There is no predefined type named
AnyType. I am guessing you have come across a poorly named type parameter variable. The naming convention for type variables is suggested to be single uppercase letters, to avoid this type of confusion.
In your case,
TestRpn<AnyType extends Comparable<AnyType>> should be rephrased as
TestRpn<E extends Comparable<E>>, which in turn means that you can substitute any type for E that implements Comparable for it’s own type. Example: java.lang.String implements
TestRpn<String> is a valid parametrization of the above base type.