As Scott indicated, this is related to
generics in Java 1.5. Collections now use generics to safeguard the types of references they contain, and the compiler is simply warning you of potentially unsafe operations.
In this case, your type is not just Vector. It's Vector<PrintWriter>. For example...
Vector<PrintWriter> myWriters = new Vector<PrintWriter>();
(The message you posted above looks like what you would get after recompiling with -Xlint to show details of the warnings.)
[ December 07, 2005: Message edited by: marc weber ]