Hello Reza,
I extended the DBMain interface with another interface with methods that I found were missing, such as another find method. My Data.java class implemented this interface (and so indirectly the DBMain interface).
I don't understand why others are so worried about this design.
There is nothing in the instructions against it, I passed with this solution, and the
Java API is full of such solutions:
LayoutManager2 -> LayoutManager
List -> Collection
SortedSet -> Set -> Collection
It's plain O-O.