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基本上：我在 java 表单中添加了四个按钮。这些按钮使我能够在记录中向前移动、向后移动、移动到最后一条记录以及移动到第一条记录。

我正在使用 netbeans IDE 7.2.1 和内置的 Apache Derby 网络服务器。

我正在尝试滚动浏览我的数据库记录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() 方法从结果集对象加载第一行以移动光标。一切都很好，但是当我尝试通过按下表单上的下一个按钮并因此调用下一个事件方法来启动要加载的下一条记录时，我收到一条错误消息：当我尝试时，代码似乎卡在了顶部再次调用 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

以下是我从堆栈跟踪控制台窗口收到的错误消息：

Employer.Workers.access$000(Workers.java:12) 的 Employer.Workers.btnNextActionPerformed(Workers.java:211) 的线程“AWT-EventQueue-0”java.lang.NullPointerException 中的异常 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 java.awt.Component 的 javax.swing.JComponent.processMouseEvent(JComponent.java:3321)。processEvent(Component.java:6270) at java.awt.Container.processEvent(Container.java:2229) at java.awt.Component.dispatchEventImpl(Component.java:4861) at 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) at java.awt.EventQueue$3.run(EventQueue.java:680) at java.security.AccessController.doPrivileged(Native Method) at 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) at java.security.AccessController.doPrivileged(Native Method) at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76) at java.awt.EventQueue.dispatchEvent(EventQueue.java:693) at 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) 构建成功（总时间：18 秒）

谢谢你。

4

2 回答 2

2

你的问题是你的ResultSet是null。

你隐藏了你的变量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
    }
}

如果你想让你的代码工作，不要隐藏变量。也不要隐藏连接。

为了更快地获得答案，请尝试减少代码并缩短示例并专注于您的实际问题。

于 2012-12-08T21:11:13.270 回答
1

很简单的人。你已经声明ResultSet rs你只需要放

rs = stmt.executeQuery(sql);

你的工作就完成了。

于 2017-05-24T15:03:08.023 回答