我还是Java新手，想知道是否有人可以帮我解决这个错误？我现在正在关注教程：
http://www.homeandlearn.co.uk/java/database_scrolling_buttons.html
我很肯定我的代码与在btnNext和btnFirst上执行的操作完全相同。但它一直在抛出一个错误：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Employees.Workers.btnNextActionPerformed(Workers.java:167)
谁能看到我哪里出错？
编辑：建议的答案无法解决此特定问题。查看给定网站上的教程，此代码应该基于本文中提到的代码工作。我需要澄清它为什么不是？
抛出错误的具体行是：
(rs.next()) {
以下是：
private void btnNextActionPerformed(java.awt.event.ActionEvent evt) { try {
即使在DoConnect中删除原始局部变量仍然不允许按钮正常工作。因此，没有发生阴影。有人可以指出我的直接方向，这正是教程显示..：/
谢谢！
/*
* To change this license header, choose License Headers in Project Properties.
* To change this template file, choose Tools | Templates
* and open the template in the editor.
*/
package Employees;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.sql.Statement;
import java.sql.ResultSet;
import javax.swing.JOptionPane;
/**
*
* @author PC
*/
public class Workers extends javax.swing.JFrame {
Connection con;
Statement stmt;
ResultSet rs;
/**
* Creates new form Workers
*/
public Workers() {
initComponents();
DoConnect();
}
public void DoConnect( ) {
try {
// Make a connection to the database
String host = "jdbc:derby://localhost:1527/Employees";
String uName = "user1";
String uPass= "pass";
Connection con = DriverManager.getConnection( host, uName, uPass );
//Execute SQL and load the records
Statement stmt = con.createStatement();
String SQL = "SELECT * FROM Workers";
ResultSet rs = stmt.executeQuery( SQL );
//Move the cursor to the first reocrd and get the data
rs.next();
int id_col = rs.getInt("ID");
String id = Integer.toString( id_col );
String first_name = rs.getString("First_Name");
String last_name = rs.getString("Last_Name");
String job = rs.getString("Job_Title");
//Display the first record in the text field
textID.setText(id);
textFirstName.setText(first_name);
textLastName.setText(last_name);
textJobTitle.setText(job);
}
catch ( SQLException err ) {
JOptionPane.showMessageDialog(this, err.getMessage());
}
}
/**
* This method is called from within the constructor to initialize the form.
* WARNING: Do NOT modify this code. The content of this method is always
* regenerated by the Form Editor.
*/
@SuppressWarnings("unchecked")
// <editor-fold defaultstate="collapsed" desc="Generated Code">
private void initComponents() {
jSeparator1 = new javax.swing.JSeparator();
textID = new javax.swing.JTextField();
textFirstName = new javax.swing.JTextField();
textLastName = new javax.swing.JTextField();
textJobTitle = new javax.swing.JTextField();
jLabel1 = new javax.swing.JLabel();
btnFirst = new javax.swing.JButton();
btnPrevious = new javax.swing.JButton();
btnNext = new javax.swing.JButton();
btnLast = new javax.swing.JButton();
setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
jLabel1.setFont(new java.awt.Font("Tahoma", 0, 12)); // NOI18N
jLabel1.setText("Job Title");
btnFirst.setText("First");
btnFirst.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
btnFirstActionPerformed(evt);
}
});
btnPrevious.setText("Previous");
btnNext.setText("Next");
btnNext.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
btnNextActionPerformed(evt);
}
});
btnLast.setText("Last");
btnLast.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
btnLastActionPerformed(evt);
}
});
javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
getContentPane().setLayout(layout);
layout.setHorizontalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addGap(51, 51, 51)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
.addComponent(btnFirst, javax.swing.GroupLayout.DEFAULT_SIZE, 57, Short.MAX_VALUE)
.addComponent(textID, javax.swing.GroupLayout.PREFERRED_SIZE, 46, javax.swing.GroupLayout.PREFERRED_SIZE)
.addComponent(jLabel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
.addGap(48, 48, 48)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
.addGroup(layout.createSequentialGroup()
.addComponent(textFirstName, javax.swing.GroupLayout.PREFERRED_SIZE, 138, javax.swing.GroupLayout.PREFERRED_SIZE)
.addGap(33, 33, 33)
.addComponent(textLastName))
.addComponent(textJobTitle, javax.swing.GroupLayout.PREFERRED_SIZE, 311, javax.swing.GroupLayout.PREFERRED_SIZE))
.addGroup(layout.createSequentialGroup()
.addComponent(btnPrevious, javax.swing.GroupLayout.PREFERRED_SIZE, 92, javax.swing.GroupLayout.PREFERRED_SIZE)
.addGap(35, 35, 35)
.addComponent(btnNext, javax.swing.GroupLayout.PREFERRED_SIZE, 92, javax.swing.GroupLayout.PREFERRED_SIZE)
.addGap(33, 33, 33)
.addComponent(btnLast, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)))
.addContainerGap(77, Short.MAX_VALUE))
);
layout.setVerticalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addGap(18, 18, 18)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(textLastName, javax.swing.GroupLayout.PREFERRED_SIZE, 28, javax.swing.GroupLayout.PREFERRED_SIZE)
.addComponent(textFirstName, javax.swing.GroupLayout.PREFERRED_SIZE, 28, javax.swing.GroupLayout.PREFERRED_SIZE))
.addComponent(textID))
.addGap(32, 32, 32)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(textJobTitle, javax.swing.GroupLayout.PREFERRED_SIZE, 28, javax.swing.GroupLayout.PREFERRED_SIZE)
.addComponent(jLabel1, javax.swing.GroupLayout.PREFERRED_SIZE, 28, javax.swing.GroupLayout.PREFERRED_SIZE))
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 59, Short.MAX_VALUE)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(btnFirst, javax.swing.GroupLayout.PREFERRED_SIZE, 32, javax.swing.GroupLayout.PREFERRED_SIZE)
.addComponent(btnPrevious, javax.swing.GroupLayout.PREFERRED_SIZE, 32, javax.swing.GroupLayout.PREFERRED_SIZE)
.addComponent(btnNext, javax.swing.GroupLayout.PREFERRED_SIZE, 32, javax.swing.GroupLayout.PREFERRED_SIZE)
.addComponent(btnLast, javax.swing.GroupLayout.PREFERRED_SIZE, 32, javax.swing.GroupLayout.PREFERRED_SIZE))
.addGap(56, 56, 56))
);
pack();
}// </editor-fold>
private void btnNextActionPerformed(java.awt.event.ActionEvent evt) {
try {
if (rs.next()) {
int id_col = rs.getInt("ID");
String id = Integer.toString( id_col );
String first_name = rs.getString("First_Name");
String last_name = rs.getString("Last_Name");
String job = rs.getString("Job_Title");
textID.setText(id);
textFirstName.setText(first_name);
textLastName.setText(last_name);
textJobTitle.setText(job);
}
else {
rs.previous( );
JOptionPane.showMessageDialog(Workers.this, "End of File");
}
}
catch (SQLException err) {
JOptionPane.showMessageDialog(Workers.this, err.getMessage());
}
}
private void btnFirstActionPerformed(java.awt.event.ActionEvent evt) {
try {
rs.first();
int id_col = rs.getInt("ID");
String id = Integer.toString(id_col);
String first_name = rs.getString("First_Name");
String last_name = rs.getString("Last_Name");
String job = rs.getString("Job_Title");
textID.setText(id);
textFirstName.setText(first_name);
textLastName.setText(last_name);
textJobTitle.setText(job);
}
catch (SQLException err) {
JOptionPane.showMessageDialog(this, err.getMessage());
}
}
private void btnLastActionPerformed(java.awt.event.ActionEvent evt) {
// TODO add your handling code here:
}
/**
* @param args the command line arguments
*/
public static void main(String args[]) {
/* Set the Nimbus look and feel */
//<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
/* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
* For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html
*/
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(Workers.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (InstantiationException ex) {
java.util.logging.Logger.getLogger(Workers.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (IllegalAccessException ex) {
java.util.logging.Logger.getLogger(Workers.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (javax.swing.UnsupportedLookAndFeelException ex) {
java.util.logging.Logger.getLogger(Workers.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
}
//</editor-fold>
/* Create and display the form */
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
new Workers().setVisible(true);
}
});
}
// Variables declaration - do not modify
private javax.swing.JButton btnFirst;
private javax.swing.JButton btnLast;
private javax.swing.JButton btnNext;
private javax.swing.JButton btnPrevious;
private javax.swing.JLabel jLabel1;
private javax.swing.JSeparator jSeparator1;
private javax.swing.JTextField textFirstName;
private javax.swing.JTextField textID;
private javax.swing.JTextField textJobTitle;
private javax.swing.JTextField textLastName;
// End of variables declaration
}
答案 0 :(得分：0)
您在类上定义了一个实例变量ResultSet rs;。在您的DoConnect()方法中，您将使用以下方法在该方法的基础上创建新的ResultSet rs变量：
ResultSet rs = stmt.executeQuery( SQL );
在您的活动方法中，您访问的实例变量似乎无法初始化，除非我在您的代码中遗漏了它。因此，它是null。以下是您可能需要做的事情：
public class Workers extends javax.swing.JFrame {
Connection con;
Statement stmt;
ResultSet rs; // this variable will be used in DoConnect()
/**
* Creates new form Workers
*/
public Workers() {
initComponents();
DoConnect();
}
public void DoConnect( ) {
// code omitted for brevity
//Execute SQL and load the records
Statement stmt = con.createStatement();
String SQL = "SELECT * FROM Workers";
// note here I'm setting the instance variable
rs = stmt.executeQuery( SQL );
// rest of code omitted for brevity.
}
}