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Python 3 map is lazy: it returns an iterator and evaluates each mapped value as the iterator advances. Convert it to a list only when you need materialized results or repeated traversal. Lazy evaluation saves memory but exceptions may occur later when a consumer iterates.

HashMap uses an array of buckets and computes an index from the hash of the key. Each bucket may hold one or more entries. When collisions happen, Java stores entries in a linked list and later in a tree when the bucket grows large enough. The table resizes when the load factor is exceeded so the map keeps operations efficient. The key idea is balancing fast lookup with controlled memory use.

Modern Java ConcurrentHashMap does not use the fixed lock segments from older implementations. It uses non-blocking reads and coordinates updates with CAS and synchronization at individual bins when needed. This allows unrelated keys to make progress concurrently. It protects map operations, not a multi-key transaction or mutable values stored inside it.

Promise.all aggregates promises already supplied to it. Eagerly mapping a large list can start all work at once, so downstream capacity needs a separate concurrency bound.

Modern Java implementations do not use the old fixed segment array commonly described in older explanations. Reads are generally non-blocking; initialization and some updates use CAS, while contended updates coordinate at a bin. Map operations are thread-safe, but multi-key invariants and mutable values still need their own synchronization design.