Mutable objects are those whose state can be changed. For instance, an array is mutable, but a String is not. Mutable class members should never be returned to a caller or accepted and stored directly. Doing so leaves you vulnerable to unexpected changes in your class state.
Instead use an unmodifiable ``++Collection++`` (via ``++Collections.unmodifiableCollection++``, ``++Collections.unmodifiableList++``, ...) or make a copy of the mutable object, and store or return the copy instead.
