Unmodifiable Views vs Immutable Copies
intermediate12 min readLesson 77 of 180
The difference between a read-only view and an independent copy — and the Arrays.asList trap.
Unmodifiable views vs immutable copies
Three different "cannot change" guarantees:
List<String> src = new ArrayList<>(List.of("a", "b"));
List<String> view = Collections.unmodifiableList(src);
// view.set(0, "x") throws — but src.set(0, "x") changes the view too!
List<String> copy = List.copyOf(src);
// copy is independent: mutating src cannot reach it
List<String> literal = List.of("a", "b");
// immutable since birth, no copy cost, rejects nulls
List.of/Set.of/Map.of are the default for constants. List.copyOf
is the tool for sealing caller-provided collections. Unmodifiable views
are for controlled exposure — the owner keeps mutability, the caller
doesn't get it.
Arrays.asList is a trap: fixed-size but writable — set works, add
throws. Rarely what you mean.