Top LLD Interview Mistakes

The patterns that fail candidates even when they know SOLID — and how to avoid them.

Fresher9 min read

Read these first

Functional vs Non-Functional Requirements

Why this matters

Strong candidates fail on process, not because they forgot a pattern name.
Knowing SOLID without a workflow still produces unmaintainable diagrams.
Interviewers probe how you recover when stuck — mistakes are teachable moments.
Avoiding these traps raises your pass rate more than learning one extra pattern.

1. ClarifyRequirements + scope

2. EntitiesNouns, verbs, ownership

3. APIsPublic operations

4. Class diagramRelationships

5. PatternsWhere they earn their keep

6. CodeCore paths + edge cases

Spend the first third of the session on requirements and entities. Code and patterns come after the shape is agreed.

Overview

The patterns that fail candidates even when they know SOLID — and how to avoid them. Study each section below in order — every list is interview-ready reference material.

Jumping to code too early

Points to cover

Writing methods before entities → wrong abstractions
Fix: spend 8+ minutes on requirements and class diagram
Interviewer may stop you — treat that as a signal to step back

God classes and manager soup

Points to cover

ParkingLotManager that parks, bills, emails, and renders UI
Every new feature lands in the same class
Fix: facade + focused collaborators (SRP)

Ignoring concurrency upfront

Points to cover

Seat booking without discussing two users on same seat
Fix: name the contended resource; lock, version, or hold TTL

Skipping extensibility

Points to cover

Hard-coded vehicle types in one giant switch
Fix: Strategy or registry; Open/Closed extension point

Java

// CampusOS — apply "Jumping to code too early" on paper first, then in code
public final class CommonMistakesExample {
    // 1. Name entities from the problem statement
    // 2. Draw relationships before writing methods
    // 3. Introduce patterns only when a force appears
}

Quick recall

Everything you need if you only revisit this box.

Jumping to code too early — review this slice before mock interviews.
God classes and manager soup — review this slice before mock interviews.
Ignoring concurrency upfront — review this slice before mock interviews.
Skipping extensibility — review this slice before mock interviews.

Test yourself

Answer these before moving on — recall is what makes it stick.