Composition and Aggregation in OOP

What Are They?

Both composition and aggregation describe “has-a” relationships between classes, but they differ in dependency strength:

Aggregation:
1. A weaker relationship.
2. The contained object (part) can exist independently of the container (whole).
Composition:
1. A stronger relationship.
2. The contained object cannot exist without the container.

Example

Aggregation: A Library has Books – books can exist outside the library.
Composition: A House has Rooms – if the house is destroyed, the rooms no longer exist.

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Introduction to Class Relationships

In object-oriented programming (OOP), understanding how classes relate to each other is crucial for designing robust and maintainable systems. Two fundamental concepts in class relationships are composition and aggregation.

These concepts help us model real-world relationships between objects in our code.
They define how objects are connected and how they depend on each other.

AnalogyThink of class relationships like building blocks. Composition and aggregation are different ways of connecting these blocks to create complex structures.

DefinitionClass RelationshipA connection between two or more classes that defines how they interact or depend on each other.

ExampleIn a car manufacturing system, a Car class might have relationships with Engine, Wheel, and Door classes.

A1 Computer fundamentals4 subtopics

A2 Networks4 subtopics

A3 Databases4 subtopics

A4 Machine learning4 subtopics

B1 Computational thinking1 subtopic

B2 Programming5 subtopics

B3 Object-oriented programming2 subtopics

B4 Abstract data types (HL only)1 subtopic

B3.2.4 Explaining the Role of Composition and Aggregation in Class Relationships (HL only) Notes