在JFrame中显示外部文件内容的问题

Question

我正在为我的大学任务做一个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。

{
    "notification": {
        "messageMD5Sum": "ce8d9520466b3cf9d920a8d9d338f11a",
        "messageId": "33e7e635-ecc0-534a-8498-b5a442fb6db6",
        "timestamp": "2018-06-06 09:40:11.911"
    },
    "delivery": {
        "deliveryId": "cfeecbcf-8a08-59c2-83f1-985e2434b67f",
        "destination": "arn:aws:sns:ap-northeast-1:111122223333:endpoint/APNS_SANDBOX/com.myapp.domain/25b45520-c2fe-3345-9c54-e458798b74ee",
        "dwellTimeMs": 37,
        "attempts": 1,
        "token": "edf0462f0caa6e6783857774009f6266edbaf8e1b65bc1f71409652f2a47be1c"
    },
    "status": "FAILURE"
}

我想在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 } 中显示外部文件的内容，但是当我运行该程序时，它会一直给我这个错误。

JFrame

有人可以告诉我我做错了什么吗？

Answer 1

在Java中，java.lang.NullPointerException几乎总是意味着你试图对一个空对象做一些事情 - 一个尚未创建的对象，或一个已创建但未正确初始化的对象。

第57行是：

        NameText.setEditable(false);

所以首先要看的是你是否曾创建过NameText。

你把它放得更远：

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，并且类最初应该大写。

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。如果你没有其他人（和你！）在阅读你的代码时会遇到很多困难。

所以NameText应该是nameText。

既然强制性的唠叨已经结束，我可以回到强制性的初始化唠叨：D

你使用它之前必须使用新的NAME。

e.g。

        NameText = new JTextField();

请注意，您几乎肯定会遇到所有其他变量的相同问题。

此外，返回到样式唠叨 - 调用Main类使得更难理解发生了什么，因为构造函数看起来太过类似于默认的Java入口点（public static void main（String [] args）的东西这是代码开始执行的起点。）