Explain the role of composition and aggregation in class relationships

Overview

In OOP, classes can relate in different ways:

• is-a relationship (inheritance)
• has-a relationship (aggregation)
• part-of relationship (composition)

Composition and aggregation both model ownership/containment, but with different dependency strength.

Aggregation (has-a)

Aggregation is a weak ownership relationship:

• One class contains or references another
• Both objects can exist independently

Example from the notes:

• Playlist has many Song objects
• A Song can still exist without a specific Playlist

UML hint: aggregation is shown with a hollow diamond at the owner side.

Composition (part-of)

Composition is a strong ownership relationship:

• One object is a required part of another
• Part lifecycle depends on whole lifecycle
• If the whole is deleted, its parts are deleted too

Example from the notes:

• Book contains Chapter objects
• Chapter does not meaningfully exist without its Book

UML hint: composition is shown with a filled diamond at the owner side.

Aggregation vs Composition

Aspect	Aggregation	Composition
Dependency strength	Weak	Strong
Independent lifecycle	Yes	No
Ownership	Shared/loose	Exclusive/strong
Delete whole object	Parts may still exist	Parts are typically removed
Common phrase	has-a	part-of

Why This Matters in Design

• Helps model real systems correctly (loose vs tight ownership)
• Improves maintainability by clarifying responsibilities
• Prevents lifecycle bugs (e.g., orphan objects or accidental shared state)
• Makes UML and code structure consistent

Python Examples

1. Aggregation Example

class Song:
    def __init__(self, title: str):
        self.title = title


class Playlist:
    def __init__(self, name: str):
        self.name = name
        self.songs: list[Song] = []

    def add_song(self, song: Song):
        self.songs.append(song)


song = Song("Imagine")
playlist = Playlist("Favorites")
playlist.add_song(song)
# song can still exist even if playlist is deleted

2. Composition Example

class Chapter:
    def __init__(self, title: str):
        self.title = title


class Book:
    def __init__(self, title: str, chapter_titles: list[str]):
        self.title = title
        # chapters are created as an internal part of Book
        self.chapters = [Chapter(t) for t in chapter_titles]


book = Book("OOP Basics", ["Intro", "Classes", "Relationships"])