Generics and Class<? extends Enum<?>>, EnumSet.allOf(class) vs class.getEnumConstants()

Question

I have the following BeanValidation code that works fine, and permits to validate that a bean annotated with:

  @EnumValue(enumClass = MyTestEnum.class)
  privat         


        

Answer 1

My explanation on @assylias's solution:

What we want to express about the type of the class is that it's a

Class, for some E, that E <: Enum

but Java does not allow us to introduce a type variable E in a method body.

Usually, we can exploit wildcard and wildcard capture to introduce a hidden type variable

class G { ... }  // b(T) is a type expression that may contain T

G   --capture-->   G, for some T, that T <: A & b(T)

But this won't work in our case, since T in Class does not have a bound that makes it work.

So we need to introduce a new type with the desired bound

class EnumClass>   // called EnumValue in assylias's solution

    EnumClass(Class enumClass) 

    Class enumClass()

EnumClass   --capture-->    EnumClass, for some E, that E <: Enum

We then call EnumClass.enumClass() to yield a

Class, for some E, that E <: Enum

which is the goal we've been trying to achieve.

But how can we call the constructor of EnumClass? The origin of the problem is that we don't have a proper type for enumClass, yet the constructor of EnumClass wants a properly typed enumClass.

Class enumClass = ...;
new EnumClass<...>(enumClass);  // wont work

Fortunately(?) the raw type helps here which disables generics type checking

EnumClass raw = new EnumClass(enumClass);  // no generics
EnumClass wild = raw; 

So the minimum gymnastics we need to perform to cast the class to the desired type is

((EnumClass)new EnumClass(enumClass)).enumClass()