First of all I suggest you to read more attentively the documentation about CrudRepository method. The answers of your questions is clearly wrote on it. For example, for findAllById(... ids) method, doc says :

If some or all ids are not found, no entities are returned for these IDs.

So, if no ids are found, no entities will be returned and you will got an empty list. Otherwise, if some ids are found, matching entities will be returned and you will got list containing only matching entities. Your method should not have to always return a list whose size is equal to your names list size