java - 为什么我不能生成这个 GUI？

Question

所以我正在尝试为一个由按钮控制的球创建一个 GUI，以通过迷宫运行。但刚开始我想创建一个带有几个方向按钮的框架，每当我尝试在 Eclipse 中运行它时，它都会出现一个根本不是很具体的错误。我的代码如下。错误指向代码的“buttonTL、buttonTR、buttonBL 和 buttonBR”部分。感谢您提供任何帮助。

错误是：
mazeassignment.One.createGUI(One.java:55) at mazeassignment.One.main(One.java:23) 的线程“main”java.lang.NullPointerException 中的异常

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;
        }
    }

}

Answer 1

您忘记初始化 buttonPanel。在开始使用它之前添加它：

buttonPanel= new JPanel();

然后它工作正常。