Java MySQLの選択クエリ

Question

以下

私は、コネクタのコードを持って、私はviewHistoryと呼ばれる別のクラスのコードがあります。今、私がしようとしているのは、ユーザーがstudentID番号を入力できるようにすることです。その後、関連する出席履歴が表示されます。しかし、それはcompiloingですが、何かを表示するかどうかを確認するために数字を入力するとエラーメッセージが表示されます。なにか提案を？

public class Connector { 
Connection con; 
PreparedStatement stmt; 
ResultSet rs; 

Connector() 
{ 
    try{ 


     Class.forName("com.mysql.jdbc.Driver"); 

     con=DriverManager.getConnection("jdbc:mysql://localhost/luxmidatabase","root",""); 
        stmt=con.prepareStatement("select * from login where username=? and password=?"); 
        } 

    catch (Exception e) 
    { 
     System.out.println(e); 
    } 
} 
} 

第二CLASS

public class AttendanceHistory extends Connector 
{ 

public AttendanceHistory() { 
    initialize(); 
} 

public JFrame frmAttendanceHistory; 

Connector con; 
private JTextField textField; 

// initialise the frame 
private void initialize() { 
    frmAttendanceHistory = new JFrame(); 
    frmAttendanceHistory.setTitle("LEC AdminPro: Attendance History"); 
    frmAttendanceHistory.setBounds(100, 100, 323, 149); 
    frmAttendanceHistory.setDefaultCloseOperation(JFrame.HIDE_ON_CLOSE); 
    frmAttendanceHistory.setLocationRelativeTo(null); 
    // Initialising a Grey Border, which I am going to be using later to add borders to JLabels. 
    Border border = LineBorder.createGrayLineBorder(); 
    frmAttendanceHistory.getContentPane().setLayout(null); 
    con = new Connector(); 

    JPanel panel_1 = new JPanel(); 
    panel_1.setBounds(0, 0, 307, 113); 
    frmAttendanceHistory.getContentPane().add(panel_1); 
    panel_1.setBorder(border); 
    panel_1.setLayout(null); 

    JLabel lblRegister = new JLabel("View Attendance History"); 
    lblRegister.setFont(new Font("Tahoma", Font.BOLD, 12)); 
    lblRegister.setBounds(10, 11, 193, 20); 
    panel_1.add(lblRegister); 

    JLabel lblStudentId = new JLabel("Student ID"); 
    lblStudentId.setFont(new Font("Arial", Font.PLAIN, 11)); 
    lblStudentId.setBounds(10, 45, 69, 14); 
    panel_1.add(lblStudentId); 

    // Created a Cancel button along with an OptionDialog which pops up to confirm whether user wants to cancel registration or not. 
    JButton btnNewButton = new JButton("Cancel"); 
    btnNewButton.addActionListener(new ActionListener() { 
     public void actionPerformed(ActionEvent arg0) { 
      Object[] options = {"Yes", "No"}; 
      Component form = null; 
      int n = JOptionPane.showOptionDialog(form, "Would you like to cancel the progress?", "Exit Confirmation", JOptionPane.YES_NO_CANCEL_OPTION, JOptionPane.QUESTION_MESSAGE, null, options, options); 
      if(n == JOptionPane.YES_OPTION) { 
       frmAttendanceHistory.setVisible(false); 
      } 

     } 
    }); 
    btnNewButton.setBounds(159, 73, 75, 23); 
    panel_1.add(btnNewButton); 

    JButton button = new JButton("Submit"); 
    button.addActionListener(new ActionListener() { 
     public void actionPerformed(ActionEvent arg0) { 


     //String studentID = textField.getText(); 

     try { 


      con.stmt.executeQuery("SELECT StudentID, date FROM attendance WHERE StudentID = 21457");    
      while (rs.next()) { 
       String date = rs.getString("date"); 
       System.out.println(date); 
      } 

      JOptionPane.showMessageDialog(frmAttendanceHistory, "Attendance has been registered."); 
      frmAttendanceHistory.setVisible(false); 

     } catch (SQLException e) { 
     //System.out.println("Record couldn't be added!"); 
     JOptionPane.showMessageDialog(frmAttendanceHistory, "Attendance couldn't be registered. Please try again!"); 
     e.printStackTrace(); 
     } 
     } 
     }); 



    button.setBounds(81, 73, 75, 23); 
    panel_1.add(button); 

    textField = new JTextField(); 
    textField.setBounds(81, 42, 212, 20); 
    panel_1.add(textField); 
    textField.setColumns(10); 




} 




public static void main(String[] args) { 
    EventQueue.invokeLater(new Runnable() { 

     public void run() { 
      try { 
      AttendanceHistory window = new AttendanceHistory(); 
      window.frmAttendanceHistory.setVisible(true); 
      } catch (Exception e) { 
      e.printStackTrace(); 
      } 
     } 
    }); 
} 



// method to make this frame visible (may be used in other classes for retrieval) 
public void setVisible(boolean b) { 
    frmAttendanceHistory.setVisible(true); 

}  
} 

主な問題は、送信ボタン内のコードです。それは私がデータをフィルタリングしようとするところです。

これはエラーメッセージです：あなたは、エラーが発生した行がrsがnullあるのでNullPointerExceptionが起こる

while (rs.next()) { 

であると言ったコメントで

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException 
at AttendanceHistory$2.actionPerformed(AttendanceHistory.java:85) 
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source) 
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source) 
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source) 
at javax.swing.DefaultButtonModel.setPressed(Unknown Source) 
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source) 
at java.awt.Component.processMouseEvent(Unknown Source) 
at javax.swing.JComponent.processMouseEvent(Unknown Source) 
at java.awt.Component.processEvent(Unknown Source) 
at java.awt.Container.processEvent(Unknown Source) 
at java.awt.Component.dispatchEventImpl(Unknown Source) 
at java.awt.Container.dispatchEventImpl(Unknown Source) 
at java.awt.Component.dispatchEvent(Unknown Source) 
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source) 
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source) 
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source) 
at java.awt.Container.dispatchEventImpl(Unknown Source) 
at java.awt.Window.dispatchEventImpl(Unknown Source) 
at java.awt.Component.dispatchEvent(Unknown Source) 
at java.awt.EventQueue.dispatchEventImpl(Unknown Source) 
at java.awt.EventQueue.access$000(Unknown Source) 
at java.awt.EventQueue$1.run(Unknown Source) 
at java.awt.EventQueue$1.run(Unknown Source) 
at java.security.AccessController.doPrivileged(Native Method) 
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source) 
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source) 
at java.awt.EventQueue$2.run(Unknown Source) 
at java.awt.EventQueue$2.run(Unknown Source) 
at java.security.AccessController.doPrivileged(Native Method) 
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source) 
at java.awt.EventQueue.dispatchEvent(Unknown Source) 
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source) 
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source) 
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source) 
at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
at java.awt.EventDispatchThread.run(Unknown Source) 

Answer 1

。あなたのコードのどこにでもrsを初期化しているとは思えません。おそらく上記の行はされている必要があります：（？あなたは変数rsを宣言している）

ResultSet rs = con.stmt.executeQuery(...); 

。

もう1つの問題：select * from login ... SQLを使用してConnectorオブジェクトで準備したStatementオブジェクトを再利用しています。代わりにクエリを実行するには、新しいステートメントを作成する必要があります。このような何か：あなたはまた、適切にそれらにclose()を呼び出すことによってStatementとResultSetオブジェクトを破棄しなければならない

Statement st = con.con.createStatement(); 
ResultSet rs = st.executeQuery("SELECT StudentID, date from ..."); 
while (rs.hasNext()) { 
    // ... 
} 

注意。 （好ましくはfinallyブロック内にあり、例外が発生してもそれが起こるようにする）。