Java Graphics Nullpointer

Question

I have a question regarding Graphics in Java. Maybe it is a basic question. Below you can see my Code to create a Graphics object with my panel in a JFrame. I would like to create a graphics object in the constructor.(If i create the graphics object in a button it works) If I start the application I get immediately a nullpointer exception because of g.setColor(Color.RED);

But I don't understand why I get this exception. I have debugged the programm and the panel is created before I use the .. = panel.getGraphics(); command. So normally my Graphics object should be generated correctly and not null. So it should not be a problem to set a Color. Could you please explain me why I get this exception?

public class G2DTest extends JFrame {

private JPanel contentPane;
private JPanel panel;
Graphics g;


/**
 * Launch the application.
 */
public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                G2DTest frame = new G2DTest();
                frame.setVisible(true);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}

/**
 * Create the frame.
 */
public G2DTest() {

    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setBounds(100, 100, 796, 810);
    contentPane = new JPanel();
    contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
    setContentPane(contentPane);
    contentPane.setLayout(null);

    panel = new JPanel();
    panel.setBackground(Color.WHITE);
    panel.setBounds(80, 87, 309, 234);
    contentPane.add(panel);

    JRadioButton rdbtnRot = new JRadioButton("rot");
    rdbtnRot.setBounds(438, 183, 251, 41);
    contentPane.add(rdbtnRot);
    g = panel.getGraphics();
    g.setColor(Color.RED);

}

[This](http://stackoverflow.com/questions/8184913/any-alternative-to-calling-getgraphics-which-is-returning-null) may be useful. — But I'm Not A Wrapper Class, Apr 01 '16 at 16:01

AJNeufeld · Answer 1 · 2016-04-01T16:16:23.773

Your JPanel is created, true. And your G2DTest (which is a JFrame) is also created.

But until frame.setVisible(true); is called, these objects are just handles to unrealized system objects. The actual operating system windows are not created until the frame (and the child objects) are made visible. Attempting to retrieve a graphic context from a non-realized window results in null.

As an aside, you really should not be create and storing a Graphics context for the life of your G2DTest JFrame.

Any painting you do should be contained within calls to #paintComponent(Graphics g), which provides you a graphics context as an argument.
The Graphics context you hold onto is a scarce resource, possibly holding "heavy weight resources" like off-screen memory. By holding onto it, your application might not be playing nice with other applications, holding resources it is not using when (say) minimized to an icon.
Your application could break when moved from one screen to another on an multi-monitor computer. Each monitor could use a different graphics configuration; your held on to context would not be updated.

There are exceptions: a full screen, exclusive mode game might be able to hold a graphics context for an extended duration. But as a general rule, don't keep holding a reference to the Graphics context in your application.

Java Graphics Nullpointer

1 Answers1