Design a Deck of Cards

Shuffle, deal, and hand management for card games.

Expert12 min read

Read these first

Design Snake and Ladder

Why this matters

Scope — what "Card model" covers inside Deck of Cards and what it does not
Scope — what "Shuffle" covers inside Deck of Cards and what it does not
Scope — what "Deal" covers inside Deck of Cards and what it does not
Scope — what "Hand" covers inside Deck of Cards and what it does not
Games and architecture patterns signal staff+ depth.

Waiting

InProgress

Finished

Turn-based games map cleanly to State pattern — one handler per game phase.

CampusOS context

CampusOS is the fictional smart-campus platform used across this track. Treat this module as a real product slice:

Context

Belongs to CampusOS — same naming and design vocabulary as parking, library, and booking modules
Scope is one JVM — persistence is in-memory or behind a repository interface
Goal is a class diagram + core Java that an interviewer can follow in 35–45 minutes
Extension paragraph at the end links to System Design when scale exceeds one process

Requirements snapshot

Functional requirements

Standard 52-card deck with suit and rank
shuffle() randomizes order
deal(n) removes top n cards to hand
Hand holds cards; play rules depend on game

Non-functional requirements

Single JVM, in-process design for this LLD article
Core flows testable with in-memory repositories
Extension points for new rules without editing central god classes
Clear public API on facade/service layer
CampusOS extension links to System Design when scale demands distribution

Overview

This CampusOS module designs Deck of Cards as a production-ready in-process service. Read requirements point-by-point before drawing classes.

Card model

Points to cover

Scope — what "Card model" covers inside Deck of Cards and what it does not
Classes — name 2–4 classes/interfaces responsible for this slice
Flow — step-by-step: trigger → validation → state change → side effect
Edge cases — invalid input, concurrent access, empty state, timeout
Pattern — Strategy, State, Observer, or Facade if it fits naturally
Test hook — one scenario an interviewer will ask you to walk through

Shuffle

Points to cover

Scope — what "Shuffle" covers inside Deck of Cards and what it does not
Classes — name 2–4 classes/interfaces responsible for this slice
Flow — step-by-step: trigger → validation → state change → side effect
Edge cases — invalid input, concurrent access, empty state, timeout
Pattern — Strategy, State, Observer, or Facade if it fits naturally
Test hook — one scenario an interviewer will ask you to walk through

Deal

Points to cover

Scope — what "Deal" covers inside Deck of Cards and what it does not
Classes — name 2–4 classes/interfaces responsible for this slice
Flow — step-by-step: trigger → validation → state change → side effect
Edge cases — invalid input, concurrent access, empty state, timeout
Pattern — Strategy, State, Observer, or Facade if it fits naturally
Test hook — one scenario an interviewer will ask you to walk through

Hand

Points to cover

Scope — what "Hand" covers inside Deck of Cards and what it does not
Classes — name 2–4 classes/interfaces responsible for this slice
Flow — step-by-step: trigger → validation → state change → side effect
Edge cases — invalid input, concurrent access, empty state, timeout
Pattern — Strategy, State, Observer, or Facade if it fits naturally
Test hook — one scenario an interviewer will ask you to walk through

Java

// CampusOS — DeckOfCards facade (keep thin)
public final class DeckOfCardsService {
    private final DeckOfCardsRepository repository;

    public DeckOfCardsService(DeckOfCardsRepository repository) {
        this.repository = repository;
    }

    /** Orchestrate: validate → domain rules → persist → return result */
    public Result handle(Request request) {
        request.validate();
        var domain = DomainModel.from(request);
        domain.applyRules();
        return repository.save(domain);
    }
}

Quick recall

Everything you need if you only revisit this box.

Card model — review this slice before mock interviews.
Shuffle — review this slice before mock interviews.
Deal — review this slice before mock interviews.
Hand — review this slice before mock interviews.

CampusOS extension

When this outgrows one JVM

Shard inventory or state by campus ID or geographic region
Push notifications, payments, and search to dedicated services (System Design track)
Use message queues for async side effects (email, analytics)
The class diagram you drew here becomes one box on the HLD architecture diagram

Test yourself

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