Behavioural Patterns interview questions

Strategy, observer, state, command, and template method in interview-sized problems.

4 questions in Low-Level Design & OOD.

Low-Level Design & Object-Oriented DesignA 23-minute read on how the topic fits together, before the questions.Low-level design fundamentals60 short answers on one page, for revising rather than studying.

medium4

mediumDesignCoding

Design the classes for a notification service that sends email, SMS and push. Which design patterns earn their place, and which would you leave out?

Name what varies first: channel, text, the user's channel choices and retry. A notification service then needs a channel interface (a strategy) picked from a registry, templates as data, a preference policy and a retry policy splitting transient from permanent errors. Most other design patterns add classes, not value.

6 minmid, senior

mediumConceptDesign

When would you use the observer pattern, and what goes wrong with it at scale?

Observer decouples a subject from things that react to it, which is right when the subject genuinely should not know its listeners. The costs are a control flow you cannot read from the code, listener lifetimes that leak, and an exception in one observer that silently prevents the rest from running.

4 minmid, senior, staff

mediumDesignCoding

An order moves through a set of states with rules about which transitions are allowed. How would you design that so it does not become a nest of if statements?

Write the legal transitions down as a table first, then give the entity one entry point that consults it and no state setter at all. The design question is where the transition rules live and how side effects stay outside the decision that changed the state.

5 minmid, senior, staff

mediumConceptDesign

When would you use the strategy pattern instead of inheritance?

Use the strategy pattern instead of inheritance when behaviour varies independently from object identity. Composition avoids subclass explosion, enables runtime swapping and makes algorithms easier to test.

5 minmid, senior, staff