LLD — Mid · Resources

1SOLID in depth

0/6▶

Mid-level LLD grades clean class design. Interviewers listen for SOLID applied deliberately, with a concrete example each.

The five principles

S — Single Responsibility: split classes that mix concerns (parsing + persistence).

SRP.

O — Open/Closed: add a subclass/strategy, don't edit a switch.

OCP.

L — Liskov: the square-vs-rectangle violation.

LSP.

I — Interface Segregation: many small interfaces beat one fat one.

ISP.

D — Dependency Inversion: depend on abstractions, inject dependencies.

DIP.

DRY, KISS, YAGNI, and the law of Demeter as everyday guides.

Law of Demeter.

2Creational patterns

0/4▶

Know the object-creation patterns and the exact problem each removes.

Creating objects

Factory Method / Abstract Factory — create without hard-coding the class.

Refactoring Guru — Factory Method.

Builder — step-by-step construction of complex objects.

Refactoring Guru — Builder.

Singleton — one instance; thread-safety caveats; why it's often overused.

Refactoring Guru — Singleton.

Prototype — clone instead of re-construct.

Refactoring Guru — Prototype.

3Structural patterns

0/5▶

Patterns that compose objects into larger structures cleanly.

Compose objects

Adapter — make an incompatible interface fit.

Refactoring Guru — Adapter.

Decorator — add behaviour without subclass explosion.

Refactoring Guru — Decorator.

Facade — a simple front over a complex subsystem.

Refactoring Guru — Facade.

Composite — treat trees of objects uniformly.

Refactoring Guru — Composite.

Proxy — stand-in for lazy loading, access control, caching.

Refactoring Guru — Proxy.

4Behavioural patterns

0/5▶

How objects communicate and share responsibility — the most-used patterns in interviews.

Object collaboration

Strategy — swap an algorithm at runtime (payment/pricing). The OCP workhorse.

Refactoring Guru — Strategy.

Observer — publish/subscribe; events and notifications.

Refactoring Guru — Observer.

State — behaviour changes with internal state (vending machine, order).

Refactoring Guru — State.

Command — encapsulate a request (undo/redo, queues).

Refactoring Guru — Command.

Template Method & Iterator — recognise each in a sentence.

Refactoring Guru — catalog.

5Classic LLD prompts

0/7▶

These recur constantly. For each: entities, relationships, key methods, and the fitting pattern(s). Class diagram first, code the core after.

Design these end to end

Parking lot — spots, vehicles, tickets, pricing; strategy for fees, factory for vehicles.

awesome-low-level-design.

Elevator system — cars, requests, scheduling; state pattern.

awesome-low-level-design — Elevator.

Splitwise — users, groups, expenses, balances; debt simplification.

awesome-low-level-design — Splitwise.

BookMyShow — shows, seats, concurrency & locking on selection.

awesome-low-level-design — Movie booking.

Vending machine / ATM — the canonical state-machine prompt.

Refactoring Guru — State.

Chess / Snakes & Ladders — board, pieces, rules, turn flow.

awesome-low-level-design — Chess.

Food delivery / ride hailing — matching, orders, state transitions.

awesome-low-level-design.

6Machine coding

0/4▶

Some rounds ask you to build a runnable mini-system in 60–90 minutes. They grade structure, extensibility, and working code.

How to score well

Start from the interface / core entities; get something running early, then extend.

awesome-low-level-design.

Prefer composition + DI so pieces are swappable and testable.

Fowler — Dependency Injection.

Write a couple of tests or a main() demo to prove it works — visibly.

Fowler — Unit test.

Manage time: MVP first, note extensions you'd add with more time.

awesome-low-level-design.