I am having a problem changing a List that has a set-size in Java.

I understand that I can't add or remove from this list but why can't I use set? When I use set the UnsupportedOperationException is thrown as well as when I use add and remove which is expected.

set

public Object set(int index,
                  Object element)

    Replaces the element at the specified position in this list with the specified element (optional operation). 

I understand its an optional operation its just that want to replace that one element of the list with another element. Is there any way I can do this?