Create TypeToken for type argument of generic super type

Question

I'm implementing a Gson TypeAdapter for two dependend generic models.

interface ModelA<T> {}
interface ModelB<T> extends ModelA<List<T>> {}

For this I need to get the TypeToken and TypeAdapter. By doing

Type type = ((ParameterizedType) type).getActualTypeArguments()[0];
TypeToken<?> token = TypeToken.get(type);
TypeAdapter<?> adapter = gson.getAdapter(token);

I get a type token for both models of any given type AnyType and the correlating adapter. This is what I need for ModelA, but for ModelB I'd need the adapter for List<AnyType> instead, which is the result of using ModelA<List<AnyType>> directly.

Using the interface of the raw type of the original type token passed

token.getRawType().getGenericInterfaces()[0]

I only seem to get the type erased type of List.

How can I combine these two informations to get the generic List<AnyType> type or create it directly? As input it only has the type token for ModelB<AnyType>.

Example

For the given behavior I implemented a simple test showing the differences. I've taken the expected part from the answer by hahn, using the Gson internal non-API class $Gson$Types.

public class TypeTokenValidate {

    public interface ModelA<T> {}
    public interface ModelB<T> extends ModelA<List<T>> {}

    @Test
    public void genericToken() throws Exception {
        TypeToken<ModelB<String>> token = new TypeToken<ModelB<String>>() {};
        ModelB<String> m = new ModelB<String>() {};
        Type expected = $Gson$Types.resolve(ModelB.class, m.getClass(),
                ModelA.class.getTypeParameters()[0]);
        assertEquals(expected, getActual(token));
    }

    private static Type getActual(TypeToken<?> token) {
        Type type = token.getType();
        //type = token.getRawType().getGenericInterfaces()[0];
        type = ((ParameterizedType) type).getActualTypeArguments()[0];
        return TypeToken.get(type).getType();
    }
}

Answer 1

There is a $Gson$Types that you can utilize to retrieve Type of typed List. For example having:

ModelB<String> m = new ModelB<String>() {};
Type resolve = $Gson$Types.resolve(m.getClass(), m.getClass(), ModelA.class.getTypeParameters()[0]);

would return Type with rawType set to List and with typeArguments of [String]. Thus preserving the types as you need.

and then executing this block

TypeToken<?> token = TypeToken.get(resolve);
TypeAdapter<?> adapter = gson.getAdapter(token);

would result with adaptor of CollectionTypeAdaptorFactory with elementTypeAdapter.type set to class String.