CODEWITHSUNDEEP
javaswingnullpointerexceptionjpaneljava-2d
thmas
thmas

Reputation: 142

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?

Upvotes: 0

Views: 341

Answers (3)

MadProgrammer
MadProgrammer

Reputation: 347184

Custom painting in Swing is normally done by overriding the paintComponent method of any class that extend JComponent. Unless you have some need to do otherwise, it is recommended that you extend from something like a JPanel.

public class MyPanel exends JPanel {
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.BLACK);
        g.drawRect(0, 0, 800, 600);
    }

    public Dimension getPreferredSize() {
        return new Dimension(800, 600);
    }
}

Take a look at Performing custom painting and 2D Graphics for more details

Upvotes: 3

JB Nizet
JB Nizet

Reputation: 691635

Create a subclass of JPanel, and override the paintComponent(Graphics g) method. Only paint from this method, using the Graphics passed as argument to the method, that you can safely cast to Graphics2D:

JPanel background = new JPanel() {
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;
        g2d.setColor(Color.BLACK);
        g2d.drawRect(0, 0, 800, 600);
    }
}

Upvotes: 4

Eng.Fouad
Eng.Fouad

Reputation: 117579

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);
    }
};

Upvotes: 5

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