4.2Aggregation

Aggregation is an association with a whole and its parts: one object collects or groups others. The parts, however, have a life of their own.

Aggregation is a special, weaker form of "has-a": a whole contains parts, but the parts can exist independently of the whole. It is called a "has-a, but not owned" relationship.

Examples: a Department has Teachers (the teachers still exist if the department closes); a Team has Players; a Library has Books.

class Department {
    private String name;
    private java.util.List<Teacher> teachers = new java.util.ArrayList<>();

    Department(String name) { this.name = name; }

    void addTeacher(Teacher t) { teachers.add(t); }   // teacher created OUTSIDE

    void showStaff() {
        for (Teacher t : teachers) System.out.println(t.getName());
    }
}

Teacher t1 = new Teacher("Ms. Perera");     // exists independently
Department d = new Department("Computing");
d.addTeacher(t1);
d = null;                                    // department gone
System.out.println(t1.getName());            // teacher still exists

Key code pattern: the part is created outside and passed in to the whole.

Reading the Department example

• The Teacher object t1 is created before the department exists, outside the Department class.
• addTeacher(t1) stores a reference to the teacher in the department's list. No new teacher is created.
• When d is set to null, the Department object can be garbage-collected. But t1 still refers to the teacher, so the teacher stays alive, and t1.getName() still prints "Ms. Perera".

The department has teachers, but it does not own them.

Sharing parts

Because parts are created outside, the same part can belong to more than one whole at once:

Teacher t2 = new Teacher("Mr. Silva");
Department computing = new Department("Computing");
Department maths = new Department("Mathematics");
computing.addTeacher(t2);
maths.addTeacher(t2);          // the same teacher works in both departments

This is impossible in composition, where each part belongs to exactly one whole.

How to recognise aggregation

• The sentence "X has Ys" is true (a Team has Players), and
• a Y still makes sense without the X (a player can leave the team and join another), and
• the Ys are usually created somewhere else and then added to the whole.

In UML, aggregation is drawn as a line with a hollow diamond at the whole (Department) end.