When would you choose a hash map instead of an array for lookups?

Choose a hash map when you need to find values by arbitrary keys, such as a user identifier, without scanning the collection. With suitable hashing and load management, lookup and insertion are expected constant time, but collisions and implementation details affect worst-case behavior. Arrays support constant-time indexed access and often provide better locality with lower overhead. A sorted array also supports logarithmic-time binary search. Consider key type, ordering, memory, mutation frequency, and data size rather than assuming a hash map is always faster.