Question

Exception in thread "main" java.lang.NullPointerException
at java.awt.Container.addImpl(Unknown Source)
at java.awt.Container.add(Unknown Source)
at Volume.<init>(Volume.java:25)
at VolumeDriver.main(VolumeDriver.java:6)

尝试运行程序时出现上述错误。我的程序不完整，但我只想查看我的窗口是什么样的，所以我可以确保它看起来正确。这是我现在的工人班

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)
    {

    }
 }
}

这是我的驱动程序类

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

任何帮助都将不胜感激。

Answer 1

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

您尝试在topPanel方法初始化之前添加BuildTopPanel()，因此显然会导致NPE！

您的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，请务必初始化并构建GUI元素！

例如：

// 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);

Answer 2

在将组件添加到面板之前初始化组件。见文件如下：

http://docs.oracle.com/javase/7/docs/api/java/awt/Container.html#add%28java.awt.Component%29

这意味着你需要在getContentPane（）之前调用BuildMainPanel（）。add（mainPanel，BorderLayout.WEST）;

与其他组件相同。

工作者类中的Java空指针异常

2 个答案: