基本上:我在Java表单中添加了四个按钮。这些按钮将使我能够在记录中前进,后退,移至最后一条记录以及移至第一条记录。

我正在将netbeans IDE 7.2.1与内置的Apache Derby Network Server一起使用。

我试图在数据库记录中滚动并在文本字段中输出结果。最初,构造函数成功连接到数据库,并使用next()方法移动光标,从结果集对象中加载第一行。一切正常,但随后,当我尝试通过按窗体上的next按钮并由此调用next事件方法来初始化要加载的下一条记录时,我收到一条错误消息:尝试时,代码似乎卡在了上面再次调用next()方法!
任何帮助,将不胜感激。

这是我的Java表单源代码:

package Employer;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.sql.Statement;
import java.sql.ResultSet;
import javax.swing.JOptionPane;

public class Workers extends javax.swing.JFrame {

    Connection con;
    Statement stmt;
    ResultSet rs;

    public Workers() {
        initComponents();
        DoConnect();
    }

    public void DoConnect( ) {

        try{
        String host = "jdbc:derby://localhost:1527/Employer";
        String uName = "admins";
        String uPass= "admins";


        Connection con = DriverManager.getConnection( host, uName, uPass );
//        Statement stmt = con.createStatement( );
        stmt = con.createStatement( ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE );

        String SQL = "SELECT * FROM WORKERS";
        ResultSet rs = stmt.executeQuery( SQL );

        rs.next( );
        int id_col = rs.getInt("ID");
        String id=Integer.toString(id_col);
        String first= rs.getString("First_Name");
        String last = rs.getString("Last_Name");
        String job = rs.getString("Job_Title");

        textID.setText(id);
        textFirstName.setText(first);
        textLastName.setText(last);
        textJobTitle.setText(job);

        }

        catch ( SQLException err ){
               System.out.println( err.getMessage( ) );

        }

    }

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

        jPanel1 = new javax.swing.JPanel();
        textJobTitle = new javax.swing.JTextField();
        jLabel1 = new javax.swing.JLabel();
        textID = new javax.swing.JTextField();
        textFirstName = new javax.swing.JTextField();
        textLastName = new javax.swing.JTextField();
        jPanel2 = new javax.swing.JPanel();
        btnNext = new javax.swing.JButton();
        btnLast = new javax.swing.JButton();
        btnPrevious = new javax.swing.JButton();
        btnFirst = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        textJobTitle.setToolTipText("");
        textJobTitle.setName(""); // NOI18N

        jLabel1.setFont(new java.awt.Font("Tahoma", 1, 14)); // NOI18N
        jLabel1.setText("Job Title:");

        javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
        jPanel1.setLayout(jPanel1Layout);
        jPanel1Layout.setHorizontalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(jLabel1)
                    .addComponent(textID, javax.swing.GroupLayout.PREFERRED_SIZE, 42, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 127, Short.MAX_VALUE)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                    .addGroup(jPanel1Layout.createSequentialGroup()
                        .addComponent(textFirstName, javax.swing.GroupLayout.PREFERRED_SIZE, 75, javax.swing.GroupLayout.PREFERRED_SIZE)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                        .addComponent(textLastName, javax.swing.GroupLayout.PREFERRED_SIZE, 60, javax.swing.GroupLayout.PREFERRED_SIZE))
                    .addComponent(textJobTitle, javax.swing.GroupLayout.PREFERRED_SIZE, 221, javax.swing.GroupLayout.PREFERRED_SIZE)))
        );
        jPanel1Layout.setVerticalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(textID, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addComponent(textFirstName, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addComponent(textLastName, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addGap(24, 24, 24)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel1)
                    .addComponent(textJobTitle, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addContainerGap(55, Short.MAX_VALUE))
        );

        textJobTitle.getAccessibleContext().setAccessibleName("");

        btnNext.setFont(new java.awt.Font("Tahoma", 1, 14)); // NOI18N
        btnNext.setText("Next");
        btnNext.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnNextActionPerformed(evt);
            }
        });

        btnLast.setFont(new java.awt.Font("Tahoma", 1, 14)); // NOI18N
        btnLast.setText("Last");

        btnPrevious.setFont(new java.awt.Font("Tahoma", 1, 14)); // NOI18N
        btnPrevious.setText("Previous");

        btnFirst.setFont(new java.awt.Font("Tahoma", 1, 14)); // NOI18N
        btnFirst.setText("First");

        javax.swing.GroupLayout jPanel2Layout = new javax.swing.GroupLayout(jPanel2);
        jPanel2.setLayout(jPanel2Layout);
        jPanel2Layout.setHorizontalGroup(
            jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel2Layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(btnFirst)
                .addGap(29, 29, 29)
                .addComponent(btnPrevious)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 59, Short.MAX_VALUE)
                .addComponent(btnNext)
                .addGap(33, 33, 33)
                .addComponent(btnLast)
                .addContainerGap())
        );
        jPanel2Layout.setVerticalGroup(
            jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel2Layout.createSequentialGroup()
                .addGap(27, 27, 27)
                .addGroup(jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(btnNext)
                    .addComponent(btnLast)
                    .addComponent(btnPrevious)
                    .addComponent(btnFirst))
                .addContainerGap(29, 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.LEADING)
                    .addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addComponent(jPanel2, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addContainerGap(795, Short.MAX_VALUE))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGap(22, 22, 22)
                .addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGap(72, 72, 72)
                .addComponent(jPanel2, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap(78, Short.MAX_VALUE))
        );

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

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





        if ( rs.next( )) {

               int id_col = rs.getInt("ID");
                String id = Integer.toString(id_col);
                String first = rs.getString("First_Name");
                String last = rs.getString("Last_Name");
                String job = rs.getString("Job_Title");

                textID.setText(id);
                textFirstName.setText(first);
                textLastName.setText(last);
               textJobTitle.setText(job);
            }
            else {
                rs.previous( );
                JOptionPane.showMessageDialog(Workers.this, "End of File");
            }
        }
        catch (SQLException err) {
            JOptionPane.showMessageDialog(Workers.this, err.getMessage());
            System.out.println( err.getMessage( ) );
        }
    }


    public static void main(String args[]) {

        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.JPanel jPanel1;
    private javax.swing.JPanel jPanel2;
    private javax.swing.JTextField textFirstName;
    private javax.swing.JTextField textID;
    private javax.swing.JTextField textJobTitle;
    private javax.swing.JTextField textLastName;
    // End of variables declaration


这是我从堆栈跟踪控制台窗口中收到的错误消息:


  线程“ AWT-EventQueue-0”中的异常java.lang.NullPointerException
    在Employer.Workers.btnNextActionPerformed(Workers.java:211)在
  Employer.Workers.access $ 000(Workers.java:12)在
  Employer.Workers $ 1.actionPerformed(Workers.java:139)在
  javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
    在
  javax.swing.AbstractButton $ Handler.actionPerformed(AbstractButton.java:2341)
    在
  javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    在
  javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    在
  javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    在java.awt.Component.processMouseEvent(Component.java:6505)在
  javax.swing.JComponent.processMouseEvent(JComponent.java:3321)在
  java.awt.Component.processEvent(Component.java:6270)在
  java.awt.Container.processEvent(Container.java:2229)在
  java.awt.Component.dispatchEventImpl(Component.java:4861)在
  java.awt.Container.dispatchEventImpl(Container.java:2287)在
  java.awt.Component.dispatchEvent(Component.java:4687)在
  java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
    在
  java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
    在java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
    在java.awt.Container.dispatchEventImpl(Container.java:2273)在
  java.awt.Window.dispatchEventImpl(Window.java:2719)在
  java.awt.Component.dispatchEvent(Component.java:4687)在
  java.awt.EventQueue.dispatchEventImpl(EventQueue.java:723)在
  java.awt.EventQueue.access $ 200(EventQueue.java:103)在
  java.awt.EventQueue $ 3.run(EventQueue.java:682)在
  java.awt.EventQueue $ 3.run(EventQueue.java:680)在
  java.security.AccessController.doPrivileged(本机方法),位于
  java.security.ProtectionDomain $ 1.doIntersectionPrivilege(ProtectionDomain.java:76)
    在
  java.security.ProtectionDomain $ 1.doIntersectionPrivilege(ProtectionDomain.java:87)
    在java.awt.EventQueue $ 4.run(EventQueue.java:696)在
  java.awt.EventQueue $ 4.run(EventQueue.java:694)在
  java.security.AccessController.doPrivileged(本机方法),位于
  java.security.ProtectionDomain $ 1.doIntersectionPrivilege(ProtectionDomain.java:76)
    在java.awt.EventQueue.dispatchEvent(EventQueue.java:693)处
  java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:244)
    在
  java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:163)
    在
  java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151)
    在
  java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:147)
    在
  java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:139)
    在java.awt.EventDispatchThread.run(EventDispatchThread.java:97)
  成功构建(总时间:18秒)


谢谢。

最佳答案

您的问题是您的ResultSetnull

您正在隐藏变量rs

您的代码(基本上)是:

public class Sample {
    ResultSet rs; // is null
    public method() {
        ResultSet rs = stmt.execute(); // here the other rs is hidden
    }
}


如果要使代码正常工作,请不要隐藏变量。也不要隐藏连接。

为了更快地获得答案,请尝试减少代码并简化示例,并专注于实际问题。

09-30 15:04
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