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 guess is that in ? extends Enum<?> the two ? could be different whereas allOf expects a T extends Enum<T> where both T are the same.

For example, consider the following code:

static enum MyEnum {}
static class EnumValue<T extends Enum<T>> {
    Class<T> enumClass;
    EnumValue(Class<T> enumClass) {
        this.enumClass = enumClass;
    }
    Class<T> enumClass() { return enumClass; }
}

These lines will compile:

EnumValue<?> enumValue = new EnumValue(MyEnum.class); // raw constructor
Set<? extends Enum<?>> enumInstances = EnumSet.allOf(enumValue.enumClass());

because we know that the two T in enumValue.enumClass() are the same but this won't:

EnumValue enumValue = new EnumValue(MyEnum.class);
Class<? extends Enum<?>> enumSelected = enumValue.enumClass();
Set<? extends Enum<?>> enumInstances = EnumSet.allOf(enumSelected);

because you have lost information by using a Class<? extends Enum<?>> as an intermediate step.