Make `GenericType<T>` not implement `Type`.

In the same vein as #73, probably GenericType should not implement java.lang.reflect.Type. It's not really a "type" itself—it's a type holder.

I can appreciated that the original conception was to have GenericType be a "type" representing a "type with generics" (sort of like an improved ParameterizedType). But in the larger scheme of things and in hindsight, it's really just one of many duplicates of the Super Type Token pattern. And to put this in context:

The original example in Neil Gafter's famous Super Type Tokens does not implement Type.
Jackson's TypeReference does not implement Type.
Spring's ParameterizedTypeReference does not implement Type.
The Guice TypeLiteral does not implement Type.
The GeantyRef fork of gentyref has a TypeToken which does not implement Type.
Guava's TypeToken does not implement Type.

Thus ClassMate's GenericType<T> is the only one that implements java.lang.reflect.Type.

The reason I bring this up is that the whole Type system (as you explain so well) is already confusing and awkward; adding one other wrapper that itself pretends to be a Type, in contrast with all the other uses of this pattern, adds even more confusion.

Let me give an illustration of the confusion it can cause. Let's say (following the discussion in #69) that I want to make a general method to convert some "type token" to a Type (so that I can then use Jackson to convert it to a JavaType, etc.). Look closely at this code:

public static Type typeTokenToType(@Nonnull final Object typeToken) {
  //super type token (check first, because ClassMate `GenericType<T>` is also a `Type`)
  final Type superTypeTokenSuperClass = typeToken.getClass().getGenericSuperclass();
  if(superTypeTokenSuperClass instanceof ParameterizedType parameterizedType) {
    final Type[] actualTypeArguments = parameterizedType.getActualTypeArguments();
    if(actualTypeArguments.length == 1) {
      return actualTypeArguments[0];
    }
  }
  //type
  if(typeToken instanceof Type) { //types in general, if they are not super type tokens needing "unwrapping", can be returned directly  
    return (Type)typeToken;
  }
  throw new IllegalArgumentException("Type token must be an instance of `Class`, or have a super class providing a single generic type argument.");
}

You see the potential bug if the developer weren't paying attention? In a perfect world, I would first test to see if typeToken is a Type, and just return it, because the type token is already a Type (e.g. a Class<?> or a ParameterizedType). Otherwise I would see if it is a "super type token". And that approach would work with all the other super type tokens—except for ClassMate's GenericType<T>, because GenericType<T> claims it is a Type itself already!

The fix for this example is easy (as shown in the method above): just leave the instanceof Type check for last. But it would be more efficient to test for Type up front. More worrisome, I would have had to have noticed before writing the method (I actually didn't at first) that GenericType<T> implements Type, or I would have wound up with such a bug.

Lots of other bugs may crop up. A developer may write a doFoo(Type type) method, assuming the type has already been "unwrapped", and someone could send it a GenericType<T>, forgetting to unwrap/extract the Type first. I could see this happening all over the place.

At the end of the day this isn't a blocker, and it's certainly not a critical bug. And if you don't agree with me, and you see value in having GenericType<T> implement Type, that's fine. Nevertheless I thought I'd document what I see as an issue so that you can think about it. Cheers!

FasterXML / java-classmate

Make `GenericType<T>` not implement `Type`. #76