The stack trace shows you that the problem is here at Game.checkRoyalFlush(Game.java:294).
It also shows you that you are trying to sort an array whose element does not implements the Comparable interface. Check the sort() method of the java.util.Arrays class for more details.
slots is of type GVstack, so the question is : Does GVstack implements Comparable ?
So, what can I do so that my array elements implement the comparable interface?
Well, you can actually "implement the java.lang.Comparable interface"? The purpose of the interface is to provide a method that allows your element to be compared to another element -- this is needed, because it is not possible to sort an array, if the sort algorithm can't tell which element comes first.