LLD interview expectations - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does LLD stand for in system design interviews?

LLD stands for Low-Level Design. It focuses on designing the internal structure and components of a system or module.

beginner

Name three key elements interviewers expect in an LLD interview.

1. Clear class and object design<br>2. Proper use of design patterns<br>3. Handling of edge cases and scalability

intermediate

Why is it important to discuss design patterns during an LLD interview?

Design patterns provide reusable solutions to common problems. Discussing them shows understanding of best practices and clean design.

intermediate

What role does UML play in LLD interviews?

UML diagrams help visually communicate the design. They clarify relationships between classes and components for better understanding.

advanced

How should you handle scalability concerns in an LLD interview?

Explain how your design can handle growth, such as adding new features, managing more users, or improving performance.

What is the main focus of an LLD interview?

A Estimating system capacity

B Choosing cloud providers

C Designing detailed classes and methods

D Writing SQL queries

LLD interviews focus on detailed design of classes, methods, and interactions.

Which diagram is commonly used to represent class relationships in LLD?

A Sequence Diagram

B Class Diagram

C Use Case Diagram

D Deployment Diagram

Class diagrams show classes and their relationships, essential for LLD.

Why should you discuss edge cases in an LLD interview?

A To show you can handle unexpected situations

B To increase code length

C To avoid writing tests

D To impress with complex code

Discussing edge cases shows thoroughness and robustness in design.

Which design pattern is useful for creating objects without specifying exact classes?

A Decorator

B Singleton

C Observer

D Factory

Factory pattern helps create objects without exposing the creation logic.

What should you do if you realize your initial design is too complex during the interview?

A Explain and simplify it step-by-step

B Ignore and continue

C Start over without explanation

D Add more classes to fix it

Explaining and simplifying shows problem-solving and communication skills.

Describe the key steps you would take to prepare for an LLD interview.

Think about how you break down a problem into smaller parts.

Explain how you would communicate your design clearly during an LLD interview.

Imagine teaching your design to a friend.

Practice

1. What is the main focus of a Low-Level Design (LLD) interview?

easy

A. Choosing cloud providers and infrastructure

B. Estimating system capacity and scaling

C. Designing detailed components and their interactions

D. Writing complex SQL queries

Solution

1. Step 1: Understand LLD interview scope

   LLD interviews focus on designing small parts of a system clearly and in detail.

2. Step 2: Compare options with LLD focus

   Estimating capacity and scaling, choosing cloud providers and infrastructure, and writing database queries relate to other areas, not the main focus of LLD interviews.

3. Final Answer:

   Designing detailed components and their interactions -> Option C

4. Quick Check:

   LLD = detailed component design [OK]

Hint: LLD = detailed design of small system parts [OK]

Common Mistakes:

• Confusing LLD with high-level system design
• Thinking LLD focuses on infrastructure
• Mixing LLD with database query writing

2. Which of the following is the correct way to represent a relationship in a class diagram during an LLD interview?

easy

A. Using arrows to show inheritance and lines for associations

B. Using SQL queries to define relationships

C. Using cloud icons to represent services

D. Using flowcharts to show data flow

Solution

1. Step 1: Identify class diagram conventions

   Class diagrams use arrows for inheritance and lines for associations between classes.

2. Step 2: Eliminate unrelated options

   SQL queries, cloud icons, and flowcharts are not used to represent class relationships in LLD diagrams.

3. Final Answer:

   Using arrows to show inheritance and lines for associations -> Option A

4. Quick Check:

   Class diagram = arrows + lines [OK]

Hint: Class diagrams use arrows and lines for relationships [OK]

Common Mistakes:

• Confusing class diagrams with flowcharts
• Using database or cloud symbols incorrectly
• Ignoring inheritance arrows

3. Consider this simplified class design snippet in an LLD interview:

class User {
  int id;
  String name;
  List<Order> orders;
}

class Order {
  int orderId;
  double amount;
}

What is the relationship between User and Order?

medium

A. Inheritance

B. Aggregation

C. Composition

D. No relationship

Solution

1. Step 1: Analyze the class fields

   User has a list of Orders, meaning User contains Orders but Orders can exist independently.

2. Step 2: Determine relationship type

   Since Orders can exist without User, this is aggregation, not composition or inheritance.

3. Final Answer:

   Aggregation -> Option B

4. Quick Check:

   User "has" Orders = Aggregation [OK]

Hint: List field means aggregation, not inheritance [OK]

Common Mistakes:

• Confusing aggregation with composition
• Thinking it's inheritance due to class names
• Ignoring independence of Order objects

4. You designed a class diagram but forgot to show the multiplicity on associations. What issue might this cause in an LLD interview?

medium

A. Ambiguity about how many objects relate to each other

B. Syntax error in the diagram

C. The system will not compile

D. No impact, multiplicity is optional

Solution

1. Step 1: Understand multiplicity role

   Multiplicity shows how many instances of one class relate to another, clarifying design details.

2. Step 2: Consequence of missing multiplicity

   Without it, the design is unclear about object counts, causing ambiguity.

3. Final Answer:

   Ambiguity about how many objects relate to each other -> Option A

4. Quick Check:

   Missing multiplicity = unclear relationships [OK]

Hint: Multiplicity clarifies object counts in relationships [OK]

Common Mistakes:

• Thinking diagram syntax breaks without multiplicity
• Assuming multiplicity is always optional
• Confusing multiplicity with inheritance

5. During an LLD interview, you are asked to design a simple library system with Books and Members. Which design choice best follows LLD interview expectations?

hard

A. Design a single class Library with all methods and no separation

B. Use complex inheritance hierarchies for Book types without explaining choices

C. Focus only on database schema without class diagrams

D. Create classes Book and Member with clear attributes and a borrowing relationship using aggregation

Solution

1. Step 1: Identify good LLD design practices

   LLD expects clear classes with attributes and simple relationships like aggregation.

2. Step 2: Evaluate options against LLD expectations

   Creating classes Book and Member with clear attributes and a borrowing relationship using aggregation matches best by defining classes and relationships clearly. A single class lacks separation, database schema skips class diagrams, and unexplained complex inheritance overcomplicates without clarity.

3. Final Answer:

   Create classes Book and Member with clear attributes and a borrowing relationship using aggregation -> Option D

4. Quick Check:

   Clear classes + simple relationships = LLD [OK]

Hint: Clear classes + simple relationships = good LLD design [OK]

Common Mistakes:

• Overcomplicating with inheritance
• Ignoring class separation
• Skipping diagrams and explanations