convert or cast output of JpaRepository

I have three interfaces that extend JpaRepository and a custom interface that has two functions. These two functions have their special output in each of three interfaces.
I implement it with List in the custom interface, so I have to use same output in the three interfaces, but I need to convert them to their special type.
Here is the custom interface:

public interface DataRepo {

  List<Object> findAllEmptyData();
  List<Object> findAllByCreatedAtBefore(String createdAt);
}

And here are the three interfaces:

public interface bTableRepo extends JpaRepository<bTable, Integer>, DataRepo {

  @Query("SELECT b FROM bTable b WHERE b.value is null AND b.data")
  List<Object> findAllEmptyData();

  @Query("SELECT b FROM bTable b WHERE b.createdAt < ?1")
  List<Object> findAllByCreatedAtBefore(LocalDate createdAt); //I need to convert List<Object> to List<bTable>
}

public interface cTableRepo extends JpaRepository<cTable, Integer>, DataRepo {

  @Query("SELECT c FROM cTable c WHERE c.value is null AND c.dData is null AND c.date is null")
  List<Object> findAllEmptyData();

  @Query("SELECT c FROM cTable c WHERE c.createdAt < ?1")
  List<Object> findAllByCreatedAtBefore(LocalDate createdAt); //I need to convert List<Object> to List<cTable>
}

public interface dTableRepo extends JpaRepository<dTable, Integer>, DataRepo {

  @Query("SELECT d FROM dTable d WHERE d.value is null AND d.date is null AND d.DateAndTime is null")
  List<Object> findAllEmptyData();

  @Query("SELECT d FROM cTable d WHERE d.createdAt < ?1")
  List<Object> findAllByCreatedAtBefore(LocalDate createdAt); //I need to convert List<Object> to List<dTable>
}

>Solution :

Make your custom repo generic:

public interface DataRepo<T> {

  List<T> findAllEmptyData();
  List<T> findAllByCreatedAtBefore(String createdAt);
}

And then use it for example with bTable:

public interface bTableRepo extends JpaRepository<bTable, Integer>, DataRepo<bTable> {