Object Design · LLD Interview Questions — Easy

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.

• Why it matters: It eliminates architectural ambiguity, prevents costly refactors, and ensures the codebase remains modular, unit-testable, and scalable.
• Real-World Example: In designing a Parking Lot System, LLD defines the exact inheritance hierarchy between Vehicle $\to$ (Car, Motorcycle, Truck), the strategy pattern for FeeCalculationStrategy, and the thread-safe synchronization on ParkingSpot.occupy().

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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.

• Mechanism: By maintaining sorted keys in a balanced tree, lookups, range scans, and foreign key joins drop to $O(\log N)$ disk page reads.
• Real-World Example: In an e-commerce order database, adding a composite index on (customer_id, order_date DESC) allows the user profile page to fetch the last 10 orders in 2ms instead of scanning 50 million rows.

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Answer:

1. Encapsulation: Bundling internal state and behaviors within a class while restricting direct access via access modifiers (private, protected). Prevents invalid state mutations (e.g., preventing a BankAccount.balance from becoming negative directly).
2. Abstraction: Hiding complex internal implementation details behind clean, minimal public interfaces (e.g., calling paymentGateway.charge(amount) without exposing TLS socket handshakes).
3. Inheritance: Reusing common fields and methods from a base class in derived classes to model "is-a" relationships (e.g., SavingsAccount extends BankAccount).
4. Polymorphism: Enabling objects of different types to respond to the same method invocation with custom runtime behavior (e.g., iterating through a list of Notification objects and calling send(), where email, SMS, and push notifications execute their own logic).

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Answer: In multi-threaded environments, concurrent execution without proper synchronization leads to race conditions, deadlocks, lost updates, and memory visibility bugs.

• Mechanism: LLD uses mutexes, read-write locks (ReentrantReadWriteLock), atomic variables (AtomicInteger), or optimistic concurrency controls (Compare-And-Swap).
• Real-World Example: In a ticket booking system, if two threads attempt to book seat B14 at the exact same millisecond, concurrency control ensures only one thread successfully acquires the seat lock while the other receives a SeatAlreadyBookedException.

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Answer: UML Behavioral Diagrams visualize the dynamic runtime interactions, state changes, and time-ordered message flows between system components.

• Primary Types:
  • Sequence Diagrams: Depict step-by-step lifecycles and asynchronous RPC/method calls between actors and services over time.
  • State Machine Diagrams: Model an entity's lifecycle through discrete states and transition triggers.
  • Activity Diagrams: Model complex procedural workflows and parallel execution forks (like an order fulfillment pipeline).

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