many to many

[Nawar Gailani]

Hi there,
If I have many to many relation, how can I map this in Java?
For example, i have students and courses, every student may have one or more courses, and every course may have one or more students.
When I write a class represent student, I write it this way?
class Student {
private String name;
private Course course;
......
}

or like this?
class Student {
private String name;
private Vector courses;
......
}

The second seems more logical to me when I search for a specific student, but I don't know if I'm missing something!

Thanks in advance

[Jeanne Boyarsky]

Nawar,
You would need the second one as a student can be in multiple courses. If the course needs to know who is registered, it would need to have a Collection of students.

I'm moving this to our OO forum since it is about modeling Java rather than databases.

[Joel McNary]

...But it could be about modeling either!

Typically, in the database, there is a cross-relation table (which can be invisible to the object model) So your database structure would be something like:

CREATE TABLE COURSE (
    COURSE_ID INTEGER NOT NULL,
    COURSE_NAME VARCHAR(255) NULL
);
 
CREATE TABLE STUDENT (
    STUDENT_ID INTEGER NOT NULL,
    STUDENT_NAME VARCHAR(255) NULL
);
 
--
-- This is our many-to-many resolution table
--
CREATE TABLE STUDENT_COURSE (
    STUDENT_COURSE_ID INTEGER NOT NULL,
    STUDENT_ID INTEGER NOT NULL,
    COURSE_ID INTEGER NOT NULL
);

While the code could look something like:

  public class Course{
 
    // * ++++++++++++++++++++++++++++++++++++ * //
    // * ++++++++  MEMBER VARIABLES  ++++++++ * //
    // * ++++++++++++++++++++++++++++++++++++ * //
 
    /**
     * This is the primary key for this table.
     */
    protected int courseId;
 
    protected java.lang.String courseName;
 
    protected java.util.List studentList;
}
 

  public class Student{
 
    // * ++++++++++++++++++++++++++++++++++++ * //
    // * ++++++++  MEMBER VARIABLES  ++++++++ * //
    // * ++++++++++++++++++++++++++++++++++++ * //
 
    /**
     * This is the primary key for this table.
     */
    protected int studentId;
 
    protected java.lang.String studentName;
 
    protected java.util.List courseList;
}

(Accessors left out as an exercise for the user.)

Note that student has a courseList and course has a studentList.

[Nawar Gailani]

Jeanne:
Yes, I thought the second is the correct. Thanks Jeanne.
Sorry about the posting in the wrong forum or section.
Joel:
Yes, I already did the linking table, but I didn't mentioned it for simplicity. Thanks Joel for your time explaining with code.