LESSON 11.1

The machine coding approach

Master the 90-minute low-level design interview: requirement scoping, entity modeling, API design, pattern selection, and test-driven implementation.

6 min read

The Machine Coding Round is the standard practical interview for mid-to-senior software engineers at companies like Uber, Flipkart, Swiggy, and Razorpay. In 90 to 120 minutes, you must design and code a fully working, extensible, and tested object-oriented application from scratch.

Unlike algorithm rounds (LeetCode), Machine Coding tests real-world software craftsmanship: can you design clean entities, adhere to SOLID, pick the right patterns, and ship working code under time constraints?

The 5-Phase Time Plan (90 Minutes)

Clarify Requirements & Bounds (15 min):
- Identify actors and primary use cases (e.g. ParkVehicle, UnparkVehicle, CalculateFee).
- Define constraints: single-floor or multi-floor? In-memory or database? Thread-safe?
Entity & Interface Modeling (15 min):
- Identify nouns (Entities, Value Objects) and verbs (Operations).
- Define interfaces for repositories, strategies, and external services.
Core Business Logic & Pattern Integration (40 min):
- Implement domain entities with invariant enforcement.
- Inject strategies (pricing, allocation, dispatch).
Driver / CLI / Integration (10 min):
- Wire up the controller or command runner to execute user inputs.
Testing & Edge Cases (10 min):
- Write automated unit tests verifying core paths and boundary failures.

Laying out the diagram…

The 4-stage machine coding pipeline: requirements -> entities -> logic -> tests.

In code

Common Traps to Avoid

The Database Trap: Do not spend 45 minutes configuring SQLite, Prisma, or Docker unless explicitly asked. An in-memory Map with a Repository interface is expected and preferred.
The Perfection Trap: Do not over-engineer with 15 GoF patterns on line 1. Write the simplest working domain code first, then refactor.
The Silent Coder Trap: Talk through your trade-offs with the interviewer as you type.

Strategy: For pluggable allocation algorithms (nearest to entrance, lowest floor).
State: For entity lifecycles (available, reserved, maintenance).

Quick check

If you run short on time in a machine coding round, what should you prioritize?

Try it

Parking Lot LLD

Design a multi-level parking lot with support for multiple vehicle types and dynamic hourly pricing.

Check your understanding

1.You have 90 minutes in a Machine Coding interview. What is the recommended time allocation?
2.What do interviewers evaluate most in a Machine Coding round?
3.What should you do if requirements are ambiguous (e.g. 'design an elevator system')?

Practice it

Parking Lot LLDModel spots, tickets, and pricing with clear responsibilities and safe allocation.Vending Machine LLDModel selection, payment, dispensing and refunds with guarded state transitions.

Your notes

1 // Skeleton of a clean Machine Coding structure: 2 // 1. Domain Entities 3 class Vehicle { 4 constructor(licensePlate, type) { 5 this.licensePlate = licensePlate; 6 this.type = type; 7 } 8 } 9 10 class ParkingSpot { 11 #vehicle = null; 12 constructor(id, supportedType) { 13 this.id = id; 14 this.supportedType = supportedType; 15 } 16 isAvailable() { return this.#vehicle === null; } 17 park(vehicle) { 18 if (!this.isAvailable()) throw new Error("Spot occupied"); 19 if (vehicle.type !== this.supportedType) throw new Error("Type mismatch"); 20 this.#vehicle = vehicle; 21 } 22 vacate() { this.#vehicle = null; } 23 get currentVehicle() { return this.#vehicle; } 24 } 25 26 // 2. Service Orchestrator 27 class SimpleParkingService { 28 #spots = []; 29 30 constructor(spots) { 31 this.#spots = spots; 32 } 33 34 park(vehicle) { 35 const spot = this.#spots.find((s) => s.isAvailable() && s.supportedType === vehicle.type); 36 if (!spot) throw new Error("Parking full for type: " + vehicle.type); 37 spot.park(vehicle); 38 return spot.id; 39 } 40 } 41 42 // 3. Test verification 43 const spots = [new ParkingSpot("S1", "CAR"), new ParkingSpot("S2", "BIKE")]; 44 const service = new SimpleParkingService(spots); 45 46 const car = new Vehicle("KA-01-1234", "CAR"); 47 const spotId = service.park(car); 48 console.assert(spotId === "S1"); 49 console.log("Vehicle parked at spot:", spotId);

1 public class Example { 2 enum VehicleType { CAR, BIKE } 3 record Vehicle(String license, VehicleType type) {} 4 5 static final class ParkingSpot { 6 final String id; 7 final VehicleType supportedType; 8 private Vehicle currentVehicle; 9 10 ParkingSpot(String id, VehicleType type) { this.id = id; this.supportedType = type; } 11 boolean isAvailable() { return currentVehicle == null; } 12 13 void park(Vehicle v) { 14 if (!isAvailable() || v.type() != supportedType) throw new IllegalStateException("Cannot park"); 15 currentVehicle = v; 16 } 17 } 18 19 static final class ParkingService { 20 private final java.util.List<ParkingSpot> spots; 21 ParkingService(java.util.List<ParkingSpot> spots) { this.spots = spots; } 22 23 String park(Vehicle v) { 24 for (var s : spots) { 25 if (s.isAvailable() && s.supportedType == v.type()) { 26 s.park(v); 27 return s.id; 28 } 29 } 30 throw new IllegalStateException("Full"); 31 } 32 } 33 34 public static void main(String[] args) { 35 var spots = java.util.List.of( 36 new ParkingSpot("S1", VehicleType.CAR), 37 new ParkingSpot("S2", VehicleType.BIKE) 38 ); 39 var service = new ParkingService(spots); 40 var car = new Vehicle("KA-01-1234", VehicleType.CAR); 41 var id = service.park(car); 42 43 if (!"S1".equals(id)) throw new AssertionError(); 44 System.out.println("Parked vehicle successfully at: " + id); 45 } 46 }

The 5-Phase Time Plan (90 Minutes)

In code

Common Traps to Avoid

Related patterns

Check your understanding

Practice it

The 5-Phase Time Plan (90 Minutes)

In code

Common Traps to Avoid

Related patterns

Check your understanding

Practice it