Question

I have been following the Java Game Programming for Beginners tutorial series, and wished to experiment by applying a background image. Unfortunately, when I render it through the paintComponent method, it moves with my sprite (albeit at one unit continuously as opposed to five); and when I render it through the paint method, I get a strange, flickering box that matches the color designated in the setBackground (color) property of the JFrame and it moves with the sprite identically to that of the prior instance (within paintComponent).

How might I code the image so as to remain static, as a background should be?

Code:

public class JavaGame extends JFrame{

int x, y;
private Image dbImage;
private Graphics dbg;
Image ghost;
Image bg;


public class AL extends KeyAdapter{
    public void keyPressed(KeyEvent e){
        int keyCode = e.getKeyCode();

        if(keyCode == e.VK_LEFT){
            if(x <= 8)
                x = 8;
            else
                x += -5;
        }
        if(keyCode == e.VK_RIGHT){
            if(x >= 235)
                x = 235;
            else
                x += +5;
        }
        if(keyCode == e.VK_UP){
            if(y <= 18)
                y = 18;
            else
                y += -5;
        }
        if(keyCode == e.VK_DOWN){
            if(y >= 235)
                y = 235;
            else
                y += +5;
        }
    }
    public void keyReleased(KeyEvent e){

    }

}

public JavaGame(){
    //Load images
    ImageIcon i = new ImageIcon("C:/Users/Taylor/workspace/Java game/src/ghost.png");
    ghost = i.getImage();

    ImageIcon j = new ImageIcon("C:/Users/Taylor/workspace/Java game/src/bg.png");
    bg = j.getImage();

    //Game properties
    addKeyListener(new AL());
    setTitle("Java Game");
    setSize(500, 500);
    setResizable(false);
    setVisible(true);
    setBackground(Color.GRAY);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    x = 150;
    y = 150;
}

public void paint(Graphics g){
    g.drawImage(bg, 0, 0, null);

    dbImage = createImage(getWidth(), getHeight());
    dbg = dbImage.getGraphics();
    paintComponent(dbg);
    g.drawImage(dbImage, x, y, this);
}

public void paintComponent(Graphics g){
    g.setColor(Color.WHITE);
    g.drawImage(ghost, x, y, this);

    repaint();
}


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

}

Pictures:

Answer 1

Were you copy/pasting code at random? That is what it looked like. There were so many odd aspects to that code that I did not document them all (a good one for code review, maybe). The example uses an asynchronous method to load the images (in order to get the animated image, animating). Use ImageIO.read(URL) for a synchronous way to load static images.

Here are some brief tips:

1. By the time this becomes deployed, the images will likely become an embedded resource and will not be accessible by File object. Add them to the run-time class-path and access them by URL.
2. Swing GUIs should be started and altered on the EDT (see the change to the main()).
3. Always call super.paint(g); (or paintComponent(g)) at the start of the method.
4. Don’t extend frame, don’t paint to a top level component. Instead extend panel and override paintComponent(). Add the panel to the frame.

Code

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

public class JavaGame extends JPanel {

    int x, y;
    private Image dbImage;
    private Graphics dbg;
    Image ghost;
    Image bg;

    public class AL extends KeyAdapter {
        public void keyPressed(KeyEvent e) {
            int keyCode = e.getKeyCode();

            if (keyCode == e.VK_LEFT) {
                if (x <= 8)
                    x = 8;
                else
                    x += -5;
            }
            if (keyCode == e.VK_RIGHT) {
                if (x >= 235)
                    x = 235;
                else
                    x += +5;
            }
            if (keyCode == e.VK_UP) {
                if (y <= 18)
                    y = 18;
                else
                    y += -5;
            }
            if (keyCode == e.VK_DOWN) {
                if (y >= 235)
                    y = 235;
                else
                    y += +5;
            }
        }

        public void keyReleased(KeyEvent e) {
        }
    }

    public JavaGame() throws Exception {
        // Load images
        //ImageIcon i = new ImageIcon(
            //  "C:/Users/Taylor/workspace/Java game/src/ghost.png");
        URL urlGhost = new URL("http://1point1c.org/gif/thum/plnttm.gif");
        ghost = Toolkit.getDefaultToolkit().createImage(urlGhost);

        //ImageIcon j = new ImageIcon(
            //  "C:/Users/Taylor/workspace/Java game/src/bg.png");
        URL urlBG = new URL("http://pscode.org/media/stromlo2.jpg");
        bg = Toolkit.getDefaultToolkit().createImage(urlBG);
        
        setFocusable(true);

        // Game properties
        addKeyListener(new AL());
        x = 150;
        y = 150;
        
        ActionListener al = new ActionListener() {
            public void actionPerformed(ActionEvent ae) {
                repaint();
            }
        };
        Timer timer = new Timer(50,al);
        timer.start();
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawImage(bg, 0, 0, null);

        //dbImage = createImage(getWidth(), getHeight());
        //dbg = dbImage.getGraphics();
        //paintComponent(dbg);
        g.drawImage(dbImage, x, y, this);

        g.setColor(Color.WHITE);
        g.drawImage(ghost, x, y, this);

        //repaint();
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                try {
                    JFrame f = new JFrame("Java Game");
                    f.setSize(500, 500);
                    f.setResizable(false);
                    f.setVisible(true);
                    f.setBackground(Color.GRAY);
                    f.setContentPane(new JavaGame());
                    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }
}