How to get a rectangle to stop when you release a key that moves it?

Question

I\'ve created a JFrame with a rectangle in the center of it that moves when I press certain keys. It\'s all nice and dandy, but the rectangle keeps going when I release the keys

Answer 1

• Avoid KeyListener, seriously, they are more trouble than they are worth, use the key bindings API instead. How to Use Key Bindings

There are a number of ways you might achieve this. One of the better way is to use a indirect approach. That is, the user presses a key and you raise a flag to indicate which is pressed, they release the key, you reset the flag, indicating that the key is no longer pressed.

You then use some kind of update loop to change the position of the object based which keys are currently active.

But why go to so much hassle I hear you ask. When a user presses a key, they is a short delay between the first key press and repeated key notification (while the key is down, the OS will send you key events until it is released), this makes the movement look a little "staggered".

Instead, we raise flag and use a constant update loop to make changes to the state of the object based on the state of the flags, which smooths out the key events, for example...

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import javax.swing.AbstractAction;
import javax.swing.Action;
import javax.swing.ActionMap;
import javax.swing.InputMap;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.KeyStroke;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class Test{

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

    public Test() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public static class TestPane extends JPanel {

        public enum HorizontalMovement {
            NONE,
            LEFT,
            RIGHT
        }

        private HorizontalMovement horizontalMovement = HorizontalMovement.NONE;

        private int xPos = 0;

        public TestPane() {
            addKeyPressedBinding("left.pressed", KeyEvent.VK_LEFT, new MoveHorizontialAction(HorizontalMovement.LEFT));
            addKeyPressedBinding("right.pressed", KeyEvent.VK_RIGHT, new MoveHorizontialAction(HorizontalMovement.RIGHT));
            addKeyReleasedBinding("left.relesed", KeyEvent.VK_LEFT, new MoveHorizontialAction(HorizontalMovement.NONE));
            addKeyReleasedBinding("right.relesed", KeyEvent.VK_RIGHT, new MoveHorizontialAction(HorizontalMovement.NONE));

            Timer timer = new Timer(40, new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    switch (horizontalMovement) {
                        case LEFT:
                            xPos--;
                            break;
                        case RIGHT:
                            xPos++;
                            break;
                    }
                    if (xPos < 0) {
                        xPos = 0;
                    } else if (xPos + 50 > getWidth()) {
                        xPos = getWidth() - 50;
                    }
                    repaint();
                }
            });
            timer.start();

        }

        protected void addKeyPressedBinding(String name, int keyCode, Action action) {
            KeyStroke ks = KeyStroke.getKeyStroke(keyCode, 0, false);
            addKeyBinding(name, ks, action);
        }

        protected void addKeyReleasedBinding(String name, int keyCode, Action action) {
            KeyStroke ks = KeyStroke.getKeyStroke(keyCode, 0, true);
            addKeyBinding(name, ks, action);
        }

        protected void addKeyBinding(String name, KeyStroke ks, Action action) {
            InputMap im = getInputMap(WHEN_IN_FOCUSED_WINDOW);
            ActionMap am = getActionMap();

            im.put(ks, name);
            am.put(name, action);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            Rectangle box = new Rectangle(xPos, (getHeight() - 50) / 2, 50, 50);
            g2d.setColor(Color.BLUE);
            g2d.fill(box);
            g2d.dispose();
        }

        protected void addKeyBinding(String left, int VK_LEFT, MoveHorizontialAction moveHorizontialAction) {
            throw new UnsupportedOperationException("Not supported yet.");
        }

        protected class MoveHorizontialAction extends AbstractAction {

            private HorizontalMovement movement;

            public MoveHorizontialAction(HorizontalMovement movement) {
                this.movement = movement;
            }

            @Override
            public void actionPerformed(ActionEvent e) {
                horizontalMovement = movement;
            }

        }

    }

}