Android Room Persistence Library - How to find entities with ids contained in list of ids?

You need parentheses:

@Query("SELECT * FROM objects WHERE obj_id IN (:ids)")
List<Object> queryObjects(List<String> ids);

(and FWIW, I filed an issue to try to get a better error message here)

You can set either list of input or array of input.

and then make a query like this

e.g. String[] ids or List ids

Query = @Query("select * from objects where obj_id in (:ids)")