Worker类中的Java空指针异常

这是我的驾驶课

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


public class Volume extends JFrame
{
private static final long serialVersionUID = 1L;
private JPanel topPanel;
private JPanel bottomPanel;
private JPanel rightPanel;
private JPanel mainPanel;
private JLabel message;
private final int width = 500;
private final int height = 400;

public Volume()
{
    setTitle("Sphere and Box Volumes");
    setSize(width,height);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    getContentPane().setLayout(new BorderLayout());
    getContentPane().add(topPanel, BorderLayout.NORTH);
    BuildTopPanel();
    getContentPane().add(bottomPanel, BorderLayout.SOUTH);
    BuildBotPanel();
    getContentPane().add(rightPanel, BorderLayout.EAST);
    BuildRightPanel();
    getContentPane().add(mainPanel, BorderLayout.WEST);
    BuildMainPanel();

    setVisible(true);
}
private void BuildTopPanel()
    {
        topPanel = new JPanel(new FlowLayout());
        topPanel.setBackground(Color.WHITE);
        JTextField reqVolume = new JTextField(8);
        message = new JLabel("Enter the required amount of volume:");
        topPanel.add(message);
        topPanel.add(reqVolume);
    }
private void BuildBotPanel()
    {
        bottomPanel = new JPanel(new FlowLayout());
        bottomPanel.setBackground(Color.WHITE);
        JButton intialQuant = new JButton("Set Initial Quantities");
        intialQuant.setActionCommand("I");
        intialQuant.addActionListener(new ButtonListener());
        JButton calcVolume = new JButton("Calculate Volumes");
        calcVolume.setActionCommand("V");
        calcVolume.addActionListener(new ButtonListener());
        JButton close = new JButton("Close");
        close.setActionCommand("C");
        close.addActionListener(new ButtonListener());
        bottomPanel.add(intialQuant);
        bottomPanel.add(calcVolume);
        bottomPanel.add(close);
    }
private void BuildRightPanel()
{
    rightPanel = new JPanel(new FlowLayout());
    rightPanel.setBackground(Color.WHITE);
}
private void BuildMainPanel()
{
    mainPanel = new JPanel(new FlowLayout());
    mainPanel.setBackground(Color.WHITE);
}
private class ButtonListener implements ActionListener
{
    public void actionPerformed(ActionEvent e)
    {

    }
 }
}

任何帮助都将不胜感激

public class VolumeDriver 
{
 public static void main(String[] args)
 {
    Volume frame = new Volume();
    frame.setVisible(true);
 }
}

您尝试在

BuildTopPanel（）

方法中初始化之前添加

topPanel

，因此它显然会导致NPE

其他构建方法也是如此。在使用GUI元素或将它们添加到窗口之前，请确保实际初始化并构建GUI元素

例如：

getContentPane().add(topPanel, BorderLayout.NORTH); // Uh oh! topPanel is not initialized!
BuildTopPanel();

---

在将组件添加到面板之前初始化组件。见以下文件：

这意味着您需要在getContentPane（）之前调用BuildMainPanel（）.add（mainPanel，BorderLayout.WEST）

---

与其他组件相同。

topPanel

为

null

。您可能需要使用调试器并找出答案。或移动

BuildTopPanel（）一行。还请注意，通常java方法名称以小写字母开头。您可能希望在方法的第一个字符上使用小写字母。
getContentPane().add(topPanel, BorderLayout.NORTH); // Uh oh! topPanel is not initialized!
BuildTopPanel();

// Initialize GUI elements first:
BuildTopPanel();
BuildBotPanel();
BuildRightPanel();
BuildMainPanel();
// Then add the GUI elements to the window:
getContentPane().add(topPanel, BorderLayout.NORTH);
getContentPane().add(bottomPanel, BorderLayout.SOUTH);
getContentPane().add(rightPanel, BorderLayout.EAST);
getContentPane().add(mainPanel, BorderLayout.WEST);