Create swap.java

TheswapHelper method knows that list is a List<E>. Therefore, 
it knowst hat any value it gets out of this list is of type E, 
and that it’s safe to put any valueof type E into the list. This
 slightly convoluted implementation of swap compiles cleanly. It
 allows us to export the nice wildcard-based declaration of swap,
 while taking advantage of the more complex generic method internally