Generics and Class<? extends Enum<?>>, En

问题:

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

  @EnumValue(enumClass = MyTestEnum.class)
  private String field;

  public enum MyTestEnum {
    VAL1, VAL2;
  }

Will be validated only if the field value is "VAL1" or "VAL2".

public class EnumNameValidator implements ConstraintValidator<EnumValue, String> {

  private Set<String> AVAILABLE_ENUM_NAMES;

  @Override
  public void initialize(EnumValue enumValue) {
    Class<? extends Enum<?>> enumSelected = enumValue.enumClass();
    Set<? extends Enum<?>> enumInstances = Sets.newHashSet(enumSelected.getEnumConstants());
    AVAILABLE_ENUM_NAMES = FluentIterable
            .from(enumInstances)
            .transform(PrimitiveGuavaFunctions.ENUM_TO_NAME)
            .toImmutableSet();
  }

  @Override
  public boolean isValid(String value, ConstraintValidatorContext context) {
    if ( value == null ) {
      return true;
    } else {
      return AVAILABLE_ENUM_NAMES.contains(value);
    }
  }

}

What I don't understand is why my first attempt failed. Using instead of the enumSelected.getEnumConstants() above the following code:

Set<? extends Enum<?>> enumInstances = EnumSet.allOf(enumSelected);

Intellij 12 doesn't highlight any error, but the compiler says:

java: method allOf in class java.util.EnumSet<E> cannot be applied to given types;
  required: java.lang.Class<E>
  found: java.lang.Class<capture#1 of ? extends java.lang.Enum<?>>
  reason: inferred type does not conform to declared bound(s)
    inferred: capture#1 of ? extends java.lang.Enum<?>
    bound(s): java.lang.Enum<capture#1 of ? extends java.lang.Enum<?>>

---

I don't understand the problem, and I also have that code which works fine:

  private static <T extends Enum<T> & EnumAlternativeName> T safeGetByAlternativeName(Class<T> enumClass, String alternativeName) {
    for ( T t : EnumSet.allOf(enumClass) ) {
      if ( t.getAlternativeName().equals(alternativeName) ) {
        return t;
      }
    }
    return null;
  }

回答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.

回答2:

My explanation on @assylias's solution:

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

Class<E>, for some E, that E <: Enum<E>

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<T extends b(T)> { ... }  // b(T) is a type expression that may contain T

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

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

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

class EnumClass<E extends Enum<E>>   // called EnumValue in assylias's solution

    EnumClass(Class<E> enumClass) 

    Class<E> enumClass()

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

We then call EnumClass<E>.enumClass() to yield a

Class<E>, for some E, that E <: Enum<E>

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<not-proper> 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()