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Strategy, observer, state, command, and template method in interview-sized problems.
3 questions in Low-Level Design & OOD.
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.
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.
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.