JButton btnNewButton = new JButton("Login");
    btnNewButton.setBackground(SystemColor.textHighlight);
    btnNewButton.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent arg0) {
            try {
                PreparedStatement pstmt;
                String query ="select student_id from students where username=? and password=?";
                pstmt = connection.prepareStatement(query);
                pstmt.setString(1, textField.getText());
                pstmt.setString(2, textField_1.getText());
               ResultSet rs = pstmt.executeQuery();
                if(rs.next())
                {
                    JOptionPane.showMessageDialog(null,"Login success");
                    st_id = rs.getInt("student_id"); /* I want to use this
                     value in another form or class ? */
                }
                else if(textField.getText().equals("") && textField_1.getText().equals(""))
                {
                    JOptionPane.showMessageDialog(null, "Field Empty");
                }
                else
                {
                    JOptionPane.showMessageDialog(null, "Username or Password Invalid");
                }
            } catch (SQLException e) {
            e.printStackTrace();
            }

        }
    });