code: public TypeMirror extractElementType(TypeMirror type) {
  		if (!this.env.getTypeUtils().isAssignable(type, this.collectionType)) {
  			return null;
  		}
  		return getCollectionElementType(type);
  	}
docs: Extract the target element type from the specified container type or {@code null}
  if no element type was found.
  @param type a type, potentially wrapping an element type
  @return the element type or {@code null} if no specific type was found