Swing 的 KeyListener 和同时按下的多个键

Question

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 eg 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. But it looks like a kludge.

Answer 1

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<Integer> pressedKeys = new HashSet<>();

    @Override
    public synchronized void keyPressed(KeyEvent e) {
        pressed.add(e.getKeyCode());
        Point offset = new Point();
        if (!pressedKeys.isEmpty()) {
            for (Iterator<Integer> it = pressedKeys.iterator(); it.hasNext();) {
                switch (it.next()) {
                    case KeyEvent.VK_W:
                    case KeyEvent.VK_UP:
                        offset.y = -1;
                        break;
                    case KeyEvent.VK_A:
                    case KeyEvent.VK_LEFT:
                        offset.x = -1;
                        break;
                    case KeyEvent.VK_S:
                    case KeyEvent.VK_DOWN:
                        offset.y = 1;
                        break;
                    case KeyEvent.VK_D:
                    case KeyEvent.VK_RIGHT:
                        offset.x = 1;
                        break;
                }
            }
        }
        System.out.println(offset); // Do something with the offset.
    }

    @Override
    public synchronized void keyReleased(KeyEvent e) {
        pressedKeys.remove(e.getKeyCode());
    }

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

Answer 2

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

Answer 3

If after 7 years I tried to do this (just to see if it is possible), someone else might as well...

The code below controls the movement with 8 axis direction, explanation in the comments. But basically, the KeyListener just define where it is possible to move, then a Thread will combine the possible destinations and move the JLabel .

package tests;

import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

import javax.swing.JFrame;
import javax.swing.JLabel;

public class Move8Axis extends JFrame {

    private static final long serialVersionUID = 7722803326073073681L;

    private boolean left = false;
    private boolean up = false;
    private boolean down = false;
    private boolean right = false;

    private JLabel lbl = new JLabel("#");


    public Move8Axis() {
        // Just setting up the window and objects
        setSize(400, 400);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setVisible(true);
        lbl.setBounds(100, 100, 20, 20);
        add(lbl);
        setLocationRelativeTo(null);

        // Key listener, will not move the JLabel, just set where to
        addKeyListener(new KeyListener() {
            @Override
            public void keyTyped(KeyEvent e) {}
            @Override
            public void keyReleased(KeyEvent e) {
                if (e.getKeyCode() == KeyEvent.VK_LEFT) left = false;
                if (e.getKeyCode() == KeyEvent.VK_RIGHT) right = false;
                if (e.getKeyCode() == KeyEvent.VK_UP) up = false;
                if (e.getKeyCode() == KeyEvent.VK_DOWN) down = false;
            }
            @Override
            public void keyPressed(KeyEvent e) {
                if (e.getKeyCode() == KeyEvent.VK_LEFT) left = true;
                if (e.getKeyCode() == KeyEvent.VK_RIGHT) right = true;
                if (e.getKeyCode() == KeyEvent.VK_UP) up = true;
                if (e.getKeyCode() == KeyEvent.VK_DOWN) down = true;
            }
        });

        // This thread will read the 4 variables left/right/up/down at every 30 milliseconds
        // It will check the combination of keys (left and up, right and down, just left, just up...) 
        // And move the label 3 pixels
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {

                    while (true) {  
                        if (left && up) {
                            lbl.setBounds(lbl.getX() - 3, lbl.getY() - 3, 20, 20);
                        } else if (left && down) {
                            lbl.setBounds(lbl.getX() - 3, lbl.getY() + 3, 20, 20);
                        } else if (right && up) {
                            lbl.setBounds(lbl.getX() + 3, lbl.getY() - 3, 20, 20);
                        } else if (right && down) {
                            lbl.setBounds(lbl.getX() + 3, lbl.getY() + 3, 20, 20);
                        } else if (left) {
                            lbl.setBounds(lbl.getX() - 3, lbl.getY(), 20, 20);
                        } else if (up) {
                            lbl.setBounds(lbl.getX(), lbl.getY() - 3, 20, 20);
                        } else if (right) {
                            lbl.setBounds(lbl.getX() + 3, lbl.getY(), 20, 20);
                        } else if (down) {
                            lbl.setBounds(lbl.getX(), lbl.getY() + 3, 20, 20);
                        } 

                        Thread.sleep(30);
                    }

                } catch (Exception ex) {
                    ex.printStackTrace();
                    System.exit(0);
                }
            }
        }).start();
    }

    public static void main(String[] args) {
        new Move8Axis();
    }
}