Past exam of the mathematics course of the University of Cambridge / 2006 / ia / Paper 5 / 9 / d / Solution

A growable-array library should store its backing Java array and logical size in private fields in Java. Public methods such as add, get and remove can enforce bounds and keep the size consistent with the stored elements. The representation can later change without requiring clients to modify direct field accesses.

The benefit is preserving invariants through encapsulation. If clients could assign the size or replace the backing Java array directly, they could create negative sizes, report elements that do not exist, or modify entries without the library's checks. Accessor methods should not expose a mutable backing object indiscriminately, since a private field alone does not prevent that indirect route. Private fields also do not provide a general security boundary against privileged reflection or native code; the point here is ordinary library access discipline.