swing - Java keyboard input - game development

Question

I have a specific "problem" with a game I'm creating for class.

The game is an implementation of "Break it". To move the platform at the bottom I just used a key listener. The problem is that after the first key press there is a short "lag" or "stutter" before the platform starts moving. How could I prevent this to get a smooth response? Is there another way than KeyListener? KeyBindings?

Here is the key listener implementation

private class KeyControl implements KeyListener {

    private int dx = 20;

    public void keyPressed(KeyEvent e) {
        if(e.getKeyCode() == KeyEvent.VK_RIGHT) {
            if(dx < 0 )
                dx = -dx;
            gamePanel.movePlatform(dx);
        }

        if(e.getKeyCode() == KeyEvent.VK_LEFT) {
            if(dx > 0 )
                dx = -dx;
            gamePanel.movePlatform(dx);
        }

        if(e.getKeyCode() == KeyEvent.VK_SPACE) {
            System.out.println("space");
            gamePanel.play();
        }

        if(e.getKeyCode() == KeyEvent.VK_ESCAPE) {
            gamePanel.pause();
        }

    }
}

and here is the method that moves the platform around

public void movePlatform(int dx) {
        int nextDX = dx;

        if(paused || init) {
            dx = 0;
        }

        // make sure platform doesnt exceed right border
        if(platform.getX() + platform.getWidth()  + dx> size.getWidth()) {
            if(nextDX < 0)
                dx = nextDX;
            else
                dx = 0;
        }

        // make sure platform doesnt exceed left border
        if(platform.getX() + dx <= 0) {
            if(nextDX > 0)
                dx = nextDX;
            else
                dx = 0;
        }

        platform.setFrame(platform.getX() + dx, platform.getY(), platform.getWidth(), platform.getHeight());
        platformIntervalX = new Interval((int)platform.getX(), (int)(platform.getX() + platform.getWidth()));
        platformIntervalY = new Interval((int)(platform.getY() - platform.getHeight()), (int)platform.getY());
        repaint();
    }

Answer 1

The solution is not to use the KeyListener's key press for moving your sprite. The key is not to rely on the hardware-specific key press frequency, to use a Swing Timer to create your own frequency. Instead use Key Bindings and a Swing Timer. Start the time on key press and stop it on key release.

For example, run this code and press and release the up-arrow key:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;

import javax.swing.*;

@SuppressWarnings("serial")
public class KeyBindingEg extends JPanel {
   private static final String UP_KEY_PRESSED = "up key pressed";
   private static final String UP_KEY_RELEASED = "up key released";
   private static final int UP_TIMER_DELAY = 50;
   private static final Color FLASH_COLOR = Color.red;

   private Timer upTimer;
   private JLabel label = new JLabel();

   public KeyBindingEg() {
      label.setFont(label.getFont().deriveFont(Font.BOLD, 32));
      label.setOpaque(true);
      add(label);

      setPreferredSize(new Dimension(400, 300));

      int condition = WHEN_IN_FOCUSED_WINDOW;
      InputMap inputMap = getInputMap(condition);
      ActionMap actionMap = getActionMap();
      KeyStroke upKeyPressed = KeyStroke.getKeyStroke(KeyEvent.VK_UP, 0, false);
      KeyStroke upKeyReleased = KeyStroke.getKeyStroke(KeyEvent.VK_UP, 0, true);

      inputMap.put(upKeyPressed, UP_KEY_PRESSED);
      inputMap.put(upKeyReleased, UP_KEY_RELEASED);

      actionMap.put(UP_KEY_PRESSED, new UpAction(false));
      actionMap.put(UP_KEY_RELEASED, new UpAction(true));

   }

   private class UpAction extends AbstractAction {
      private boolean onKeyRelease;

      public UpAction(boolean onKeyRelease) {
         this.onKeyRelease = onKeyRelease;
      }

      @Override
      public void actionPerformed(ActionEvent evt) {
         if (!onKeyRelease) {
            if (upTimer != null && upTimer.isRunning()) {
               return;
            }
            System.out.println("key pressed");
            label.setText(UP_KEY_PRESSED);

            upTimer = new Timer(UP_TIMER_DELAY, new ActionListener() {

               @Override
               public void actionPerformed(ActionEvent e) {
                  Color c = label.getBackground();
                  if (FLASH_COLOR.equals(c)) {
                     label.setBackground(null);
                     label.setForeground(Color.black);
                  } else {
                     label.setBackground(FLASH_COLOR);
                     label.setForeground(Color.white);
                  }
               }
            });
            upTimer.start();
         } else {
            System.out.println("Key released");
            if (upTimer != null && upTimer.isRunning()) {
               upTimer.stop();
               upTimer = null;
            }
            label.setText("");
         }
      }

   }

   private static void createAndShowGui() {
      KeyBindingEg mainPanel = new KeyBindingEg();

      JFrame frame = new JFrame("KeyBindingEg");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

---

Edit

Or a better example, one that moves a sprite in any direction based on a key press of one of the arrow keys. No delay encountered:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.awt.image.BufferedImage;
import java.util.EnumMap;
import java.util.Map;

import javax.swing.*;

@SuppressWarnings("serial")
public class KeyBindingEg2 extends JPanel {
   enum Dir {
      LEFT("Left", KeyEvent.VK_LEFT, -1, 0),
      RIGHT("Right", KeyEvent.VK_RIGHT, 1, 0),
      UP("Up", KeyEvent.VK_UP, 0, -1),
      DOWN("Down", KeyEvent.VK_DOWN, 0, 1);

      private String name;
      private int keyCode;
      private int deltaX;
      private int deltaY;
      private Dir(String name, int keyCode, int deltaX, int deltaY) {
         this.name = name;
         this.keyCode = keyCode;
         this.deltaX = deltaX;
         this.deltaY = deltaY;
      }
      public String getName() {
         return name;
      }
      public int getKeyCode() {
         return keyCode;
      }
      public int getDeltaX() {
         return deltaX;
      }
      public int getDeltaY() {
         return deltaY;
      }      
   }
   public static final int TIMER_DELAY = 10;
   public static final int DELTA_X = 2;
   public static final int DELTA_Y = DELTA_X;
   public static final int SPRITE_WIDTH = 10;
   public static final int SPRITE_HEIGHT = SPRITE_WIDTH;
   private static final String PRESSED = "pressed";
   private static final String RELEASED = "released";
   private static final int PREF_W = 800;
   private static final int PREF_H = 650;
   private Map<Dir, Boolean> dirMap = new EnumMap<>(Dir.class);
   private int spriteX = 0;
   private int spriteY = 0;
   private BufferedImage sprite;
   private Timer animationTimer = new Timer(TIMER_DELAY, new AnimationListener());

   public KeyBindingEg2() {
      for (Dir dir : Dir.values()) {
         dirMap.put(dir, Boolean.FALSE);
      }
      sprite = createSprite();
      setKeyBindings();
      animationTimer.start();
   }

   private BufferedImage createSprite() {
      BufferedImage sprt = new BufferedImage(SPRITE_WIDTH, SPRITE_HEIGHT, BufferedImage.TYPE_INT_ARGB);
      Graphics g = sprt.getGraphics();
      g.setColor(Color.RED);
      g.fillRect(0, 0, SPRITE_WIDTH, SPRITE_HEIGHT);
      g.dispose();
      return sprt;
   }

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

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      if (sprite != null) {
         g.drawImage(sprite, spriteX, spriteY, this);
      }
   }

   private void setKeyBindings() {
      int condition = WHEN_IN_FOCUSED_WINDOW;
      InputMap inputMap = getInputMap(condition);
      ActionMap actionMap = getActionMap();

      for (Dir dir : Dir.values()) {
         KeyStroke keyPressed = KeyStroke.getKeyStroke(dir.getKeyCode(), 0, false);
         KeyStroke keyReleased = KeyStroke.getKeyStroke(dir.getKeyCode(), 0, true);

         inputMap.put(keyPressed, dir.toString() + PRESSED);
         inputMap.put(keyReleased, dir.toString() + RELEASED);

         actionMap.put(dir.toString() + PRESSED, new DirAction(dir, PRESSED));
         actionMap.put(dir.toString() + RELEASED, new DirAction(dir, RELEASED));
      }

   }

   private class AnimationListener implements ActionListener {
      @Override
      public void actionPerformed(ActionEvent e) {
         int newX = spriteX;
         int newY = spriteY;
         for (Dir dir : Dir.values()) {
            if (dirMap.get(dir)) {
               newX += dir.getDeltaX() * DELTA_X;
               newY += dir.getDeltaY() * DELTA_Y;
            }
         }
         if (newX < 0 || newY < 0) {
            return;
         }
         if (newX + SPRITE_WIDTH > getWidth() || newY + SPRITE_HEIGHT > getHeight()) {
            return;
         }
         spriteX = newX;
         spriteY = newY;
         repaint();         
      }
   }

   private class DirAction extends AbstractAction {

      private String pressedOrReleased;
      private Dir dir;

      public DirAction(Dir dir, String pressedOrReleased) {
         this.dir = dir;
         this.pressedOrReleased = pressedOrReleased;
      }

      @Override
      public void actionPerformed(ActionEvent evt) {
         if (pressedOrReleased.equals(PRESSED)) {
            dirMap.put(dir, Boolean.TRUE);
         } else if (pressedOrReleased.equals(RELEASED)) {
            dirMap.put(dir, Boolean.FALSE);
         }
      }

   }

   private static void createAndShowGui() {
      KeyBindingEg2 mainPanel = new KeyBindingEg2();

      JFrame frame = new JFrame("KeyBindingEg");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}