ButtonClick上的Java Graphics repaint（）

Question

I have this code and want to repaint my graphic when the Button gets pressed:

public class JDraw extends JFrame {

/**
 * Draws a figur in a JFrame.
 * The color of it is random and can get changed by a button.
 */

public static JButton okButton = new JButton("change color");

public JDraw(String newTitel) {
    super.setTitle(newTitel);
}

//Main method
public static void main(String str[]) {
    JDraw window = new JDraw("Graphic");
    window.setSize(300, 450);
    window.setVisible(true);
    window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    window.add(okButton, BorderLayout.SOUTH);

    okButton.addActionListener(new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent e) {
            //JDraw.repaint(); <-- problem
        }
    });
}



@Override
public void paint(final Graphics g) {

    super.paint(g);

    Random rand = new Random();
    int r1 = rand.nextInt(255);
    int b1 = rand.nextInt(255);
    int g1 = rand.nextInt(255);
    g.setColor(new Color(r1, g1, b1));

    //head
    g.drawOval(100, 50, 100, 100);
    g.fillOval(100, 50, 100, 100); // filled
    //more drawing stuff.....

}
}

However I have no idea how to do it, because I cant do the repaint in my ActionPerfomed. Error: non-static method repaint() cannot be referenced from a static context

I hope somebody can help. :)

Answer 1

You need to make the following call in your actionPerformed :

window.repaint();

To be able to reference window from within you actionPerformed , you need to make your window variable final :

final JDraw window = ...;

However, if I can suggest a few improvements:

1. Don't extend JFrame
2. Don't override paint(Graphics) of JFrame
3. Instead create a class that extends JComponent or JPanel and set it as the content pane of the JFrame
4. Instead of overrding paint(Graphics) , rather override paintComponent(Graphics)
5. Your okButton should not be static . Instead move all your code in a non-static method like initUI() and have a code like new JDraw().initUI();
6. Wrap the code starting your UI in a SwingUtilities.invokeLater(Runnable) so that your UI is properly initiated from the Event Dispatching Thread.

Answer 2

You can't refer to the class JDraw. You should use an object instead. The object in your case is window . So, use:

window.repaint();

It is like saying: Human, walk to the door. Human is the class. You can't tell Human to do something, you need an instance of Human, like Obama or Santa Claus. In your case: you can't tell JDraw to repaint, but the object of type JDraw, ie: window .