UML: how to implement Association class in Java

问题:

I have this UML Association class. Note that: horizontal line is a solid line and the vertical line is a dashed line.

 ---------                  ---------
|         |*(a)        *(b)|         |
| CLASS   |________________|  CLASS  |
|STUDENT  |     |          |  COURSE |
 ---------      |           ---------
                |*(c)
          ______|______
         |             |
         |             |
         |  CLASS      |
         | TRANSCRIPT  |
         |_____________|

I understand this relationship but I have met some problems when implement this UML to code. I can implement relation between class Student and class Course to code. Here is my code:

class Student {
  Vector<Course> b;
}

class Course {
   Vector<Student> a;
}

But, at class Transcript, I don't understand so much, how to use this class in code. Is it the property of both class Student and Course. So, if that's true then the code will be:

class Student {
  Vector<Course> b;
  Vector<Transcript> c;
}

class Course {
  Vector<Student> a;
  Vector<Transcript> c;
}

Is it true? If this is wrong, please teach me how to implement this UML.

Thanks :)

回答1:

First of all, don't use Vector, as it's an old class that shouldn't be used anymore for more than 10 years. Use either a Set or a List.

If the Transcript class contains information about the way a student attends a course (for example, the date of its subscription to the course), you could implement it like this:

class Student {
    Set<Transcript> transcripts;
}

class Transcript {
    Student student;
    Course course;
    Date subscriptionDate;
}

class Course {
    Set<Transcript> transcripts;
}

That doesn't prevent you from providing a method in Student that returns all his courses: 

public Set<Course> getCourses() {
    Set<Course> result = new HashSet<Course>();
    for (Transcript transcript : transcripts) {
        result.add(transcript.getCourse());
    }
    return result;
}

If Transcript doesn't contain any information, then it's probably there to model how these classes would be mapped in database tables, where the only way to have a many-to-many association between two tables is to use a join table holding the IDs of the two associated tables.

回答2:

I know this question is very old but I have a way more convenient than embedding n..1 relations in the association class :

public class Transcript {
    //Transcript's properties
} 

public class Course {

    private Map<Student, Transcript> transcriptsByStudent;
}

public class Student {

    private Map<Course, Transcript> transcriptsByCourse;
}

回答3:

Just to add on, on your model you denote multiplicity on the association class. This is unnecessary because the association class IS the association itself, and the multiplicity relationship of the original two classes will satisfy.