使用java窗体的数据库滚动按钮-调用下一个方法向上移动光标Resultset对象时出错

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ApacheDerby网络服务器

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，但是当我试图通过按下表单上的next按钮来启动下一个要加载的记录，从而调用next event方法时，我得到一条错误消息：当我再次尝试调用next（）方法时，代码似乎卡在了某个点上！ 任何帮助都将不胜感激

以下是我的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                   

包业主；
导入java.sql.Connection；
导入java.sql.DriverManager；
导入java.sql.SQLException；
导入java.sql.Statement；
导入java.sql.ResultSet；
导入javax.swing.JOptionPane；
公共类工作者扩展了javax.swing.JFrame{
连接con；
报表stmt；
结果集rs；
公职人员（）{
初始化组件（）；
DoConnect（）；
}
公共无效文档连接（）{
试一试{
String host=“jdbc:derby://localhost:1527/Employer";
字符串uName=“admins”；
字符串uPass=“admins”；
Connection con=DriverManager.getConnection（主机、uName、uPass）；
//语句stmt=con.createStatement（）；
stmt=con.createStatement（ResultSet.TYPE\u SCROLL\u不敏感，ResultSet.CONCUR\u可更新）；
String SQL=“SELECT*FROM WORKERS”；
结果集rs=stmt.executeQuery（SQL）；
rs.next（）；
int id_col=rs.getInt（“id”）；
String id=Integer.toString（id\u col）；
String first=rs.getString（“first_Name”）；
字符串last=rs.getString（“last_Name”）；
String job=rs.getString（“job_Title”）；
textID.setText（id）；
textFirstName.setText（第一个）；
textLastName.setText（last）；
textJobTitle.setText（作业）；
}
捕获（SQLException错误）{
System.out.println（err.getMessage（））；
}
}
@抑制警告（“未选中”）
//                           
私有组件（）{
jPanel1=newjavax.swing.JPanel（）；
textJobTitle=newjavax.swing.JTextField（）；
jLabel1=newjavax.swing.JLabel（）；
textID=newjavax.swing.JTextField（）；
textFirstName=newjavax.swing.JTextField（）；
textLastName=newjavax.swing.JTextField（）；
jPanel2=newjavax.swing.JPanel（）；
btnNext=newjavax.swing.JButton（）；
btnLast=newjavax.swing.JButton（）；
btnPrevious=newjavax.swing.JButton（）；
btnFirst=newjavax.swing.JButton（）；
setDefaultCloseOperation（javax.swing.WindowConstants.EXIT_ON_CLOSE）；
textJobTitle.setToolTipText（“”）；
textJobTitle.setName（“”；//NOI18N
jLabel1.setFont（新的java.awt.Font（“Tahoma”，1，14））；//NOI18N
jLabel1.setText（“职务：”）；
javax.swing.GroupLayout jPanel1Layout=新的javax.swing.GroupLayout（jPanel1）；
setLayout（jPanel1Layout）；
jPanel1Layout.setHorizontalGroup(
jPanel1Layout.createParallelGroup（javax.swing.GroupLayout.Alignment.LEADING）
.addGroup（jpanellayout.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_值）
.addGroup（jPanel1Layout.createParallelGroup（javax.swing.GroupLayout.Alignment.LEADING，false）
.addGroup（jpanellayout.createSequentialGroup（）
.addComponent（textFirstName，javax.swing.GroupLayout.PREFERRED_SIZE，75，javax.swing.GroupLayout.PREFERRED_SIZE）
.addPreferredGap（javax.swing.LayoutStyle.ComponentPlacement.RELATED，javax.swing.GroupLayout.DEFAULT\u SIZE，Short.MAX\u值）
.addComponent（textLastName，javax.swing.GroupLayout.PREFERRED\u SIZE，60，javax.swing.GroupLayout.PREFERRED\u SIZE））
.addComponent（textJobTitle，javax.swing.GroupLayout.PREFERRED_SIZE，221，javax.swing.GroupLayout.PREFERRED_SIZE）））
);
jPanel1Layout.setVerticalGroup(
jPanel1Layout.createParallelGroup（javax.swing.GroupLayout.Alignment.LEADING）
.addGroup（jpanellayout.createSequentialGroup（）
.addContainerGap（）
.addGroup（jPanel1Layout.createParallelGroup（javax.swing.GroupLayout.Alignment.BASELINE）
.addComponent（textID，javax.swing.GroupLayout.PREFERRED\u SIZE，javax.swing.GroupLayout.DEFAULT\u SIZE，javax.swing.GroupLayout.PREFERRED\u SIZE）
.addComponent（textFirstName、javax.swing.GroupLayout.PREFERRED\u SIZE、javax.swing.GroupLayout.DEFAULT\u SIZE、javax.swing.GroupLayout.PREFERRED\u SIZE）
.addComponent（textLastName、javax.swing.GroupLayout.PREFERRED\u SIZE、javax.swing.GroupLayout.DEFAULT\u SIZE、javax.swing.GroupLayout.PREFERRED\u SIZE））
.addGap（24,24,24）
.addGroup（jPanel1Layout.createParallelGroup（javax.swing.GroupLayout.Alignment.BASELINE）
.addComponent（jLabel1）
.addComponent（textJobTitle、javax.swing.GroupLayout.PREFERRED\u SIZE、javax.swing.GroupLayout.DEFAULT\u SIZE、javax.swing.GroupLayout.PREFERRED\u SIZE））
.addContainerGap（55，简称最大值））
);
文本作业
public class Sample {
    ResultSet rs; // is null
    public method() {
        ResultSet rs = stmt.execute(); // here the other rs is hidden
    }
}
rs = stmt.executeQuery(sql);