Design Tic-Tac-Toe

Board, win detection, and two-player loop.

Expert12 min read

Read these first

Design a Chess Game

Why this matters

3×3 board; two players X and O
Scope — what "Board" covers inside Tic-Tac-Toe and what it does not
Scope — what "Win check" covers inside Tic-Tac-Toe and what it does not
Scope — what "Player turn" covers inside Tic-Tac-Toe 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

3×3 board; two players X and O
Detect win on row, column, or diagonal
Detect draw when board full
Reject illegal moves (occupied cell, wrong turn)

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 Tic-Tac-Toe as a production-ready in-process service. Read requirements point-by-point before drawing classes.

Requirements

Points to cover

3×3 board; two players X and O
Detect win on row, column, or diagonal
Detect draw when board full
Reject illegal moves (occupied cell, wrong turn)

Board

Points to cover

Scope — what "Board" covers inside Tic-Tac-Toe 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

Win check

Points to cover

Scope — what "Win check" covers inside Tic-Tac-Toe 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

Player turn

Points to cover

Scope — what "Player turn" covers inside Tic-Tac-Toe 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 — TicTacToe facade (keep thin)
public final class TicTacToeService {
    private final TicTacToeRepository repository;

    public TicTacToeService(TicTacToeRepository 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.

Requirements — review this slice before mock interviews.
Board — review this slice before mock interviews.
Win check — review this slice before mock interviews.
Player turn — 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.