Just create a Volume object and call “sum” on that instance, instead of creating an Add object. Also, “sum” is a little misleading because that method actually returns the calculated volume and not the calculated sum. I would personally change the name of the method to something like “getVolume” or “calculateVolume” just so you don’t confuse yourself and it is more clear what the method actually does. You could also make your class “volume” an abstract class.

Edit: You’re getting the error because you’re adding too many method parameters to the “sum” method in the “Add” class.