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javaswingnullpointerexceptionjpaneljava-2d

Using graphics on a JPanel

I'm trying to draw a rectangle to a JPanel using the following code:

JPanel background = new JPanel();
Graphics2D g = null;
Graphics2D g2d = (Graphics2D) g;
g2d.setColor(Color.BLACK);
g2d.drawRect(0, 0, 800, 600);

When I try to compile it I get the error

java.lang.NullPointerException on the set colour line.

I have also tried this but i get the same bug

JPanel background = new JPanel();
Graphics bg = background.getGraphics();
bg.setColor(Color.BLACK);
bg.drawRect(0, 0, 800, 600);

can anyone help me fix this bug?

Solution

  • To draw on a JPanel, you need to override paintComponent(). You can override it on the fly as follows or create a 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);
    }
};