Given the abstract class, RentableDwelling, containing:
a (private) double instance variable, rentPerUnit
a constructor accepting a double used to initialize rentPerUnit
a method, getRentPerUnit that returns the value of rentPerUnit
an abstract method getRentalIncome that returns a double
write a (non-abstract) subclass, ApartmentHouse, containing:
an int instance variable numberOfApartments
a constructor accepting a double, passed to the superclass constructor, and an int used to initialize numberOfApartments
the method getRentalIncome calculated as the product of numberOfApartments and the rent per unit. getRentalIncome returns a double.