Relationships (association, aggregation, composition) in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define an association relationship between two classes.

LLD

class Teacher:
    def __init__(self, name):
        self.name = name

class Student:
    def __init__(self, name, teacher):
        self.name = name
        self.teacher = [1]

Drag options to blanks, or click blank then click option'

Ateacher

Bname

CStudent

Dself

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'name' instead of 'teacher' causes the student to store a string, not the teacher object.

Using 'self' or 'Student' is incorrect as they do not refer to the teacher instance.

The student holds a reference to the teacher object, representing an association.

2fill in blank

medium

Complete the code to show aggregation where a Library has many Books.

LLD

class Book:
    def __init__(self, title):
        self.title = title

class Library:
    def __init__(self):
        self.books = []

    def add_book(self, book):
        self.books.[1](book)

Drag options to blanks, or click blank then click option'

Aremove

Bappend

Cclear

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'remove' or 'pop' deletes items instead of adding.

Using 'clear' empties the list, which is not intended.

Aggregation means the library holds references to book objects. We add books to the list using append.

3fill in blank

hard

Fix the error in the composition example where a Car has an Engine.

LLD

class Engine:
    def __init__(self, power):
        self.power = power

class Car:
    def __init__(self, power):
        self.engine = [1](power)

Drag options to blanks, or click blank then click option'

Apower

Bengine

CEngine

DCar

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'engine' or 'power' as a function causes errors.

Using 'Car' tries to create a Car inside Car, which is incorrect.

In composition, the Car creates and owns the Engine instance. We must call the Engine class to create it.

4fill in blank

hard

Fill both blanks to complete the aggregation example where a Team has multiple Players.

LLD

class Player:
    def __init__(self, name):
        self.name = name

class Team:
    def __init__(self):
        self.players = []

    def add_player(self, player):
        self.players.[1](player)

    def remove_player(self, player):
        self.players.[2](player)

Drag options to blanks, or click blank then click option'

Aappend

Bpop

Cremove

Dclear

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop to remove by value causes errors.

Using clear removes all players, not just one.

To add a player, use append. To remove a specific player, use remove.

5fill in blank

hard

Fill all three blanks to complete the composition example where a House has Rooms with names and sizes.

LLD

class Room:
    def __init__(self, name, size):
        self.name = [1]
        self.size = [2]

class House:
    def __init__(self):
        self.rooms = []

    def add_room(self, name, size):
        room = [3](name, size)
        self.rooms.append(room)

Drag options to blanks, or click blank then click option'

Aname

Bsize

CRoom

Dself

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up parameter names or using 'self' incorrectly.

Not calling the Room class to create a new object.

The Room constructor assigns name and size to its attributes. The House creates Room instances to compose rooms.

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