Immutable copies in Java
Java interview exercise on Modern Java, difficulty 1 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.
Write frozen(items) that returns an immutable copy of the list: later changes to the original must not show in the copy, and adding to the copy must throw UnsupportedOperationException. List.copyOf does both (and it rejects null elements, which is fine here).
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.