Inheritance - Absolute Java - page 446

TIP: (continued)

The line

super (originalObject);

is an invocation of a constructor for the base class Employee . The class Employee has
no constructor with a parameter of type HourlyEmployee , but originalObject is of
type HourlyEmployee . Fortunately, every object of type HourlyEmployee is also of
type Employee . So, this invocation of super is an invocation of the copy constructor
for the class Employee .
The fact that every object is not only of its own type but is also of the type of
its ancestor classes simply refl ects what happens in the everyday world. An hourly
employee is an employee as well as an hourly employee. This sometimes is referred to
as the “is a” relationship : For example, an HourlyEmployee is an Employee .
Display 7.6 contains a program demonstrating that an HourlyEmployee and a
SalariedEmployee are also Employee objects. The method showEmployee requires
an argument of type Employee . The objects joe and sam are of type Employee because
they are instances of classes derived from the class Employee and so are suitable argu-
ments for showEmployee .

“is a”
relationship

■

An Object of a Derived Class Has More than One Type

An object of a derived class has the type of the derived class, and it also has the type of the
base class. More generally, a derived class has the type of every one of its ancestor classes.
So, you can assign an object of a derived class to a variable of any ancestor type (but not
the other way around). You can plug in a derived class object for a parameter of any of its
ancestor types. More generally, you can use a derived class object anyplace you can use an
object of any of its ancestor types.