Collections.unmodifiableList and defensive copy

Question

If I write

List a1 = Arrays.asList(1, 2, 3);
List a2 = Collections.unmodifiableList(a1);

a2 is r

Answer 1

The statement:

Collections.unmodifiableList(a1);

returns a wrapper over the original collection whose modifier methods throw UnsupportedOperationException.

The wrapper is read-through, meaning that if you modify a1, the changes reflect on the wrapped collection a2.