91 curated questions graded from Foundations (Easy) to Practical Patterns (Medium) and Internals & Architecture (Hard).
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Foundations & Core Concepts
Answer: Low-Level Design (LLD) transforms abstract high-level architectural requirements into concrete, implementable software blueprints - specifying class structures, object relationships, design patterns, method signatures, and state transitions before writing production code.
Vehicle $\to$ (Car, Motorcycle, Truck), the strategy pattern for FeeCalculationStrategy, and the thread-safe synchronization on ParkingSpot.occupy().Answer: Database indexing constructs an auxiliary data structure (typically a B+ Tree, Hash Table, or GiST/GIN index) that maps key column values to their physical disk block addresses, eliminating slow $O(N)$ full table scans.
(customer_id, order_date DESC) allows the user profile page to fetch the last 10 orders in 2ms instead of scanning 50 million rows.Answer:
private, protected). Prevents invalid state mutations (e.g., preventing a BankAccount.balance from becoming negative directly).paymentGateway.charge(amount) without exposing TLS socket handshakes).SavingsAccount extends BankAccount).Notification objects and calling send(), where email, SMS, and push notifications execute their own logic).Answer: In multi-threaded environments, concurrent execution without proper synchronization leads to race conditions, deadlocks, lost updates, and memory visibility bugs.
ReentrantReadWriteLock), atomic variables (AtomicInteger), or optimistic concurrency controls (Compare-And-Swap).B14 at the exact same millisecond, concurrency control ensures only one thread successfully acquires the seat lock while the other receives a SeatAlreadyBookedException.Answer: UML Behavioral Diagrams visualize the dynamic runtime interactions, state changes, and time-ordered message flows between system components.
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Core concepts, definitions, basic syntax, and first principles expected in round 1 screening.
Real-world mechanisms, state management, edge cases, performance trade-offs, and practical coding.
Deep runtime internals, memory models, distributed design, concurrency failure modes, and architectural decisions.