Collectors.toList() returns a mutable ArrayList carrying the stream's results. It is the workhorse collector that has shipped since Java 8 and is still useful when you need to keep adding to or sorting the list afterwards. The lambda n -> n * n is mapped over the stream, then materialised into a list. Don't return this list from a public method without copying or wrapping with Collections.unmodifiableList(...) if callers should not mutate it.

Collectors.toUnmodifiableList() (Java 10+) wraps the result in a list whose mutators throw UnsupportedOperationException. Reach for this in API boundaries where you do NOT want callers mutating internal state. It also enables some downstream optimisations because the runtime knows the list will not change. Note that the elements themselves are not deeply frozen, only the list structure is locked.

Java 16 added Stream.toList() directly on the stream, returning an unmodifiable List. It is the preferred form going forward because it is shorter and signals intent. The runtime difference from Collectors.toUnmodifiableList() is negligible. On older JDKs (13 in our test runner here), use Collectors.toUnmodifiableList() and migrate when your project bumps. Both forms preserve encounter order and behave like the upstream stream regarding nulls.