static int idFrom(Class<?> clazz)
    {
        final Integer id = __nonPublicMaps.get(clazz);
        if (id == null)
            throw new RuntimeException("Unknown map: " + clazz);

        return id.intValue();
    }
Used when the type is an interface (Map/SortedMap).

@author David Yu
@created Apr 24, 2012
protostuff-runtime/src/main/java/io/protostuff/runtime/PolymorphicMapSchema.java