Lists

I've been reading a book java generics which has a nice section on java collections.
I'm wondering why they bother with the List/Map/set interface stuff if they don't expect people to create new List/Map/Set classes from them.
So what to subclass surely the things you could be subclass are subclassed?

I remembered that FORTRAN had a nice disk backed array - I like the notion of a simple disk-based array in Java which would appear to map a very large array into a much smaller in memory object. The basic idea is to avoid all that reading/writing out just use it directly.

I was thinking that it might be possible to use reflection on simple predictably sized objects ( ints, shorts, floats, booleans doubles, chars , enums ) to 'parse'