Two Valid Ways to Create an Immutable List in Java

Quick Answer

List.of('a', 'b') is correct because it uses the factory method introduced in Java 9 that builds a genuinely immutable list directly, rather than wrapping an already-existing mutable list. Any attempt to add, remove, or set an element on the result throws UnsupportedOperationException, and unlike many older collection APIs, List.of() also rejects null elements outright rather than silently accepting them. It helps to contrast this with Collections.unmodifiableList(), which sits alongside it in this topic area: that method does not create a new immutable list at all, it returns a view wrapping an existing list, so the view itself blocks structural changes, but the underlying list can still be mutated directly and those changes will show up through the view. List.of() has no such backdoor, because there is no separate backing list left for anyone to mutate. When an exam question asks you to identify genuinely immutable list creation, look for factory methods that construct a new list from scratch, such as List.of() and its relatives, rather than methods that merely wrap or restrict access to an existing collection. The naming can be misleading, since both approaches prevent modification through the reference you are given, but only the factory-method approach guarantees the data itself can never change from any angle.