Immutable Collections

Java 9 factory methods (List.of, Set.of, Map.of) create truly immutable collections with no nulls, while Collections.unmodifiableList creates only an unmodifiable view that still reflects changes to the underlying mutable list.

List.of() creates an immutable list that throws UnsupportedOperationException on any mutation attempt, and List.copyOf() creates an immutable snapshot from any collection to protect against changes to the original.

Arrays.asList returns a fixed-size list backed by the original array where set() works but add/remove do not, while List.of returns a truly immutable list where no mutation is possible, and Arrays.asList has a dangerous primitive array gotcha.