Inheritance - Java - page 449

Self-Test Exercises
14. Redefine the toString method of the class SalariedEmployee (Display 7.5) so
that it uses super.toString() . This new definition of toString will be equiva-
lent to the one given in Display 7.5.
15. Redefine the equals method for the class HourlyEmployee (Display 7.3) using
super.equals to invoke the equals method of the base class Employee .
16. Is the following program legal? The relevant classes are defined in Displays 7.2
and 7.3.

public class EmployeeDemo
{
public static void main( String[] args)
{
HourlyEmployee joe =
new HourlyEmployee("Joe Young",
new Date("Feb", 1, 2004), 10.50, 40);
String nameNDate = joe. super .toString();
System.out.println(nameNDate);
}
}

17. Suppose you add the following defined constant to the class Employee (Display 7.2):

public static final int STANDARD_HOURS = 160; //per month

Would it then be legal to add the following method to the class HourlyEmployee
(Display 7.3)?

public void setHoursToStandard()
{
hours = STANDARD_HOURS;
}

The Class Object
Java has a class that is an ancestor of every class. In Java, every class is a derived class of a
derived class of . . . (for some number of iterations of “a derived class of ”) of the class
Object . So, every object of every class is of type Object , as well as being of the type of its
class (and also of the types of all its ancestor classes). Even classes that you define yourself
are descendent classes of the class Object . If you do not make your class a derived class of
some class, then Java will automatically make it a derived class of the class Object .
The class Object allows you to write Java code for methods with a parameter of
type Object that can be replaced by an object of any class whatsoever. You will eventu-
ally encounter library methods that accept an argument of type Object and hence can
be used with an argument that is an object of absolutely any class.

Object
class