java - Using graphics on a JPanel -


i'm trying draw rectangle jpanel using following code:

jpanel background = new jpanel(); graphics2d g = null; graphics2d g2d = (graphics2d) g; g2d.setcolor(color.black); g2d.drawrect(0, 0, 800, 600);

when try compile error

java.lang.nullpointerexception on set colour line.

i have tried same bug

jpanel background = new jpanel(); graphics bg = background.getgraphics(); bg.setcolor(color.black); bg.drawrect(0, 0, 800, 600);

can me fix bug?

to draw on jpanel, need override paintcomponent(). can override on fly follows or create subclass:

jpanel background = new jpanel() {     @override     public void paintcomponent(graphics g)     {         super.paintcomponent(g);         graphics2d g2d = (graphics2d) g;         g2d.setcolor(color.black);         g2d.drawrect(0, 0, 800, 600);     } };

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