When a form is submitted the only thing sent to the server is strings: unless the server side knows how to convert these strings into actual objects you'll not get what you expect.
There are several options, including (a) convert the
string representation of the list into an actual List and (b) store the list in session scope. S2's type conversion may help with (a), depending on the string representation of the list--which is what you'll get by putting the list into an <s:hidden...> tag.