Problem Statement
Explain the difference between an immutable collection and an unmodifiable view, and why it matters.
Explanation
An immutable collection cannot change—no element can be added, removed, or mutated through any reference. An unmodifiable view disallows mutations through that view, but the backing collection may still change elsewhere and those changes will appear. Immutability is safer for sharing across threads and for defensive returns; unmodifiable views are lightweight guards around a mutable source.