使用java表单的数据库滚动按钮 - 调用next方法将光标向上移动ResultSet对象时出错 [英] Database scrolling buttons using java forms - Error when calling next method to move cursor up Resultset object

问题描述

基本上：我添加了四个按钮到java表单。

Basically: I am adding four buttons to a java form. The buttons will enable me to move forward through the records, move back, move to the last record, and move to the first record.

我使用的是netbeans IDE 7.2。这些按钮将使我可以向前移动通过记录，移回到最后一条记录，并移动到第一条记录。 1和内置的Apache Derby网络服务器。

I am using netbeans IDE 7.2.1 together with the inbuilt Apache Derby Network Server.

我试图滚动浏览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，以移动游标。所有罚款和dandy，但然后当我尝试通过按下窗体上的下一个按钮启动下一个记录，从而调用下一个事件方法，我得到一个错误消息：代码似乎顶部卡在的时候，当我尝试再次调用next（）方法！

I am trying to scroll through my database records and output results in text fields. Initially the constructor successfully connects to the database and loads the first row from the result set object using the next() method to move the cursor. All fine and dandy, but then when I try to initiate the next record to load by pressing the next button on the form and thereby calling the next event method I get an error message: the code seems top get stuck at the point when I try to call the next() method again! Any Help would be appreciated.

这是我的java表单源代码：

Here is my java form source code:

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                   

这里是我从堆栈跟踪控制台窗口：

And here are the error messages that I get from the Stack trace console window:

线程AWT-EventQueue-0中的异常java.lang.NullPointerException
Employer.Workers .btnNextActionPerformed（Workers.java:211）at
Employer.Workers.access $ 000（Workers.java:12）at
Employer.Workers $ 1.actionPerformed（Workers.java:139）at
javax.swing.AbstractButton.fireActionPerformed（AbstractButton.java:2018）
at
javax.swing.AbstractButton $ Handler.actionPerformed（AbstractButton.java:2341）
at
javax。 swing.DefaultButtonModel.fireActionPerformed（DefaultButtonModel.java:402）
at
javax.swing.DefaultButtonModel.setPressed（DefaultButtonModel.java:259）
at
javax.swing.plaf。 basic.ButtonListener.mouseReleased（BasicButtonListener.java:252）
在java.awt.Component.processMouseEvent（Component.java:6505）at
javax.swing.JComponent.processMouseEvent（JComponent.java:3321）
java.awt.Component.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）at
java.awt.Component.dispatchEvent（Component.java:4687）at
java.awt.LightweightDispatcher.retargetMouseEvent（Container.java:4832）
at
java.awt.LightweightDispatcher.processMouseEvent（Container.java:4492）
在java.awt.LightweightDispatcher.dispatchEvent Container.java:4422）
在java.awt.Container.dispatchEventImpl（Container.java:2273）at
java.awt.Window.dispatchEventImpl（Window.java:2719）at
java .awt.Component.dispatchEvent（Component.java:4687）at
java.awt.EventQueue.dispatchEventImpl（EventQueue.java:723）at
java.awt.EventQueue.access $ 200（EventQueue.java： 103）at
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）
at
java.security.ProtectionDomain $ 1.doIntersectionPrivilege（ProtectionDomain.java： 87）
at java.awt.EventQueue $ 4.run（EventQueue.java:696）at
java.awt.EventQueue $ 4.run（EventQueue.java:694）at
java.security .AccessController.doPrivileged（Native Method）at
java.security.ProtectionDomain $ 1.doIntersectionPrivilege（ProtectionDomain.java:76）
在java.awt.EventQueue.dispatchEvent（EventQueue.java:693）at
java.awt.EventDispatchThread.pumpOneEventForFilters（EventDispatchThread.java:244）
at
java.awt.EventDispatchThread.pumpEventsForFilter（EventDispatchThread.java:163）
在
java。 awt.EventDispatchThread.pumpEventsForHierarchy（EventDispatchThread.java:151）
at
java.awt.EventDispatchThread.pumpEvents（EventDispatchThread.java:147）
at
java.awt.EventDispatchThread。 pumpEvents（EventDispatchThread.java:139）
在java.awt.EventDispatchThread.run（EventDispatchThread.java:97）
BUILD SUCCESSFUL（总时间：18秒）

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at Employer.Workers.btnNextActionPerformed(Workers.java:211) at Employer.Workers.access$000(Workers.java:12) at Employer.Workers$1.actionPerformed(Workers.java:139) at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018) at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341) at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402) at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259) at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252) at java.awt.Component.processMouseEvent(Component.java:6505) at javax.swing.JComponent.processMouseEvent(JComponent.java:3321) at java.awt.Component.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) at java.awt.Component.dispatchEvent(Component.java:4687) at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832) at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492) at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422) at java.awt.Container.dispatchEventImpl(Container.java:2273) at java.awt.Window.dispatchEventImpl(Window.java:2719) at java.awt.Component.dispatchEvent(Component.java:4687) at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:723) at java.awt.EventQueue.access$200(EventQueue.java:103) at 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) at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87) at java.awt.EventQueue$4.run(EventQueue.java:696) at 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) at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:163) at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:147) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:139) at java.awt.EventDispatchThread.run(EventDispatchThread.java:97) BUILD SUCCESSFUL (total time: 18 seconds)

谢谢。

推荐答案

c> ResultSet 是 null 。

You problem is that your ResultSet is null.

您正在隐藏您的变量 rs 。

您的代码（基本上）是：

Your code (basically) is:

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。

If you want to make your code working don't hide the variable. Also don't hide the connection.

要更快地获得答案尝试减少您的代码，使示例更短，专注于您的真正问题。

To get answers faster try to reduce your code and make the example shorter and concentrate on your real problem.

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