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javaswingawtdouble-buffering

Simple custom painting mistake

I just started coding again and I guess I forgot how to double buffer. This is the code I have now and I'm not sure what I am missing. When I start it there is just a white screen, no oval.

What is the mistake in rendering?

import java.awt.Graphics;
import java.awt.Image;

import javax.swing.JFrame;

public class Graphs extends JFrame {

private Image dbImage;
private Graphics dbg;

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

public Graphs() {
    setSize(1000, 600);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setResizable(false);
    setTitle("Graphs");
    setLocationRelativeTo(null);
    setVisible(true);
}

public void paint(Graphics g) {
    dbImage = createImage(getWidth(), getHeight());
    dbg = dbImage.getGraphics();
    paintComponent(dbg);
    dbg.drawImage(dbImage, 0, 0, this);
}

public void paintComponent(Graphics g) {
    g.drawOval(200, 200, 200, 200);
    repaint();
}
}

Update: compilation error on @Override

The method  paintComponent(Graphics) of type Graphs must override or implement a supertype method.

1 quick fix available:
-> Remove '@Override' annotation
Solution

  • The reason that you're not seeing the oval is that you're drawing the image onto its own Graphics object. Replace:

    dbg.drawImage(dbImage, 0, 0, this);

    with

    g.drawImage(dbImage, 0, 0, this);

    Also better not to override paint in a top-level container but rather override paintComponent in a sub-classed JComponent. Also remember to call

    super.paintComponent(g);