Class diagrams in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Identify the correct relationship type in a class diagram

In a class diagram, which relationship best represents a 'Car' class having an 'Engine' class as a part of it, where the engine cannot exist without the car?

A Composition

B Association

C Aggregation

D Inheritance

💡 Hint

Think about whether the 'Engine' can exist independently of the 'Car'.

Composition means a strong ownership where the part (Engine) cannot exist without the whole (Car). Aggregation allows the part to exist independently.

❓ Architecture

intermediate

Determine the correct multiplicity in a class diagram

A 'Library' class contains multiple 'Book' objects. Which multiplicity notation correctly represents this in a class diagram?

A Library 1 ----- * Book

B Library 1 ----- 1 Book

C Library * ----- 1 Book

D Library 0..1 ----- 0..1 Book

💡 Hint

Consider how many books a library can have.

The library can have many books, so the multiplicity on the Book side is '*', meaning many.

❓ scaling

advanced

Choose the best class diagram design for a scalable user management system

You need to design a user management system that supports different user roles with specific permissions. Which class diagram design is best for scalability?

A Separate classes for each user role inheriting from User class with hardcoded permissions

B User class with a single string attribute listing all permissions

C User class with Role class and Permission class linked via associations

D Single User class with many boolean attributes for each permission

💡 Hint

Think about flexibility and ease of adding new roles or permissions.

Using separate Role and Permission classes linked to User allows easy addition and management of roles and permissions without changing the User class.

❓ tradeoff

advanced

Tradeoff between inheritance and composition in class diagrams

Which statement best describes a tradeoff when choosing inheritance over composition in class diagrams?

A Inheritance offers more flexibility to change behavior at runtime than composition

B Composition leads to tighter coupling between classes than inheritance

C Composition requires more code duplication than inheritance

D Inheritance can cause rigid hierarchies making future changes harder compared to composition

💡 Hint

Consider how easy it is to modify or extend behavior later.

Inheritance creates fixed class hierarchies that can be hard to change, while composition allows more flexible design by assembling behaviors.

❓ estimation

expert

Estimate the number of classes in a class diagram for an online shopping system

For an online shopping system with features: user accounts, product catalog, shopping cart, order processing, payment, and reviews, estimate the minimum number of distinct classes needed in a well-designed class diagram.

A Around 5 classes

B Around 10 classes

C Around 20 classes

D More than 30 classes

💡 Hint

Think about main entities and supporting classes for each feature.

A well-designed system would have classes like User, Product, Cart, CartItem, Order, Payment, Review, plus supporting classes, totaling around 10.

Practice

1. What does a class diagram primarily represent in system design?

easy

A. The structure of classes and their relationships

B. The flow of data between servers

C. The user interface layout

D. The sequence of method calls

Solution

1. Step 1: Understand the purpose of class diagrams

   Class diagrams show the static structure of a system by representing classes and their relationships.

2. Step 2: Differentiate from other diagrams

   Flow of data, UI layout, and method sequences are shown in other diagram types like sequence or activity diagrams.

3. Final Answer:

   The structure of classes and their relationships -> Option A

4. Quick Check:

   Class diagram = structure and relationships [OK]

Hint: Class diagrams show classes and links, not flow or UI [OK]

Common Mistakes:

• Confusing class diagrams with sequence diagrams
• Thinking class diagrams show UI design
• Assuming class diagrams show runtime behavior

2. Which of the following is the correct way to represent inheritance in a class diagram?

easy

A. A dashed line with a hollow circle pointing to the child class

B. A dashed line with an open arrow pointing to the child class

C. A solid line with a filled diamond pointing to the parent class

D. A solid line with a hollow triangle arrow pointing to the parent class

Solution

1. Step 1: Recall UML notation for inheritance

   Inheritance is shown by a solid line with a hollow triangle arrow pointing to the parent class.

2. Step 2: Identify incorrect options

   Filled diamonds represent composition, dashed lines with open arrows represent dependencies, and hollow circles are not standard UML symbols.

3. Final Answer:

   A solid line with a hollow triangle arrow pointing to the parent class -> Option D

4. Quick Check:

   Inheritance = solid line + hollow triangle [OK]

Hint: Inheritance uses solid line + hollow triangle arrow [OK]

Common Mistakes:

• Mixing composition and inheritance symbols
• Using dashed lines for inheritance
• Pointing arrow to child instead of parent

3. Given this class diagram snippet:
Class A - attr1: int + method1()
What does the '-' before attr1 and '+' before method1() mean?

medium

A. '-' means protected attribute; '+' means private method

B. '-' means private attribute; '+' means public method

C. '-' means public attribute; '+' means protected method

D. '-' means static attribute; '+' means abstract method

Solution

1. Step 1: Understand UML visibility symbols

   In UML, '-' means private visibility, '+' means public visibility.

2. Step 2: Apply to attributes and methods

   So, attr1 is private attribute, method1() is public method.

3. Final Answer:

   '-' means private attribute; '+' means public method -> Option B

4. Quick Check:

   Visibility: '-' private, '+' public [OK]

Hint: '-' = private, '+' = public in UML [OK]

Common Mistakes:

• Confusing visibility symbols with data types
• Assuming '-' means protected
• Mixing static/abstract with visibility

4. Identify the error in this class diagram description:
Class Car - engine: Engine - wheels: List<Wheel> -- engine "1" --- "4" wheels
What is wrong with the association notation?

medium

A. The association should use a hollow diamond for aggregation

B. The multiplicity numbers are reversed; wheels should be "4" and engine "1"

C. The association line uses double dashes instead of a single solid line

D. The class names should be connected with a dashed line

Solution

1. Step 1: Check association line style

   Associations use a single solid line, not double dashes.

2. Step 2: Verify multiplicity and line type

   Multiplicity "1" to "4" is correct; aggregation uses hollow diamond but not required here.

3. Final Answer:

   The association line uses double dashes instead of a single solid line -> Option C

4. Quick Check:

   Association = single solid line [OK]

Hint: Association lines are single solid lines, not double dashes [OK]

Common Mistakes:

• Using double dashes for associations
• Confusing aggregation with association
• Reversing multiplicity numbers

5. You need to design a class diagram for a library system where a Library has many Book objects, and each Book can have multiple Authors. Which of the following best represents this relationship?

hard

A. Library "1" --- "*" Book; Book "*" --- "*" Author with an association class for author details

B. Library "*" --- "1" Book; Book "1" --- "*" Author with inheritance

C. Library "1" --- "1" Book; Book "1" --- "1" Author with composition

D. Library "*" --- "*" Book; Book "*" --- "1" Author with aggregation

Solution

1. Step 1: Analyze multiplicity between Library and Book

   A library has many books, so multiplicity is "1" to "*".

2. Step 2: Analyze multiplicity between Book and Author

   A book can have multiple authors, and authors can write multiple books, so "*" to "*" is correct, often modeled with an association class for author details.

3. Step 3: Check other options for correctness

   Other options have incorrect multiplicities or wrong relationships like inheritance or composition.

4. Final Answer:

   Library "1" --- "*" Book; Book "*" --- "*" Author with an association class for author details -> Option A

5. Quick Check:

   Library to Book = 1 to many; Book to Author = many to many [OK]

Hint: Many-to-many needs association class; library has many books [OK]

Common Mistakes:

• Using inheritance instead of association
• Incorrect multiplicity numbers
• Confusing composition with aggregation