基本数据库/GUI应用程序错误-Java

Question

我还是Java新手，想知道是否有人能帮我解决这个错误？我现在正在遵循这里的教程：

http://www.homeandlearn.co.uk/java/database_scrolling_buttons.html

我确信我的代码与在btnNext和btnFirst上执行的操作中所述的完全一样。但它不断抛出一个错误：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Employees.Workers.btnNextActionPerformed(Workers.java:167)

有人看到我哪里出错了吗？

编辑：建议的答案并不能解决这个特定的问题。看看网站上的教程，这个代码应该基于本文中提到的代码工作。我需要澄清为什么不是？

抛出错误的具体线路是：

(rs.next()) {

如下所示：

private void btnNextActionPerformed(java.awt.event.ActionEvent evt) { try {

即使在删除DoConnect中的原始局部变量时，该按钮仍然无法正常工作。因此，不会出现阴影。有人能告诉我直接的方向吗？这正是教程所示的。：/

谢谢！

/*
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 * 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                   
    }

Answer 1

在类上定义了一个实例变量ResultSet rs;。在您的DoConnect()方法中，您正在创建一个新的ResultSet rs变量，该变量位于该方法的本地，如下所示：

ResultSet rs = stmt.executeQuery( SQL );

在您的事件方法中，您正在访问实例变量，该变量似乎从未初始化过，除非我在代码中遗漏了它。因此，它为空。以下是您可能需要做的：

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.
    }
}