Java IF ELSE语句

Question

Quick question. I got a code below, which checks with a database whether a given ID is available in the database, if so, it retrieves all attendance histories for the ID, otherwise if the ID didn't exist in the database it should display an error message. The code I got however, always says that the record has been attached, and then displays the error message as well regardless whether the ID exists or not. I think I probably have to change the order of the code or such? Could you please advise?

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


        String studentID = textField.getText(); 

        try {

            Statement st = con.con.createStatement();
            ResultSet rs = st.executeQuery("SELECT StudentID, date FROM attendance WHERE StudentID ="+textField.getText());


            while ( rs.next() ) {
                String student = rs.getString("StudentID");
                String date = rs.getString("date");
                System.out.println(student+"");
                System.out.println(date);
            }

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

            if (!rs.next()) {
            JOptionPane.showMessageDialog(frmAttendanceHistory, "A record for the given Student ID doesn't exist");
            }

        } 


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

Answer 1

You're repeatedly calling rs.next() until it returns false (as otherwise you'll never get out of the while loop) - then you're calling rs.next() again. Why would you expect it to return true then?

I suspect you want something like:

if (rs.next()) {
    String student = rs.getString("StudentID");
    String date = rs.getString("date");
    System.out.println(student+"");
    System.out.println(date);
    JOptionPane.showMessageDialog(frmAttendanceHistory,
       "Attendance has been registered.");
    frmAttendanceHistory.setVisible(false);
} else {
    JOptionPane.showMessageDialog(frmAttendanceHistory,
        "A record for the given Student ID doesn't exist");
}

Answer 2

You're doing a second rs.next, which would go to a second record if there was one. If there's only one (which is already consumed further up in your app), then it fails giving you a success followed by a failure.

Answer 3

while ( rs.next() ) {

followed by

if (!rs.next()) {

Doesn't make any sense as the if will always execute. Your loop will continue until rs.next() returns false.