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ResultSet
是null
。
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。