Why can't I generate this GUI?

Question

So I'm trying to create a GUI for a ball that is controlled by buttons to run through a maze. But just to begin with I want to create a frame with a few directional buttons and whenever I try to run this in eclipse it comes up with an error that is not very specific at all. My code is below. The error points to the "buttonTL, buttonTR, buttonBL and buttonBR" sections of the code. Thank you for any help you can provide.

The error is:
Exception in thread "main" java.lang.NullPointerException at mazeassignment.One.createGUI(One.java:55) at mazeassignment.One.main(One.java:23)

import java.awt.*;
import java.awt.event.*;
import javax.imageio.ImageIO;
import javax.swing.*;

public class One extends JFrame implements ActionListener//, ChangeListener 
{
    private JButton buttonRight, buttonLeft, buttonUp, buttonDown, buttonPlay, buttonExit, buttonReset, buttonGrid, buttonTL, buttonTR, buttonBL, buttonBR, buttonM, buttonOption1, buttonOption3, buttonCompass;
    private JPanel panelCentre, panelRight, panelBottom, buttonPanel, compassPanel, optionsPanel, debugPanel;
    private JSlider sliderSpeed;
    private JLabel appSpeed;
    private JButton [] JBArray = new JButton[630];
    private int bLocation = 290;
    public int ballLocation = 287;
    private Icon bImage, iconBall, tile, iconWall, iconCompassNorth;

    public static void main (String[] args)
    {
        One frame = new One();
        frame.setSize(875, 600);
        frame.createGUI();
        frame.setVisible(true);
    }

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

        panelCentre = new JPanel();
        panelCentre.setPreferredSize(new Dimension(625, 450));
        panelCentre.setBackground(Color.BLUE);
        window.add(panelCentre);
        panelCentre.setLayout(new GridLayout(21, 30));

        panelRight = new JPanel();
        panelRight.setPreferredSize(new Dimension(200, 450));
        panelRight.setBackground(Color.GRAY);
        window.add(panelRight, BorderLayout.EAST);

        panelBottom = new JPanel();
        panelBottom.setPreferredSize(new Dimension(875, 100));
        panelBottom.setBackground(Color.GREEN);
        window.add(panelBottom, BorderLayout.SOUTH);

        debugPanel = new JPanel();
        debugPanel.setPreferredSize(new Dimension(200, 100));
        debugPanel.setBackground(Color.WHITE);
        panelRight.add(debugPanel);

        buttonTL = new JButton("");
        buttonPanel.add(buttonTL);
        buttonTL.setEnabled(false);

        buttonUp = new JButton("^");
        buttonPanel.add(buttonUp);
        buttonUp.addActionListener(this);

        buttonTR = new JButton("");
        buttonPanel.add(buttonTR);
        buttonTR.setEnabled(false);

        buttonLeft = new JButton("<");
        buttonPanel.add(buttonLeft);
        buttonLeft.addActionListener(this);

        buttonM = new JButton("");
        buttonPanel.add(buttonM);
        buttonM.setEnabled(false);

        buttonRight = new JButton(">");
        buttonPanel.add(buttonRight);
        buttonRight.addActionListener(this);

        buttonBL = new JButton("");
        buttonPanel.add(buttonBL);
        buttonBL.setEnabled(false);

        buttonDown = new JButton("v");
        buttonPanel.add(buttonDown);
        buttonDown.addActionListener(this);

        for(int i=0; i<630; i++)
        {
            JBArray[i] = new JButton();
            buttonGrid = new JButton(""+i);
            panelCentre.add(JBArray[i]);
            JBArray[i].setBorderPainted(false);
        }
    }

    public void actionPerformed(ActionEvent event)
    {
        Object source = event.getSource();

        JBArray[bLocation].setIcon(iconBall);
        JBArray[bLocation].setIcon(tile);

        if (source == buttonRight)
        {
            JBArray[bLocation+1].setIcon(iconBall);
            JBArray[bLocation-1].setIcon(tile);
            bLocation=bLocation+1;
        }
        if (source == buttonLeft)
        {
            JBArray[bLocation-1].setIcon(iconBall);
            JBArray[bLocation+1].setIcon(tile);
            bLocation=bLocation-1;
        }
        if (source == buttonUp)
        {
            JBArray[bLocation-30].setIcon(iconBall);
            JBArray[bLocation+30].setIcon(tile);
            bLocation=bLocation-30;
        }
        if (source == buttonDown)
        {
            JBArray[bLocation+30].setIcon(iconBall);
            JBArray[bLocation-30].setIcon(tile);
            bLocation=bLocation+30;
        }
    }

}

Exception in thread "main" java.lang.NullPointerException at mazeassignment.One.createGUI(One.java:55) at mazeassignment.One.main(One.java:23) — , Apr 29 '15 at 16:27
So which is line 55? What could you be using on that line which might be `null`? — greg-449, Apr 29 '15 at 16:48
also, look at your actionPerformed. There you can easily go outside the array boundaries, you dont check exact location in the array element. what if it is the first in the array? you cant do -1 and etc. — Ophelia, Apr 29 '15 at 17:05

score 0 · Accepted Answer · answered Apr 29 '15 at 17:04

0

You forgot to initialize buttonPanel. Add this before you start to use it:

buttonPanel= new JPanel();

Then it works fine.

answered Apr 29 '15 at 17:04

Alex

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This was exactly it. Realised I had told those buttons where to go, but that place didn't exist yet, so I had to create it. thank you! – Apr 29 '15 at 17:12

Why can't I generate this GUI?

1 Answers1