Java - GUI (swing) - Null Pointer Exception

Question

I got 2 classes:
- 1st. makes a frame (JFrame) and adds a panel (JPanel) on it
- second one makes the panel and draws a rectangle on it (at least i thought it would)

this is the first class

class Frame {
JFrame frame;
Panel panel;

void draw() {
    frame = new JFrame ("qwertz");
    frame.setVisible(true);
    frame.setLocationRelativeTo(null);
    frame.setSize(300,200);

    panel = new Panel();
    panel.setLayout(null);
    panel.paint();
    frame.add(panel);
    }}

and the second

class Panel extends JPanel {
void paint() {
    Graphics g = getGraphics();

    g.drawRect(50,50,90,70);
}}

when i call the draw() method from the first class it throws this exception at me:

java.lang.NullPointerException  
          at Panel.paint(Panel.java:8) ((  g.drawRect(50,50,90,70);  ))
          at Frame.draw(Frame.java:15) ((  panel.paint();            ))

Panel is reserved Java name for awt.Panel, change that to MyPanel — mKorbel
– mKorbel, Commented May 3, 2013 at 18:42

Leo Izen · Accepted Answer · 2013-05-03 19:10:06Z

2

That's not how you're supposed to paint. To paint a component, override the paintComponent(Graphics g) method of the JPanel then call repaint();

class MyPanel extends JPanel {
    @Override // <-- this makes a compiler error if you typod the method name
    public void paintComponent(Graphics g) {
        g.drawRect(50,50,90,70);
    }
}

and

panel = new MyPanel();
panel.setLayout(null);
panel.repaint(); // <<---- Look here! It says repaint() not paint()
frame.add(panel);

Also, if all you have to do is paint on this panel, I'd consider using a plain-old Component, and overriding paint(Graphics g) instead of paintComponent(Graphics g). paintComponent(Graphics g) is exclusively for swing components.

edited May 3, 2013 at 19:10

answered May 3, 2013 at 18:47

Leo Izen

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2 Comments

user2347921 Over a year ago

what exactly happens, when you override the paintComponent(Graphics g) method and don't add super.paintComponent(Graphics g) to the first line of the method implementation ?

Leo Izen Over a year ago

It won't do any of the other painting that the class would normally do, had you not overwritten that method, such as painting childred added to a container.

Houbie · Accepted Answer · 2013-05-03 18:45:07Z

1

instead of implementing the paint method, you should implement the paintComponent(Graphics g) method. This way, the graphics object you have is valid.

http://docs.oracle.com/javase/6/docs/api/javax/swing/JComponent.html#paintComponent(java.awt.Graphics)

answered May 3, 2013 at 18:45

Houbie

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Comments

Lukas Eichler · Accepted Answer · 2013-05-03 18:43:41Z

0

You are trying to draw the panel before it is added to the Frame. Try to move frame.paint(); below frame.add(panel);. Additionally if you are using Swing you should use JPanel instead of Panel.

answered May 3, 2013 at 18:43

Lukas Eichler

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