Error reading file

OK, so I get another error right here. I have class B and class C extend abstract class A. I create a ArrayList<A> lst = new ArrayList<A>() then add both B and C objects type into that ArrayList then write that ArrayList to file data.dat. When I read it, only attributes from B and C is read to the ArrayList and attributes that B and C inherit from A is gonna be "null" and 0.
I would ask if this problem is caused by that I use generic type ArrayList<A> so only the attributes of B and C are read into ArrayList or by other reason. Finnaly, how to solve it? Thanks :)

OK, seem to be solved by adding serialVersion for class A :)