null pointer exception in graphics of component

Question

I have DrawOutput class, which extends JComponent. this.getGraphics whic I pass to paint is null here. How can I get Grapics of this class?

public class DrawOutput extends JComponent {

Here is constructor of class.

 DrawOutput (MinDistances requiredMinDistances, MainMatrix matrix){
            super();
            getRequiredMedoidsArray(requiredMinDistances);
            paint(this.getGraphics(), requiredMinDistances, matrix);
        }

content is null here

  public void paint(Graphics content, MinDistances requiredMinDistances, MainMatrix matrix)        {
    ...

}

private float[] setColor (int colorID){
           float[]hsbValues=new float[3];
    if(colorID == 1){
        hsbValues =  Color.RGBtoHSB(0,255,255,hsbValues);
    }
    else if(colorID == 2){
        hsbValues =  Color.RGBtoHSB(255,0,255,hsbValues);
    }
    else if(colorID == 3){
        hsbValues =  Color.RGBtoHSB(0,255,0,hsbValues);
    }
    else if(colorID == 4){
        hsbValues =  Color.RGBtoHSB(255,255,0,hsbValues);
    }
    else if(colorID == 5){
        hsbValues =  Color.RGBtoHSB(255,0,0,hsbValues);
    }
    else if(colorID == 6){
        hsbValues =  Color.RGBtoHSB(255,255,255,hsbValues);
    }
    else{
        hsbValues =  Color.RGBtoHSB(0, 0, 0,hsbValues);
    }
    return hsbValues;
}

private void getRequiredMedoidsArray(MinDistances distancesCell){
   ...
}

}

Any suggestions?

maybe you should override `paint()` and wait until it is called automatically? — micha, Apr 27 '13 at 15:08
"Swing programs should override `paintComponent()` instead of overriding `paint()`."—[*Painting in AWT and Swing: The Paint Methods*](http://www.oracle.com/technetwork/java/painting-140037.html#callbacks). — trashgod, Apr 27 '13 at 16:46

score 0 · Answer 1 · answered Apr 27 '13 at 15:12

Do not do it in the constructor, keep the paining in the paint, or do the painting using active rendering.

My suggestion is

Create a BufferedImage in constructor.
Draw on the BufferedImage whenever you want.
Draw the BufferedImage to screen in paint.

Global state:

BufferedImage offscreen;
Graphics offscreenGraphics;

In constructor:

offscreen = new BufferedImage(width, height, BufferedImage.TYPE_INT_ARGB);
offscreenGraphics = offscreen.getGraphics();

then you can just draw on offscreenGraphics whenever you want without issue.

Then in paint:

g.drawImage(offscreen, 0, 0, width, height, null);

Hope this helps.

null pointer exception in graphics of component

1 Answers1