Weird behaviour when returning list from Mysql database call

Question

I wrote a method to copy external Mysql data into my current database as Student DAOs. When I write to the current database directly in the persistence layer, I get correct results, but if I return a list to use it in the logic layer weird things happen and the "courses" field of Students with the same name is the same where it should be different.

Anyone else has experienced weird behavior when returning lists from a database?

Here is my code:

    List<Student> fetchlist = new ArrayList<Student>();

    try {
        //Connecting to external DB
        Class.forName("com.mysql.jdbc.Driver");
        Connection connect = DriverManager.getConnection(dbstring);

        //GET STUDENTS
        Map<String, Student> studentMap = new HashMap<String, Student>();
        Statement statementStudents = connect.createStatement();
        ResultSet resultSetStudents = statementStudents.executeQuery("select * from students2");

        while (resultSetStudents.next()) {
            Student student = new Student();

            String sid = resultSetStudents.getString("sid");

            String firstname = resultSetStudents.getString("firstname");
            String lastname = resultSetStudents.getString("lastname");
            String email = resultSetStudents.getString("email");

            student.setFirstname(StringEscapeUtils.unescapeHtml(firstname));
            student.setName(StringEscapeUtils.unescapeHtml(lastname));
            student.setEmail(email);

            studentMap.put(sid, student);
        }


        Statement statementBookings = connect.createStatement();
        ResultSet resultSetBookings = statementBookings.executeQuery("select * from bookings2");

        //Go through bookings in order to complete Student and add
        while (resultSetBookings.next()) {
            String cid = resultSetBookings.getString("cid");
            String sid = resultSetBookings.getString("sid");

            String course = "";

                if (cid.equals("1521"))
                    course = "Course 1";
                else if (cid.equals("1522"))
                    course = "Course 2";
                else if (cid.equals("1523"))
                    course = "Course 3";
                else if (cid.equals("1524"))
                    course = "Course 4";

                Student student = studentMap.get(sid);

                student.setCourse(course);

                fetchlist.add(student);

        }
    } catch (ClassNotFoundException e) {
        log.error("Could not find Mysql JDBC class");

    } catch (SQLException e) {
        log.error("Error executing SQL statement");
    }

    return fetchlist;

Does Student class has a hashMap and equals methods? If not create it and test again. — Jorge Campos, Dec 12 '13 at 03:21
what kind of hashmap? Student is a DAO and must be serializable — javanoob, Dec 12 '13 at 16:13
I suppose you mean this: http://stackoverflow.com/questions/27581/overriding-equals-and-hashcode-in-java — javanoob, Dec 12 '13 at 16:20
thanks now I understand: Student.id-field is only created when writing to the Database but not when just doing new Student() so they are not comparable. — javanoob, Dec 12 '13 at 16:29
Sadly enough that didn't solve the problem - returned list is still created wrong :( — javanoob, Dec 13 '13 at 03:45
The error can't be the equals method because Student is just the value, not the key in the hashmap. — javanoob, Dec 13 '13 at 04:53
Try to close your statements and resultsets after you fetchs it. — Jorge Campos, Dec 13 '13 at 05:22
okay I reviewed the code and by creating a new student object with correct data and closing the statements and resultsets it works now :) — javanoob, Dec 13 '13 at 20:35
Great! Put an answer to your question, it can help others that may have the same problem. — Jorge Campos, Dec 13 '13 at 23:06

score 0 · Answer 1 · answered Dec 16 '13 at 02:21

0

For list-adding I created new Student objects and filled them with the data from the objects from the HashMap. Also I closed all my statements and resultsets where possible. Then I didn't have any wrong data in the list anymore.

answered Dec 16 '13 at 02:21

javanoob

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Weird behaviour when returning list from Mysql database call

1 Answers1