Low-Level Design Course

Welcome to Low-Level Design (LLD) on TechLearningPRO — a hands-on course for engineers preparing for object-oriented design interviews and building maintainable Java systems.

High-Level Design (HLD) answers what the system is made of at scale: services, databases, caches, queues, and how they talk across a network.

A repeatable design process prevents panic in timed interviews and reduces rework in production.

Functional requirements describe what the system must do: park a car, issue a ticket, transfer money.

Object-oriented design organizes software around classes (templates) and objects (runtime instances).

Encapsulation hides internal representation and exposes a controlled interface.

Abstraction focuses on essential behavior while hiding irrelevant detail.

Inheritance models is-a relationships: SavingsAccount is an Account, Truck is a Vehicle.

Polymorphism lets one interface invoke different behaviors depending on runtime type.

Composition builds objects by combining other objects (has-a).

The Single Responsibility Principle (SRP) states a class should have one reason to change — one axis of responsibility.

The Open/Closed Principle (OCP) says software entities should be open for extension but closed for modification.

The Liskov Substitution Principle (LSP) requires subtypes to behave as expected wherever the base type appears.

The Interface Segregation Principle (ISP) says clients should not depend on methods they do not use.

The Dependency Inversion Principle (DIP) says high-level modules should not depend on low-level modules; both depend on abstractions.

Unified Modeling Language (UML) is the standard notation for sketching object-oriented designs.

A class diagram shows types, fields, methods, and relationships — the static blueprint of your LLD.

Sequence diagrams show object interactions over time — the perfect follow-up after a class diagram when the interviewer says 'walk me through parking a car.' Vertical lifelines…

Activity diagrams model workflows with decisions, forks, and parallel steps — useful when business logic branches heavily: ATM menu, order fulfillment, game turn sequence.

State diagrams (state machines) model object lifecycle: ticket valid → paid → used; elevator idle → moving → door open.

The Singleton pattern ensures a class has one instance and provides global access — common for configuration registries, in-memory id generators, or connection pool facades in s…

Factory patterns encapsulate object creation — hiding whether you construct Car, Truck, or future Scooter.

The Builder pattern constructs complex objects step by step — ideal when many optional fields, validation rules, or immutable products make telescoping constructors unreadable.

The Strategy pattern defines a family of algorithms, encapsulates each, and makes them interchangeable.

The Observer pattern defines one-to-many dependency: when subject state changes, observers update automatically.

The Adapter pattern converts one interface into another clients expect — integrating legacy payment SDK, third-party map API, or old printer driver without rewriting your domain…

Don't Repeat Yourself (DRY) means every piece of knowledge should have a single authoritative representation.

Keep It Simple, Stupid (KISS) urges the simplest design that satisfies requirements.

You Aren't Gonna Need It (YAGNI) says implement only what requirements demand today — not speculative features 'we might need multi-currency someday' without evidence.

Cohesion measures how closely class elements relate; coupling measures dependency between classes.

The parking lot is the most common LLD interview question.

Library management LLD covers books, members, loans, reservations, and fines — classic entity-relationship modeling with behavior.

ATM LLD models card authentication, PIN validation, cash dispensing, balance inquiry, and transaction logging — often coordinated with a remote BankHost abstraction.

Elevator LLD challenges scheduling, state management, and request queues — internal requests (inside cabin) vs external (floor up/down).

Movie ticket booking LLD involves shows, seats, concurrent locking, payments, and confirmations — similar to BookMyShow.

Cab booking (Uber/Ola style) LLD covers riders, drivers, trips, location matching, fare calculation, and trip state — usually scoped to one city and in-process matching for inte…

Splitwise LLD models groups, expenses, split strategies (equal, exact, percentage), and simplified balances between members — who owes whom.

Snake and Ladder is a compact LLD problem testing game board modeling, player movement, jump rules, win detection, and turn management — common as a lighter interview or warm-up.

A structured LLD interview strategy maximizes signal in forty-five to sixty minutes.

Searchable LLD interview question bank — OOP, SOLID, UML, design patterns, parking lot, ATM, elevator, and interview strategy.

This design exercise walks through a timed LLD session for an online auction system — bidders, items, bids, timers, and winner notification.

Congratulations on completing Low-Level Design on TechLearningPRO.