Low Level Design Introduction Tutorial from Scratch (2026)
Low Level Design (LLD) is the phase of software development where you translate high-level architectural decisions into detailed, implementable class designs. After reviewing thousands of codebases and conducting hundreds of system design interviews, I can confidently say that LLD skills are what separate engineers who can talk about systems from engineers who can actually build them.
This introduction covers what LLD is, when to apply it, how to approach class design, and practical code organization strategies. Whether you are preparing for technical interviews or looking to improve your real-world design skills, this tutorial provides a solid foundation.
What is Low Level Design?
Low Level Design (LLD), also called detailed design, is the process of designing the individual components, classes, interfaces, and data structures that implement the high-level architecture. While High Level Design (HLD) answers "what components do we need?", LLD answers "exactly how will each component work?" It involves defining class hierarchies, relationships (inheritance, composition, association), method signatures, error handling strategies, and state management.
LLD is typically done before writing code, but agile teams often iterate between LLD and coding in short cycles. The level of detail depends on the complexity and criticality of the system — safety-critical systems like medical devices or aircraft control require very detailed LLD, while simple CRUD apps may need very little.
// Example of LLD output: class diagram translated to code
public class ParkingLot {
private List levels;
private Map activeTickets;
public ParkingLot(int numberOfLevels, int spotsPerLevel) {
this.levels = new ArrayList<>();
this.activeTickets = new HashMap<>();
for (int i = 0; i < numberOfLevels; i++) {
levels.add(new ParkingLevel(i, spotsPerLevel));
}
}
public ParkingTicket parkVehicle(Vehicle vehicle) {
for (ParkingLevel level : levels) {
if (level.hasAvailableSpot(vehicle.getType())) {
ParkingSpot spot = level.parkVehicle(vehicle);
ParkingTicket ticket = new ParkingTicket(vehicle, spot);
activeTickets.put(ticket.getId(), ticket);
return ticket;
}
}
throw new ParkingFullException("No available spots");
}
} Class Design Principles
Good class design follows several well-established principles. The Single Responsibility Principle (SRP) dictates that a class should have only one reason to change. The Open-Closed Principle (OCP) states that classes should be open for extension but closed for modification. The Liskov Substitution Principle (LSP) ensures that subclasses can replace their parent classes without breaking the system.
Beyond SOLID, you should also consider invariants — conditions that must always hold true for your objects. For example, a BankAccount should never have a negative balance (unless overdraft is explicitly allowed). Enforce invariants through constructor validation and by controlling mutation via domain methods rather than public setters.
// Applying SRP and invariants
public class TemperatureSensor {
private final String id;
private final double minTemp;
private final double maxTemp;
private double currentTemp;
public TemperatureSensor(String id, double minTemp, double maxTemp) {
this.id = id;
this.minTemp = minTemp;
this.maxTemp = maxTemp;
this.currentTemp = (minTemp + maxTemp) / 2; // safe default
}
public void updateTemperature(double newTemp) {
if (newTemp < minTemp || newTemp > maxTemp) {
throw new IllegalArgumentException("Temperature out of range");
}
this.currentTemp = newTemp;
}
public double getCurrentTemp() {
return currentTemp;
}
}Code Organization and Package Structure
How you organize code into packages and modules has a massive impact on maintainability. A common approach is package-by-feature: grouping classes by the feature they support rather than by their technical layer. For example, instead of having separate packages for all controllers, all services, and all repositories, each feature gets its own package containing its controller, service, repository, and domain model.
Package-by-feature improves cohesion, makes it easier to navigate the codebase, and supports better encapsulation (package-private access). For larger systems, consider a layered approach on top of feature packages: presentation, application, domain, and infrastructure layers, each with clear dependency direction.
// Package structure example (package-by-feature)
// com.example.parking/
// +-- common/ (shared utilities, base classes)
// +-- vehicle/ (feature: vehicle management)
// | +-- Vehicle.java
// | +-- VehicleRepository.java
// | +-- VehicleService.java
// | +-- VehicleController.java
// +-- parking/ (feature: parking operations)
// | +-- ParkingLot.java
// | +-- ParkingTicket.java
// | +-- ParkingService.java
// +-- payment/ (feature: payment processing)
// +-- PaymentGateway.java
// +-- PaymentService.javaFrequently Asked Questions
What is the difference between LLD and HLD?
HLD (High Level Design) focuses on system architecture — components, data flow, technology choices — while LLD (Low Level Design) focuses on detailed class design, method signatures, data structures, and algorithms within each component.
Do I need LLD for every project?
Not every project requires formal LLD. Small CRUD apps or prototypes can skip it. However, for complex systems, production-grade software, or interview preparation, LLD is essential.
What are common LLD interview topics?
Common LLD interview topics include designing a parking lot, elevator system, library management system, chess game, vending machine, ATM, task management system, and logging framework.
Which language is best for LLD interviews?
Java is the most common choice for LLD interviews due to its rich OOP features, widespread use, and strong typing. C++ and Python are also popular alternatives.
Originally published on Ayodhyyya. Last updated June 1, 2026.