Buffered Keyboard Input

Question

I am trying to write a 2d graphical game. In this game I have keyboard inputs to move a square block on a black plane. I order to ensure smooth key motions I want to use Buffered key input. In order to do this I use a boolean array that saves the key strokes.

import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.util.Timer;

import javax.swing.JFrame;
import javax.swing.JPanel;


public class Main extends JFrame{

/**
 * @param args
 */
class Entity
{
    int x = 150,y = 150;
    int  Speed = 5;

}

Entity user = new Entity();
boolean[] keys = new boolean[KeyEvent.KEY_TYPED];

public Main()
{
    setSize(800,600);
    setLocationRelativeTo(null);

    final JPanel display = new JPanel()
    {
        protected void paintComponent(Graphics g)
        {

            g.setColor(Color.black);
            g.fillRect(0, 0, getWidth(), getHeight());
            g.setColor(Color.RED);
            g.fillRect(user.x, user.y, 30, 30);
        }
    };


    addKeyListener(new KeyAdapter() 
    {

        public void keyPressed(KeyEvent arg0)
        {

            keys[arg0.getKeyCode()] = true;

            if(keys[KeyEvent.VK_UP])
            {
                user.y -= user.Speed;

            }
            if(keys[KeyEvent.VK_DOWN])
            {
                user.y += user.Speed;
            }
            if(keys[KeyEvent.VK_LEFT])
            {
                user.x -= user.Speed;
            }
            if(keys[KeyEvent.VK_RIGHT])
            {
                user.x += user.Speed;

            }

            setFocusable(true);
            repaint();

        }
    });


    //add a action listener
    //remember to set the focusable 


    add(display);
}
public static void main(String[] args) {
    // TODO Auto-generated method stub


    Thread t = new Thread();
    try {
        t.sleep(100);
        Main m = new Main();
        m.setVisible(true);
        m.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }   
}

}

In this input the block moves in the correct direction but just twice, then It moves in a different motion and it even Stops moving.

I have searched in all my Java books that I have and it does not help much. How can I get this error fixed?

Answer 1

Make the boolean[] keys a local variable so that it does not contain previously pressed keys, which will impact the x,y coordinates.

public void keyPressed(KeyEvent arg0)
{
    boolean[] keys = new boolean[KeyEvent.KEY_TYPED];
    keys[arg0.getKeyCode()] = true;

    if(keys[KeyEvent.VK_UP])
    {
        user.y -= user.Speed;
    }
    else if(keys[KeyEvent.VK_DOWN])
    {
        user.y += user.Speed;
    }
    else if(keys[KeyEvent.VK_LEFT])
    {
        user.x -= user.Speed;
    }
    else if(keys[KeyEvent.VK_RIGHT])
    {
        user.x += user.Speed;

    }

    setFocusable(true);
    repaint();
}

If you just want to clear the keys

public void keyPressed(KeyEvent arg0)
{
        keys = new boolean[KeyEvent.KEY_TYPED];
        keys[arg0.getKeyCode()] = true;
       /* Rest of code */