Collections.unmodifiable* vs List.of() vs List.copyOf()

Choose the right unmodifiable/immutable collection factory and understand the differences in mutability guarantees.

Java offers several ways to create read-only collections, with different guarantees:

Collections.unmodifiableList = putting a 'DO NOT TOUCH' sign on a whiteboard (someone can still erase it from behind). List.of = a printed poster (truly cannot be changed).

Key Concepts

1. Collections.unmodifiableList(list): returns a VIEW over the original list. Mutations through the view throw UnsupportedOperationException, but the backing list can still be modified directly (and changes are visible through the view). Not truly immutable.

2. List.of(elements...) (Java 9): creates a truly immutable list. No backing list. Null elements not allowed. Provides value-based equality. Iteration order matches argument order.

3. List.copyOf(collection) (Java 10): creates an immutable copy. If the source is already an immutable list, may return the same instance (optimization). Null elements not allowed.

4. Arrays.asList(array): returns a fixed-size list backed by the array. Can set() elements but can't add/remove. Changes to the array are visible in the list and vice versa.

Map and Set equivalents: - Map.of(k1, v1, k2, v2), Set.of(e1, e2) — immutable - Map.copyOf(map), Set.copyOf(set) — immutable copies - Map.ofEntries(Map.entry(k, v), ...) — for > 10 entries

Null policy: - List.of() / Map.of() / Set.of(): null not allowed (NullPointerException) - Collections.unmodifiable*: null allowed (wraps whatever the backing collection has)