Relationships (association, aggregation, composition) in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is association in system design?

Association is a relationship where two objects know about each other and can interact, but neither owns the other. It's like two friends who can talk but live separately.

beginner

Define aggregation with a real-life example.

Aggregation is a 'has-a' relationship where one object contains or uses another, but the contained object can exist independently. For example, a library has many books, but books can exist without the library.

intermediate

What makes composition different from aggregation?

Composition is a strong 'part-of' relationship where the contained object cannot exist without the container. Like a house and its rooms: if the house is destroyed, rooms don't exist separately.

intermediate

In UML diagrams, how is composition represented?

Composition is shown with a filled diamond shape at the container end connecting to the part, indicating strong ownership and lifecycle dependency.

beginner

Explain why understanding these relationships is important in system design.

Knowing association, aggregation, and composition helps design clear object interactions, manage lifecycles properly, and build maintainable, scalable systems.

Which relationship implies that the part can exist independently of the whole?

A Aggregation

B Composition

C Association

D Inheritance

Aggregation allows the part to exist independently, unlike composition where the part depends on the whole.

In which relationship does the lifecycle of the part depend entirely on the whole?

A Composition

B Aggregation

C Dependency

D Association

Composition means the part cannot exist without the whole, so their lifecycles are tied.

What symbol represents aggregation in UML diagrams?

A Filled diamond

B Hollow diamond

C Arrow

D Dashed line

Aggregation is shown with a hollow diamond at the container end.

Which relationship best describes two objects that interact but do not own each other?

A Composition

B Aggregation

C Association

D Generalization

Association is a loose relationship where objects know each other but have no ownership.

If a car has an engine, and the engine cannot exist without the car, what is this relationship?

A Association

B Aggregation

C Dependency

D Composition

The engine is part of the car and depends on it, so this is composition.

Describe the differences between association, aggregation, and composition with simple examples.

Think about friends, libraries with books, and houses with rooms.

Explain how understanding these relationships helps in designing software systems.

Consider how parts depend on wholes in real life.

Practice

1. Which relationship describes two objects connected without ownership, where both can exist independently?

easy

A. Inheritance

B. Association

C. Composition

D. Aggregation

Solution

1. Step 1: Understand relationship types

   Association means objects are linked but do not own each other and can exist separately.

2. Step 2: Compare with other relationships

   Aggregation and composition imply ownership; inheritance is a different concept.

3. Final Answer:

   Association -> Option B

4. Quick Check:

   Association = connected without ownership [OK]

Hint: No ownership means association, not aggregation or composition [OK]

Common Mistakes:

• Confusing aggregation with association
• Thinking composition allows independent existence
• Mixing inheritance with relationships

2. Which of the following correctly shows a composition relationship in UML notation?

easy

A. A solid diamond at the owner end connected to the part

B. A hollow diamond at the owner end connected to the part

C. A plain line with no diamonds between objects

D. A dashed arrow pointing from part to owner

Solution

1. Step 1: Recall UML notation for composition

   Composition uses a solid diamond at the owner end to show strong ownership.

2. Step 2: Differentiate from aggregation and association

   Aggregation uses hollow diamond; association uses plain line; dashed arrow is for dependency.

3. Final Answer:

   A solid diamond at the owner end connected to the part -> Option A

4. Quick Check:

   Composition = solid diamond [OK]

Hint: Solid diamond means composition, hollow means aggregation [OK]

Common Mistakes:

• Mixing hollow and solid diamonds
• Confusing association line with composition
• Using dashed arrows for composition

3. Consider this code snippet representing relationships:

class Engine:
    pass

class Car:
    def __init__(self):
        self.engine = Engine()

What type of relationship does Car have with Engine?

medium

A. Association

B. Aggregation

C. Inheritance

D. Composition

Solution

1. Step 1: Analyze ownership in code

   Car creates and owns Engine instance inside its constructor, implying strong ownership.

2. Step 2: Match with relationship definitions

   Since Engine cannot exist meaningfully without Car here, this is composition.

3. Final Answer:

   Composition -> Option D

4. Quick Check:

   Car owns Engine fully = Composition [OK]

Hint: Creating owned object inside means composition [OK]

Common Mistakes:

• Thinking it's aggregation because of ownership
• Confusing association with composition
• Assuming inheritance from code

4. Identify the error in this aggregation example:

class Wheel:
    pass

class Car:
    def __init__(self, wheels):
        self.wheels = wheels

car = Car([Wheel(), Wheel()])

What is wrong with this aggregation?

medium

A. Wheels cannot exist independently in aggregation

B. Car should not hold wheels as a list

C. No error; this is correct aggregation

D. Wheels should be created inside Car for composition

Solution

1. Step 1: Understand aggregation ownership

   Aggregation means Car owns wheels but wheels can exist independently.

2. Step 2: Check code for ownership and independence

   Wheels are created outside Car and passed in, so they can exist alone, matching aggregation.

3. Final Answer:

   No error; this is correct aggregation -> Option C

4. Quick Check:

   Aggregation allows independent parts [OK]

Hint: Aggregation parts created outside owner, passed in [OK]

Common Mistakes:

• Thinking wheels must be created inside Car
• Confusing aggregation with composition
• Assuming list usage is wrong

5. You are designing a library system where a Library contains many Books. Books can exist outside the library and be moved between libraries. Which relationship best fits this scenario and why?

hard

A. Aggregation, because library owns books but books can exist alone

B. Composition, because books depend fully on the library

C. Association, because library and books are loosely connected

D. Inheritance, because books inherit properties from library

Solution

1. Step 1: Analyze ownership and existence

   Books can exist independently and move between libraries, so they are not fully owned.

2. Step 2: Match relationship type

   Aggregation fits because library owns books but books can exist alone and be shared.

3. Final Answer:

   Aggregation, because library owns books but books can exist alone -> Option A

4. Quick Check:

   Aggregation = ownership with independent parts [OK]

Hint: If parts can exist alone, use aggregation [OK]

Common Mistakes:

• Choosing composition thinking books depend fully
• Selecting association ignoring ownership
• Confusing inheritance with ownership