Can somebody help me find a bug in my applet?

Question

Hi,
I am in the process of creating a RPG program using Java. I am not very experienced with java, however the problem i am currently facing is something i can't seem to figure out. I would like to draw a simple grid where a character (indicated by a filled circle) moves around this grid collecting items (indicated by a red rectangle). When the character moves on top of the grid with the item, i would like it to disappear. Right now i am not worrying about the fact that the item will reappear after the character moves away again, because sometimes, when the character moves over the item, nothing happens/another item disappears. i have been at this for 4 days and still cannot figure out what is goign on. can somebody please help me? it would be most appreciated.
Thanks

Answer 1

State is the key. Here the rects array keeps track of things. Of course, there are lots of ways to put these things together. Use the arrow keys to move the token and the x key to reset the rectangles.
import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import javax.swing.*;
public class RPG extends JApplet
    public void init()
       RPGPanel rpgPanel = new RPGPanel();
       Container cp = getContentPane();
       cp.setLayout(new BorderLayout());
       cp.add(rpgPanel);
    public static void main(String[] args)
        JApplet applet = new RPG();
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(applet);
        f.setSize(400,400);
        f.setLocation(200,200);
        applet.init();
        f.setVisible(true);
class RPGPanel extends JPanel
    final int
        GRID_SIZE = 8,
        PAD       = 20;
    Rectangle[] rects;
    int rectSize;
    Ellipse2D.Double token;
    int dx, dy;
    public RPGPanel()
        TokenController controller = new TokenController(this);
        dx = 2;
        dy = 2;
        requestFocusInWindow();
    protected void paintComponent(Graphics g)
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        drawGrid(g2);
        drawRects(g2);
        g2.setPaint(Color.red);
        g2.fill(token);
    public void moveToken(int horiz, int vert)
        int x = dx * horiz;
        int y = dy * vert;
        if(isWithinBounds(x, y))
            token.x += x;
            token.y += y;
            repaint((int)(token.x - dx), (int)(token.y - dy),
                    (int)(token.x + token.width + dx),
                    (int)(token.y + token.height + dy));
        else
            Toolkit.getDefaultToolkit().beep();
        // check for contact with rects
        checkForContact();
    public void reset()
        for(int j = 0; j < rects.length; j++)
            if(rects[j].width == 0)
                rects[j].width = rectSize;
        token.x = getWidth()/2 - token.width/2;
        token.y = getHeight()/2 - token.height/2;
        repaint();
    private boolean isWithinBounds(int x, int y)
        int w = getWidth();
        int h = getHeight();
        Rectangle r = token.getBounds();
        if(r.x + x < 0 || r.x + r.width + x > w)
            return false;
        if(r.y + y < 0 || r.y + r.height + y > h)
            return false;
        return true;
    private void checkForContact()
        for(int j = 0; j < rects.length; j++)
            Rectangle r = rects[j];
            if(token.intersects(r.x, r.y, r.width, r.height))
                r.width = 0;
        repaint();
    private void drawGrid(Graphics2D g2)
        int w = getWidth();
        int h = getHeight();
        double xInc = (double)(w - 2*PAD) / GRID_SIZE;
        double yInc = (double)(h - 2*PAD) / GRID_SIZE;
        double x1 = PAD, y1 = PAD, x2 = w - PAD, y2;
        // horizontal lines
        for(int j = 0; j <= GRID_SIZE; j++)
            g2.draw(new Line2D.Double(x1, y1, x2, y1));
            y1 += yInc;
        // vertical lines
        y1 = PAD; y2 = h - PAD;
        for(int j = 0; j <= GRID_SIZE; j++)
            g2.draw(new Line2D.Double(x1, y1, x1, y2));
            x1 += xInc;
    private void drawRects(Graphics2D g2)
        if(rects == null)
            initRectsAndToken();
        g2.setPaint(Color.blue);
        for(int row = 0; row < GRID_SIZE; row++)
            for(int col = 0; col < GRID_SIZE; col++)
                int index = row * GRID_SIZE + col;
                g2.fill(rects[index]);
    private void initRectsAndToken()
        int w = getWidth();
        int h = getHeight();
        double xInc = (double)(w - 2*PAD) / GRID_SIZE;
        double yInc = (double)(h - 2*PAD) / GRID_SIZE;
        rectSize = (int)(0.3 * Math.min(xInc, yInc));
        rects = new Rectangle[GRID_SIZE * GRID_SIZE];
        double x0 = PAD + (xInc - rectSize)/2 + 1;
        double y0 = PAD + (yInc - rectSize)/2 + 1;
        double x = x0, y = y0;
        for(int row = 0; row < GRID_SIZE; row++)
            for(int col = 0; col < GRID_SIZE; col++)
                int index = row * GRID_SIZE + col;
                rects[index] = new Rectangle((int)x, (int)y, rectSize, rectSize);
                x += xInc;
            x = x0;
            y += yInc;
        // token
        int d = (int)(0.8 * Math.min(w, h) - 2*PAD) / GRID_SIZE;
        token = new Ellipse2D.Double(w/2 - d/2, h/2 - d/2, d, d);
class TokenController
    RPGPanel rpgPanel;
    public TokenController(RPGPanel rpgp)
        rpgPanel = rpgp;
        registerKeys();
    private Action up = new AbstractAction()
        public void actionPerformed(ActionEvent e)
            rpgPanel.moveToken(0, -1);
    private Action left = new AbstractAction()
        public void actionPerformed(ActionEvent e)
            rpgPanel.moveToken(-1, 0);
    private Action down = new AbstractAction()
        public void actionPerformed(ActionEvent e)
            rpgPanel.moveToken(0, 1);
    private Action right = new AbstractAction()
        public void actionPerformed(ActionEvent e)
            rpgPanel.moveToken(1, 0);
    private Action reset = new AbstractAction()
        public void actionPerformed(ActionEvent e)
           rpgPanel.reset();
    private void registerKeys()
        rpgPanel.getInputMap().put(KeyStroke.getKeyStroke("UP"), "UP");
        rpgPanel.getActionMap().put("UP", up);
        rpgPanel.getInputMap().put(KeyStroke.getKeyStroke("LEFT"), "LEFT");
        rpgPanel.getActionMap().put("LEFT", left);
        rpgPanel.getInputMap().put(KeyStroke.getKeyStroke("DOWN"), "DOWN");
        rpgPanel.getActionMap().put("DOWN", down);
        rpgPanel.getInputMap().put(KeyStroke.getKeyStroke("RIGHT"), "RIGHT");
        rpgPanel.getActionMap().put("RIGHT", right);
        rpgPanel.getInputMap().put(KeyStroke.getKeyStroke("X"), "X");
        rpgPanel.getActionMap().put("X", reset);
}