Extract getContainerElementType() method in $Gson$Types

[tlinkowski]

Currently, $Gson$Types class contains the following public method:

public static Type getCollectionElementType(Type context, Class<?> contextRawType)

This method works like a charm for all kind of Collections but it is unnecessarily fixed to work only on Collections, and it cannot be applied to work on other Iterables (e.g. on Streams, or any other type of containers that do not implement Collection).

I suggest extracting from this method a method like:

public static Type getContainerElementType(Type context, Class<?> contextRawType, Class<?> containerSupertype) {
  Type containerType = getSupertype(context, contextRawType, containerSupertype);

  if (containerType instanceof WildcardType) {
    containerType = ((WildcardType) containerType).getUpperBounds()[0];
  }
  if (containerType instanceof ParameterizedType) {
    return ((ParameterizedType) containerType).getActualTypeArguments()[0];
  }
  return Object.class;
}

and rewrite getCollectionElementType as:

public static Type getCollectionElementType(Type context, Class<?> contextRawType) {
  return getContainerElementType(context, contextRawType, Collection.class);
}

PS. I came upon this issue when trying to implement a TypeAdapterFactory for Streams - see https://stackoverflow.com/questions/50881488/is-it-possible-to-pass-a-java-util-stream-to-gson/50895699#50895699

[JakeWharton]

This method isn't for public use though.

[tlinkowski]

I understand - the package with the "internal" keyword (com.google.gson.internal) and the class name with dollar signs ($Gson$Types) clearly tells it's not an API that you want your users to rely on (or even use at all). I guess that if the library were modularized, this entire package wouldn't be exported.

On the other hand, when you look at #1335, you'll see that it's not possible to implement a reliable generic interface-based TypeAdapterFactory without resorting to this class. So maybe you could consider providing this one method (getContainerElementType) in a public API?

Or you could simply provide such public method in $Gson$Types (as I proposed in this issue) which would mean: "This is internal API. It may change anytime without prior notice. Use at your own risk if you must."

[inder123]

@tlinkowski We don't want you to use an internal class as a binary dependency. Please copy it over and change it to suit your needs.

[tlinkowski]

Fair enough.

I wonder, however, what you are going to do about the Multiset example in TypeAdapterFactorys JavaDoc (#1335) because I cannot see a way to make it work without resorting to $Gson$Types.getCollectionElementType.