NullPointerException using Graphics2D in custom JPanel

Question

I'm extending JPanel to make a custom drawing panel, but am getting a NullPointerException and can't work out why. I've removed code until it's pretty bare, but the error is still occuring.

package testdraw;

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import javax.swing.JPanel;

public class DrawPanel extends JPanel {

    public DrawPanel() {
        this.Draw();
    }

    public void Draw(){
        Graphics g = this.getGraphics();
        Graphics2D g2d = (Graphics2D) g;

        RenderingHints rh = new RenderingHints(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        rh.put(RenderingHints.KEY_RENDERING,RenderingHints.VALUE_RENDER_SPEED);

        g2d.setRenderingHints(rh);
    }
}

I'm getting the error:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException

from the line where I call the setRenderingHints method. Any help appreciated.

Answer 1

You're calling "Draw" in the constructor. You should wait until it's fully constructed and visibile before calling getGraphics.

Answer 2

The null comes from when you call Graphics g = this.getGraphics(); As Paul said, you shouldn't call this in the constructor because the panel doesn't exist yet. It might be better to put this code in an overridden paintComponent() method

Answer 3

Usually, the draw method is not called by the panel itself, but rather from the Java2D framework. As a consequence, it is not a Draw() with no parameters method, but rather the paint(Graphics g) method. In this case, the Graphics will never be null, and will always be a graphics2D (as far as you use a Java2 VM).