Why this matters
- Machine holds inventory of items in slots with limited quantity
- Scope — what "States" covers inside Vending Machine and what it does not
- Scope — what "Inventory" covers inside Vending Machine and what it does not
- Scope — what "Change calculation" covers inside Vending Machine and what it does not
- Machine problems test State, Strategy, and facade coordination.
Idle
Moving
DoorOpen
Idle
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
- Machine holds inventory of items in slots with limited quantity
- Customer selects item, inserts coins/cash, machine validates payment
- Dispense item and return change when payment exceeds price
- Reject selection when out of stock or insufficient payment
- Admin can restock slots and collect cash
- State-driven flow: idle → accepting money → dispensing → idle
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 Vending Machine as a production-ready in-process service. Read requirements point-by-point before drawing classes.
Requirements
Points to cover
- Machine holds inventory of items in slots with limited quantity
- Customer selects item, inserts coins/cash, machine validates payment
- Dispense item and return change when payment exceeds price
- Reject selection when out of stock or insufficient payment
- Admin can restock slots and collect cash
- State-driven flow: idle → accepting money → dispensing → idle
States
Points to cover
- Scope — what "States" covers inside Vending Machine 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
Inventory
Points to cover
- Scope — what "Inventory" covers inside Vending Machine 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
Change calculation
Points to cover
- Scope — what "Change calculation" covers inside Vending Machine 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
// CampusOS — VendingMachine facade (keep thin)
public final class VendingMachineService {
private final VendingMachineRepository repository;
public VendingMachineService(VendingMachineRepository 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.
- States — review this slice before mock interviews.
- Inventory — review this slice before mock interviews.
- Change calculation — 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.