Question

Owner Class:

class University {
  List<Student> studentCollection;
  public void addStudent();
  public void removeStudent();
}

owned Class:

class Student {
  String name;
  public void setName();
  public String getName();
}

Here University is composed of Students and if we delete university object, all its students are supposed to be deleted.

But i want to ask the Student what University it is in?
Can we change Student class to have the reference of University?

Proposed class

class Student {
  String name;
  University univRef;
  /* ... */
}

would this still be appropriate composition? In UML can this be marked as composition?
Or should it be changed to bidirectional association?

Note: Searching for the answer wasn’t successful & struggled with formatting 🙂

Answer 1

Yes, it is still a composition. The Students are still owned by the University and they are still killed when the University burns down.

Formally speaking, the first composition (without the back-reference), is modeled in a UML class diagram with a filled diamond on the University end, and an arrow on the Student end. The composition without the back-reference is then represented without the arrow. However, a lot of people are not very strict in applying the arrow, using the second notation also for a unidirectional composition.

To be clear you should model it with two arrows: One with a filled diamond from the University to the Student, the other without diamond from the Student to the University.