panels - why can't I move code to another method?

Question

I'm writing some code that at the moment draws a circle and later will move the circle when buttons are pressed. When the panel is created, I want to have a circle already drawn in the centre. The code I've written in the actionPerformed method does this when a button is pressed, but I wanted this to be already drawn when the panel initialises so I moved the code to the createGUI method. However, it doesn't work when I move it there and I get a null pointer exception.

Could someone please explain to me why? Thanks.

import java.awt.Color;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Random;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class MovingArrows extends JFrame implements ActionListener {

    private JButton buttonUp, buttonDown, buttonLeft, buttonRight;
    private JPanel panel;
    private int xCircleCentre, yCircleCentre;

    final int xCircleCentreStarting = 250, yCircleCentreStarting = 250;
    final int RADIUS = 20;

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        MovingArrows frame = new MovingArrows();
        frame.setSize(550, 600);
        frame.createGUI();
        frame.setVisible(true);

    }

    private void createGUI() {
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        Container window = getContentPane();
        window.setLayout(new FlowLayout());

        panel = new JPanel();
        panel.setPreferredSize(new Dimension(500, 500));
        panel.setBackground(Color.white);

        window.add(panel);

        buttonUp = new JButton("Up");
        buttonDown = new JButton("Down");
        buttonLeft = new JButton("Left");
        buttonRight = new JButton("Right");
        window.add(buttonUp);
        window.add(buttonDown);
        window.add(buttonLeft);
        window.add(buttonRight);
        buttonUp.addActionListener(this);
        buttonDown.addActionListener(this);
        buttonLeft.addActionListener(this);
        buttonRight.addActionListener(this);

        Graphics paper = panel.getGraphics();
        paper.setColor(Color.black);
        paper.fillOval(xCircleCentreStarting - RADIUS, yCircleCentreStarting
                - RADIUS, RADIUS * 2, RADIUS * 2);

    }

    @Override
    public void actionPerformed(ActionEvent event) {
        /*
         * Graphics paper = panel.getGraphics(); paper.setColor(Color.black);
         * paper.fillOval(xCircleCentreStarting - RADIUS, yCircleCentreStarting
         * - RADIUS, RADIUS * 2, RADIUS * 2);
         */
    }
}

Exception in thread "main" java.lang.NullPointerException at MovingArrows.createGUI(MovingArrows.java:58) at MovingArrows.main(MovingArrows.java:28) line 58 is 'paper.setColor(Color.black);' and line 28 is 'frame.createGUI();' — Zaw, Dec 16 '13 at 12:39
Obviously, paper is null. Thus, panel.getGraphics() returns null, which implies that `panel` has not been rendered yet. — Cedric Reichenbach, Dec 16 '13 at 12:47

score 0 · Accepted Answer · edited May 23 '17 at 10:32

0

The panel.getGraphics() is returning null because the component is not rendered yet.

Also see Any alternative to calling getGraphics() which is returning null

edited May 23 '17 at 10:32

Community

1
1

answered Dec 16 '13 at 12:28

bknopper

1,193
12
28

Unfortunately, I don't have 50 points yet and can't comment. I was planning to delete this "Answer"-comment wrapper after the stack trace was provided, so we could help the questioner a bit further... – bknopper Dec 16 '13 at 12:38
I see the problem now. Thanks for helping. – Zaw Dec 16 '13 at 12:54

panels - why can't I move code to another method?

1 Answers1