Java 在JFrame中显示外部文件内容时出现问题

我正在为我的大学作业做一个Java程序，但有些东西不起作用。我可以被认为是Java的新手，因为我没有太多的编码经验

package javaassignment;

import java.io.*;
import java.util.*;

public class Main extends javax.swing.JFrame 

{
private FileInputStream in;
static String name;
Scanner file;

 public Main(String filename) throws FileNotFoundException 
 {
    in = new FileInputStream(filename);
    try
    {
      file=new Scanner(new File("c:/users/MSI-USER/desktop/JavaAssignment/Student.txt"));  
    }
    catch(FileNotFoundException ea)
    {
        ea.printStackTrace();
    }
 }
 public void Read()
 {
     do
     {
         name=file.nextLine();       
     }
     while(file.hasNext());
     System.out.println(name);
     file.close();
 }


 public String getWord() throws IOException 
 {
    int c;
    StringBuffer buf = new StringBuffer();

    do 
    {
      c = in.read();
      if (Character.isSpace((char) c))
        return buf.toString();
      else
        buf.append((char) c);
    } while (c != -1);

    return buf.toString();
  }

    public Main() 
    {
        ComboPanel.add(NameText);
        NameText.setEditable(false);
        NameText.setText(name);
        initComponents();
    }


    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        ButtonPanel = new javax.swing.JPanel();
        PrintButton = new javax.swing.JButton();
        SaveButton = new javax.swing.JButton();
        CalcButton = new javax.swing.JButton();
        LabelPanel = new javax.swing.JPanel();
        StudentLabel = new javax.swing.JLabel();
        AttendanceLabel = new javax.swing.JLabel();
        ScrollPanel = new javax.swing.JPanel();
        jScrollBar1 = new javax.swing.JScrollBar();
        ComboPanel = new javax.swing.JPanel();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        PrintButton.setText("Print");
        PrintButton.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                PrintButtonActionPerformed(evt);
            }
        });

        SaveButton.setText("Save");
        SaveButton.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                SaveButtonActionPerformed(evt);
            }
        });

        CalcButton.setText("Calculate Percentage");
        CalcButton.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                CalcButtonActionPerformed(evt);
            }
        });

        javax.swing.GroupLayout ButtonPanelLayout = new javax.swing.GroupLayout(ButtonPanel);
        ButtonPanel.setLayout(ButtonPanelLayout);
        ButtonPanelLayout.setHorizontalGroup(
            ButtonPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(ButtonPanelLayout.createSequentialGroup()
                .addGap(43, 43, 43)
                .addComponent(SaveButton, javax.swing.GroupLayout.PREFERRED_SIZE, 126, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(PrintButton, javax.swing.GroupLayout.PREFERRED_SIZE, 130, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(CalcButton)
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
        );
        ButtonPanelLayout.setVerticalGroup(
            ButtonPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(ButtonPanelLayout.createSequentialGroup()
                .addContainerGap()
                .addGroup(ButtonPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(PrintButton, javax.swing.GroupLayout.DEFAULT_SIZE, 66, Short.MAX_VALUE)
                    .addComponent(CalcButton, javax.swing.GroupLayout.DEFAULT_SIZE, 66, Short.MAX_VALUE)
                    .addComponent(SaveButton, javax.swing.GroupLayout.DEFAULT_SIZE, 66, Short.MAX_VALUE))
                .addGap(21, 21, 21))
        );

        StudentLabel.setText("Student");

        AttendanceLabel.setText("Attendance Status");

        javax.swing.GroupLayout ComboPanelLayout = new javax.swing.GroupLayout(ComboPanel);
        ComboPanel.setLayout(ComboPanelLayout);
        ComboPanelLayout.setHorizontalGroup(
            ComboPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 0, Short.MAX_VALUE)
        );
        ComboPanelLayout.setVerticalGroup(
            ComboPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 0, Short.MAX_VALUE)
        );

        javax.swing.GroupLayout ScrollPanelLayout = new javax.swing.GroupLayout(ScrollPanel);
        ScrollPanel.setLayout(ScrollPanelLayout);
        ScrollPanelLayout.setHorizontalGroup(
            ScrollPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, ScrollPanelLayout.createSequentialGroup()
                .addComponent(ComboPanel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(jScrollBar1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
        );
        ScrollPanelLayout.setVerticalGroup(
            ScrollPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addComponent(jScrollBar1, javax.swing.GroupLayout.DEFAULT_SIZE, 460, Short.MAX_VALUE)
            .addComponent(ComboPanel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
        );

        javax.swing.GroupLayout LabelPanelLayout = new javax.swing.GroupLayout(LabelPanel);
        LabelPanel.setLayout(LabelPanelLayout);
        LabelPanelLayout.setHorizontalGroup(
            LabelPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(LabelPanelLayout.createSequentialGroup()
                .addComponent(StudentLabel, javax.swing.GroupLayout.PREFERRED_SIZE, 382, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(AttendanceLabel, javax.swing.GroupLayout.PREFERRED_SIZE, 378, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap(15, Short.MAX_VALUE))
            .addComponent(ScrollPanel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
        );
        LabelPanelLayout.setVerticalGroup(
            LabelPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(LabelPanelLayout.createSequentialGroup()
                .addGroup(LabelPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                    .addComponent(AttendanceLabel, javax.swing.GroupLayout.DEFAULT_SIZE, 89, Short.MAX_VALUE)
                    .addComponent(StudentLabel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(ScrollPanel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
        );

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING, false)
                    .addComponent(LabelPanel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                    .addComponent(ButtonPanel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(LabelPanel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(ButtonPanel, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap())
        );

        pack();
    }// </editor-fold>                        

    private void SaveButtonActionPerformed(java.awt.event.ActionEvent evt) {                                           
        // TODO add your handling code here:
    }                                          

    private void PrintButtonActionPerformed(java.awt.event.ActionEvent evt) {                                            

    }                                           

    private void CalcButtonActionPerformed(java.awt.event.ActionEvent evt) {                                           
     JavaAssignment J=new JavaAssignment();
     PercentageDialog P=new PercentageDialog();
     P.setVisible(true);

    }                                          




    public static void main(String args[]) throws java.io.IOException
    {




        try 
        {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) 
            {
                if ("Nimbus".equals(info.getName())) 
                {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } 
        catch (ClassNotFoundException ex) 
        {
            java.util.logging.Logger.getLogger(Main.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } 
        catch (InstantiationException ex) 
        {
            java.util.logging.Logger.getLogger(Main.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } 
        catch (IllegalAccessException ex) 
        {
            java.util.logging.Logger.getLogger(Main.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } 
        catch (javax.swing.UnsupportedLookAndFeelException ex) 
        {
            java.util.logging.Logger.getLogger(Main.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }

        java.awt.EventQueue.invokeLater(() -> {
            new Main().setVisible(true);
        });
    }
    private javax.swing.JTextField NameText;

    // Variables declaration - do not modify                     
    private javax.swing.JLabel AttendanceLabel;
    private javax.swing.JPanel ButtonPanel;
    private javax.swing.JButton CalcButton;
    private javax.swing.JPanel ComboPanel;
    private javax.swing.JPanel LabelPanel;
    private javax.swing.JButton PrintButton;
    private javax.swing.JButton SaveButton;
    private javax.swing.JPanel ScrollPanel;
    private javax.swing.JLabel StudentLabel;
    private javax.swing.JScrollBar jScrollBar1;
    // End of variables declaration                   
}

---

有人能告诉我我做错了什么吗？

在Java.lang.NullPointerException中，几乎总是意味着你试图对一个空对象做些什么-一个尚未创建的对象，或者一个已创建但未正确初始化的对象

第57行是这样的：

        NameText.setEditable(false);

因此，首先要看的是您是否创建过NameText

你把它写得更深了：

private javax.swing.JTextField NameText;

请注意，您违反了一个重要的样式原则—对象和变量最初应该是小写的，类最初应该大写

请注意，这与自然英语用法相反

Steve is a person.

在Java中：

Person steve = new Person();

你会习惯的。如果你不这样做，其他人（还有你！）将很难阅读你的代码

所以NameText应该是NameText

既然强制性的风格唠叨已经结束，我可以回到强制性的初始化唠叨：D

您必须在使用新的NAMETEXT之前创建它

e、 g

请注意，几乎可以肯定，对于所有其他变量，您都会遇到同样的问题

---

另外，返回到风格上的唠叨——调用类Main使理解发生了什么变得更加困难，因为构造函数看起来太类似于默认的Java入口点（公共静态void Main（String[]args）东西，它是代码开始执行的起点）.

在

Main（）

构造函数在

initComponents（）中初始化之前，您正在调用
ComboPanel
。因此，NPE用于调用
null
对象上的方法，因此可以在顶部初始化组件：public Main（）{initComponents（）；ComboPanel.add（NameText）；NameText.setEditable（false）；NameText.setText（name）；}Ha！我甚至没有看到initComponents，因为编辑器把它折叠起来了。漂亮的斑点。
Person steve = new Person();

        NameText = new JTextField();