPONENT);

The Key Bindings article attempts to simplify some of the key bindings concepts from the Swing tutortial.