Object-Oriented Design

The low-level design round, read as a method rather than a pattern quiz: turning fuzzy requirements into objects with clean seams — the nouns/verbs pass, composition over inheritance, invariants stated out loud, the four patterns that actually appear, and the state machine hiding inside most of the classics.

Explainers

• The OOD Interview, Decoded — The object-design round looks like it wants pattern trivia. It is really grading one thing: can you turn four fuzzy sentences into a handful of objects with clean seams? We make the method legible — the nouns/verbs pass that turns a prompt into candidate classes and methods, composition over inheritance stated honestly, the invariants you say out loud, the four patterns that actually show up in the room (factory, strategy, observer, state) each with a five-line code moment, SOLID trimmed to the letters that earn their keep, and the secret weapon: most OOD classics are state machines wearing a costume. Click the nouns in a live requirements pass, level a design against the classic failures, and leave with the 45-minute LLD script.
• Three OOD Classics, Worked — The method from the OOD interview page, run on the three prompts that show up most. The parking lot end to end — requirements, the nouns pass, the class model, and the three decisions that actually get graded: spot assignment as a strategy, the ticket as the identity object, fee calculation as a swappable strategy, with the real compact code. Then the reveal that collapses half the catalog: the vending machine and the elevator are the same problem — state machines with states, events, guards, and actions. Drive one live: insert coins, select, dispense; request floors up and down; watch the legal transitions fire and the illegal ones bounce with the guard named. Finish with the missing-guard bugs interviewers probe, and the move that spots a state machine in any prompt.