java - 为什么我的程序可以编译但不能运行？

Question

由于对 OP 代码进行了更改，这是一个后续问题。另一位用户建议我将另一个问题链接到这个问题。

OP 是:Why does the compiler not run the program even though there are not any errors?

我有一段代码可以编译但无法运行。我试图让 GUI 运行，以便我可以添加代码来执行我需要的功能。代码如下:

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

import java.awt.event.*;

public class Transaction extends JFrame {

private static final long serialVersionUID = 1L;
// JFrame frame = new JFrame("Bank Account - S Standridge");
JMenuBar menuBar;
JMenu file = new JMenu("File");
JMenu edit = new JMenu("Edit");
JMenu about = new JMenu("About");
JMenuItem transaction = new JMenuItem("Transaction");
JMenuItem summary = new JMenuItem("Summary");
JMenuItem exit = new JMenuItem("Exit");
private JPanel mp;
private JPanel tp;
private JPanel bp;
private JButton calcButton;    
private JButton exitButton; 
private JMenuItem summaryMenuItem;
private JMenuItem aboutMenuItem;
private JMenuItem exitMenuItem;

public Transaction() {
    setTitle("Bank Account - S Standridge");

    mp = new JPanel();
    tp = new JPanel();
    bp = new JPanel();

    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    setLayout(new BorderLayout());

    menuPanel();        
    transactionPanel();
    BuildButtonPanel();     

    add(mp, BorderLayout.NORTH);
    add(tp, BorderLayout.WEST);
    add(bp, BorderLayout.SOUTH);

    pack();
    setVisible(true);
}

public void summary() {

}

private void menuPanel() {
    b
    menuBar = new JMenuBar();

    setJMenuBar(menuBar);
    setVisible(true);

    menuBar.add(file);
    menuBar.add(edit);
    menuBar.add(about);

    summaryMenuItem.addActionListener(new SummaryMenuListener());
    aboutMenuItem.addActionListener(new AboutMenuListener());

    file.add(transaction);
    file.add(summaryMenuItem);
    file.add(exitMenuItem);


}

private void BuildButtonPanel() {

     // Create a panel for the buttons.
      bp = new JPanel();

      // Create the buttons.
      calcButton = new JButton("Calculate");
      exitButton = new JButton("Exit");

      // Register the action listeners.
      calcButton.addActionListener(new CalcButtonListener());
      exitButton.addActionListener(new ExitButtonListener());

      // Add the buttons to the button panel.
      bp.add(calcButton);
      bp.add(exitButton);

}

private void transactionPanel()
{
    setLayout(new FlowLayout());

    JRadioButton b1 = new JRadioButton("Checkings");
    // b1.addActionListener(this);
    add(b1);

    JRadioButton b2 = new JRadioButton("Savings");
    // b2.addActionListener(this);
    add(b2);



    ButtonGroup bg = new ButtonGroup();
    bg.add(b1);
    bg.add(b2);


    JTextField tf = new JTextField(5);
    add(tf);
  }

}

class CalcButtonListener implements ActionListener
{
   public void actionPerformed(ActionEvent e)
   {

   }
}

class SummaryMenuListener implements ActionListener
   {
      public void actionPerformed(ActionEvent e)
      {

      }
   }

class ExitButtonListener implements ActionListener
   {
      public void actionPerformed(ActionEvent e)
      {
          System.exit(0);
      }
   }

class AboutMenuListener implements ActionListener
   {
      public void actionPerformed(ActionEvent e)
      {
       // Displays Message Box
      }
   }

我在控制台中得到的错误如下:

java.lang.reflect.InvocationTargetException
IWAV0052E Invocation Target Exception creating Transaction
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
at java.lang.reflect.Constructor.newInstance(Unknown Source)
at org.eclipse.ve.internal.java.vce.launcher.remotevm.JFCLauncher$1.run(JFCLauncher.java:59)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$000(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
Caused by: java.lang.NullPointerException
at Transaction.menuPanel(Transaction.java:64)
at Transaction.<init>(Transaction.java:37)
... 19 more

Answer 1

嗯，这绝对不是您的实际代码，因为您在 menuPanel 方法的开头有一个恶意 b。但是，您基本上需要查看堆栈跟踪:

Caused by: java.lang.NullPointerException
at Transaction.menuPanel(Transaction.java:64)
at Transaction.<init>(Transaction.java:37)

查看 Transaction 中的那些行，找出空指针的位置。

看起来是这一行:

summaryMenuItem.addActionListener(new SummaryMenuListener());

您尚未初始化 summaryMenuItem，因此当您尝试根据其值调用方法时，您会遇到异常。

比修复这个特定错误更重要的是学习如何修复错误:

• 查看堆栈跟踪
• 理解异常的含义
• 如有必要，在调试器中运行代码，调试器通常会在抛出未捕获的异常时暂停
• 如果可能，编写单元测试，这样您就可以自信地修复代码