java - Breakout Game Multiple Key Handling

Question

i want my rectangle to stop when the user press both left and right key. I researched about multiple key handlings but couldnt manage to find anything.

package View;

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

public class GamePanel extends JPanel {

private final int WIDTH = 600, HEIGHT = 500;

// PROPERTIES

private Timer timer;
int x = 0;
int y = 475;
int velX = 0; // only left or right

// CONSTRUCTOR

public GamePanel() {

    setSize(WIDTH, HEIGHT);
    setBackground(new Color(240, 255, 255));

    timer = new Timer(5, new MyTimerListener());        
    addKeyListener( new MyKeyListener());

    setFocusable(true); // for key listening
    setFocusTraversalKeysEnabled(false); 

    timer.start();
}

// METHODS

@Override 
public void paintComponent(Graphics g) {

    super.paintComponent(g);
    g.setColor( new Color(201, 51, 51));
    g.fillRoundRect(x, y, 80, 20, 15, 15);      
}

public class MyTimerListener implements ActionListener {

    @Override
    public void actionPerformed(ActionEvent e) {

        if ( x < 0 )
        {
            velX = 0;
            x = 0;          
        }

        if ( x > 520 )
        {
            velX = 0;
            x = 520;
        }

        x = x + velX;

        repaint();
    }       
}

public class MyKeyListener implements KeyListener {

    @Override
    public void keyPressed(KeyEvent e) {

        int code = e.getKeyCode();

        if ( code == KeyEvent.VK_LEFT )
            velX = -1;

        if ( code == KeyEvent.VK_RIGHT )
            velX = 1;       
    }
    @Override
    public void keyReleased(KeyEvent e) {

        velX = 0;           
    }
    @Override
    public void keyTyped(KeyEvent e) {}             
}   
}

thanks for heeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeelp

![two muppets]

Answer 1

So again, use Key Bindings and not a KeyListener. You would want to use separate bindings for key press and release for both right and left arrow keys, and will want to have your key binding actions change the state of your program that will effect how the Swing Timer moves the paddle. In this example here (borrowed from another example of mine), I've created an enum, Direction to encapsulate the right and left directions, and have created a Map<Direction, Boolean> that will associate a boolean with both right and left directions. A right arrow key press will change the map to associate the RIGHT direction with a true boolean, and key release would do the opposite. The Timer would poll the Map to see where to move the paddle. For example:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.util.EnumMap;
import java.util.Map;

import javax.swing.*;

@SuppressWarnings("serial")
public class GamePanel2 extends JPanel {
   private static final int PREF_W = 600;
   private static final int PREF_H = 500;
   private static final int PADDLE_Y = 475;
   private static final int PADDLE_W = 80;
   private static final int PADDLE_H = 20;
   private static final int PADDLE_ARC = 15;
   private static final int TIME_DELAY = 15;
   public static final int VEL_X = 2;
   private static final Color PADDLE_COLOR = new Color(201, 51, 51);
   private int paddleX;
   private Timer time = new Timer(TIME_DELAY, new TimerListener());

   // key presses and releases will change the boolean values held in this Map
   // When an arrow key is pressed, the direction-corresponding boolean is set true
   // and likewise when the arrow key is released the direction corresponding boolean is false
   private Map<Direction, Boolean> dirMap = new EnumMap<>(Direction.class);


   public GamePanel2() {
      setKeyBindings();
      time.start();
   }

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

      // iterate through all the Direction enums
      for (Direction direction : Direction.values()) {
         // set all values to false
         dirMap.put(direction, false);

         // create two key strokes, one for pressed and one for released
         int keyValue = direction.getKeyValue();
         KeyStroke pressedKey = KeyStroke.getKeyStroke(keyValue, 0, false);
         KeyStroke releasedKey = KeyStroke.getKeyStroke(keyValue, 0, true);

         // create two Actions, one for pressed, one for released
         Action pressedAction = new KeyAction(direction, true);
         Action releasedAction = new KeyAction(direction, false);

         // add keystroke to inputMap and use keystroke's toString as binding link
         inputMap.put(pressedKey, pressedKey.toString());
         inputMap.put(releasedKey, releasedKey.toString());

         // link binding links to our actions
         actionMap.put(pressedKey.toString(), pressedAction);
         actionMap.put(releasedKey.toString(), releasedAction);
      }

   }

   @Override
   public Dimension getPreferredSize() {
      if (isPreferredSizeSet()) {
         return super.getPreferredSize();
      }
      return new Dimension(PREF_W, PREF_H);
   }

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      Graphics2D g2 = (Graphics2D) g;
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
            RenderingHints.VALUE_ANTIALIAS_ON);
      g.setColor(PADDLE_COLOR);
      g.fillRoundRect(paddleX, PADDLE_Y, PADDLE_W, PADDLE_H, PADDLE_ARC, PADDLE_ARC);
   }

   private class TimerListener implements ActionListener {
      @Override
      public void actionPerformed(ActionEvent e) {
         // if JPanel no longer displayed, stop the Timer
         if (!GamePanel2.this.isDisplayable()) {
            ((Timer) e.getSource()).stop();
         }
         // here's the key: iterate through the Direction enum
         for (Direction direction : Direction.values()) {
            // get corresponding boolean from dirMap
            // and if true, change location of x and y
            if (dirMap.get(direction)) {
               int possibleX = paddleX + VEL_X * direction.getDeltaX();
               if (direction == direction.RIGHT && possibleX + PADDLE_W <= getWidth()) {
                  paddleX = possibleX;
               }
               if (direction == Direction.LEFT && possibleX >= 0) {
                  paddleX = possibleX;
               }
            }
         }
         repaint();
      }
   }

   private class KeyAction extends AbstractAction {
      private Direction direction;
      private boolean pressed;

      public KeyAction(Direction direction, boolean pressed) {
         this.direction = direction;
         this.pressed = pressed;               
      }

      @Override
      public void actionPerformed(ActionEvent e) {
         dirMap.put(direction, pressed); // key press simply changes the map, that's it.
      }
   }

   private static void createAndShowGui() {
      GamePanel2 mainPanel = new GamePanel2();

      JFrame frame = new JFrame("GamePanel2");
      frame.setDefaultCloseOperation(JFrame.DISPOSE_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();
         }
      });
   }
}

enum Direction {
   LEFT("Left", KeyEvent.VK_LEFT, -1),
   RIGHT("Right", KeyEvent.VK_RIGHT, 1);

   private String text; 
   private int keyValue; // KeyEvent.VK_?
   private int deltaX; 

   Direction(String text, int keyValue, int deltaX) {
      this.text = text;
      this.keyValue = keyValue;
      this.deltaX = deltaX;
   }

   public String getText() {
      return text;
   }

   public int getKeyValue() {
      return keyValue;
   }

   @Override
   public String toString() {
      return text;
   }

   public int getDeltaX() {
      return deltaX;
   }
}