Association, Aggregation and Composition

Has-a relationships of different strengths

Objects relate to each other in several ways. Association is a general relationship where objects use or know about each other: a teacher teaches students, and both exist independently. Aggregation is a whole-part association with weak ownership: a department has professors, but if the department is closed the professors still exist and can join another; parts can be shared. Composition is a whole-part relationship with strong ownership: a house has rooms, an order has order lines; the parts belong to exactly one whole and their lifetime is tied to it, so destroying the whole destroys the parts. Dependency is the weakest link: one class temporarily uses another, for example as a method parameter. These distinctions matter for design (who creates and deletes what, what can be shared) and appear in UML diagrams. In code, composition usually means the whole creates its parts and does not expose them for others to keep; aggregation usually means parts are passed in from outside.

From association to composition

Relationships range from loose links to parts that live and die with their whole.

Aggregation versus composition in code

Who creates the parts reveals the relationship.

// aggregation: professors exist independently and are passed in
public class Department {
    private final List<Professor> professors = new ArrayList<>();
    public void addProfessor(Professor p) { professors.add(p); }   // shared, created elsewhere
}

// composition: order lines are created by and belong to one Order
public class Order {
    private final List<OrderLine> lines = new ArrayList<>();

    public void addLine(String sku, int qty, long unitPricePaise) {
        lines.add(new OrderLine(sku, qty, unitPricePaise));          // created inside
    }

    public long totalPaise() {
        return lines.stream().mapToLong(OrderLine::totalPaise).sum();
    }

    private record OrderLine(String sku, int qty, long unitPricePaise) {
        long totalPaise() { return qty * unitPricePaise; }
    }
}

A college, its departments and its students

A building and its rooms are composition: demolish the building and the rooms are gone. A club and its members are aggregation: the club can close, and the members carry on and join other clubs.

Quick check: An Order creates its OrderLine objects, and they cannot exist without it. Which relationship is this?

• Composition
• Association
• Aggregation
• Dependency