有效代码的java.lang.NullPointerException

Question

我有以下代码块工作，但现在我收到了NPE错误。我知道这意味着代码指向一个空值，我只是不明白这是怎么发生的。它仍然有效有时，我觉得很奇怪。

我在rs = myStmt .... line

上得到空指针错误

public static void listClasses() {
        System.out.println("\nStudent Enrollment\n");
        try {
            System.out.println("Enter Student ID to See What Classes they are enrolled in: ");
            user_entered_student_id = sc.nextLine();

            con = DriverManager.getConnection("jdbc:mysql://localhost:3306/ClassSelector?autoReconnect=true&useSSL=false", "root", "");

            boolean found = false;
            String listStudentEnrollmentInfo =
                    "SELECT ClassSelector.student_x_class.student_id, " +
                            "ClassSelector.students.student_name, " +
                            "ClassSelector.students.hometown, " +
                            "ClassSelector.classes.class_name, " +
                            "ClassSelector.classes.description " +
                            "FROM ClassSelector.student_x_class " +
                            "JOIN classes on ClassSelector.student_x_class.class_id = ClassSelector.classes.class_id " +
                            "JOIN students on ClassSelector.student_x_class.student_id = ClassSelector.students.student_id " +
                            "Where ClassSelector.student_x_class.student_id =" + user_entered_student_id;
            rs = myStmt.executeQuery(listStudentEnrollmentInfo);
            while (rs.next()) {
                String studentInClass = (rs.getString("student_id") + "\t" + rs.getString("student_name") + " \t " + rs.getString("class_id") + " \t" + rs.getString("class_name"));