Question

我是Java的新手，并且一直在研究这个网站上的数据库教程。 http://www.homeandlearn.co.uk/java/move_to_the_first_and_last_records.html

使用Netbeans，它会给我

的错误输出

    Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Employees.Workers.btnNextActionPerformed(Workers.java:231)
at Employees.Workers.access$200(Workers.java:19)
at Employees.Workers$3.actionPerformed(Workers.java:140)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
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:6533)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6298)
at java.awt.Container.processEvent(Container.java:2236)
at java.awt.Component.dispatchEventImpl(Component.java:4889)
at java.awt.Container.dispatchEventImpl(Container.java:2294)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
at java.awt.Container.dispatchEventImpl(Container.java:2280)
at java.awt.Window.dispatchEventImpl(Window.java:2746)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:90)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.awt.EventQueue$4.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

此代码运行正常，并按预期将数据库中的第一组详细信息加载到文本框中。

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

public void DoConnect( ) {

    try{
        String strHost = "jdbc:derby://localhost:1527/Employees";
        String strUsername = "";
        String strPassword = "";

        Connection con = DriverManager.getConnection(strHost, strUsername, strPassword);
        Statement stmt = con.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE);
        String strSQL = "SELECT * FROM Workers";
        ResultSet rs = stmt.executeQuery(strSQL);

        rs.next();
        int nId = rs.getInt("ID");
        String strId = Integer.toString(nId);
        String strFirstName = rs.getString("First_Name");
        String strLastName = rs.getString("Last_Name");
        String strJobTitle = rs.getString("Job_Title");

        textBoxId.setText(strId);
        textBoxFirstName.setText(strFirstName);
        textBoxLastName.setText(strLastName);
        textBoxJobTitle.setText(strJobTitle);

    }
    catch(SQLException err){
        JOptionPane.showMessageDialog(this, err.getMessage());
    }
}

抛出错误的代码是按下按钮，它们都给我同样的错误。

    private void btnFirstActionPerformed(java.awt.event.ActionEvent evt) {                                         
    try {
        rs.first();
        int nId = rs.getInt("ID");
        String strId = Integer.toString(nId);
        String strFirstName = rs.getString("First_Name");
        String strLastName = rs.getString("Last_Name");
        String strJobTitle = rs.getString("Job_Title");

        textBoxId.setText(strId);
        textBoxFirstName.setText(strFirstName);
        textBoxLastName.setText(strLastName);
        textBoxJobTitle.setText(strJobTitle);
    }
    catch (SQLException err) {
        JOptionPane.showMessageDialog(this, err.getMessage());
    }
}                                        

private void btnPreviousActionPerformed(java.awt.event.ActionEvent evt) {                                            
    try {
        if (rs.previous()) {
            int nId = rs.getInt("ID");
            String strId = Integer.toString(nId);
            String strFirstName = rs.getString("First_Name");
            String strLastName = rs.getString("Last_Name");
            String strJobTitle = rs.getString("Job_Title");

            textBoxId.setText(strId);
            textBoxFirstName.setText(strFirstName);
            textBoxLastName.setText(strLastName);
            textBoxJobTitle.setText(strJobTitle);
        }
        else {
            rs.next();
            JOptionPane.showMessageDialog(this, "End of File");
        }
    }
    catch (SQLException err) {
        JOptionPane.showMessageDialog(this, err.getMessage());
    }
}                                           

private void btnNextActionPerformed(java.awt.event.ActionEvent evt) {                                        
    try {
        if (rs.next()) {
            int nId = rs.getInt("ID");
            String strId = Integer.toString(nId);
            String strFirstName = rs.getString("First_Name");
            String strLastName = rs.getString("Last_Name");
            String strJobTitle = rs.getString("Job_Title");

            textBoxId.setText(strId);
            textBoxFirstName.setText(strFirstName);
            textBoxLastName.setText(strLastName);
            textBoxJobTitle.setText(strJobTitle);
        }
        else {
            rs.previous();
            JOptionPane.showMessageDialog(this, "End of File");
        }
    }
    catch (SQLException err) {
        JOptionPane.showMessageDialog(this, err.getMessage());
    }
}                                       

private void btnLastActionPerformed(java.awt.event.ActionEvent evt) {                                        
    try {
        rs.last();
        int nId = rs.getInt("ID");
        String strId = Integer.toString(nId);
        String strFirstName = rs.getString("First_Name");
        String strLastName = rs.getString("Last_Name");
        String strJobTitle = rs.getString("Job_Title");

        textBoxId.setText(strId);
        textBoxFirstName.setText(strFirstName);
        textBoxLastName.setText(strLastName);
        textBoxJobTitle.setText(strJobTitle);
    }
    catch (SQLException err) {
        JOptionPane.showMessageDialog(this, err.getMessage());
    }
}

我不认为它是数据库代码本身。我使用相同的代码将第一个条目预加载到一些文本框中，它可以正常工作。有人可以帮忙吗？

感谢。

抛出Java错误

0 个答案: