Object-Oriented Programming: Polymorphism and Interfaces - Java: How to Program Early Objects - page 407

34 return socialSecurityNumber;

35 }

36
37

// return String representation of Employee object

38

@Override

39

public String toString()

40

{

41

return String.format( "%s %s%nsocial security number: %s" ,

42

getFirstName(), getLastName(), getSocialSecurityNumber());

43

}

44
45

// abstract method must be overridden by concrete subclasses
public abstract double earnings(); // no implementation here

46

47

} // end abstract class Employee

Fig. 10.4 | Employee abstract superclass. (Part 2 of 2.)

Why did we decide to declare earnings as an abstract method? It simply does not
make sense to provide a specific implementation of this method in class Employee . We
cannot calculate the earnings for a general Employee —we first must know the specific type
of Employee to determine the appropriate earnings calculation. By declaring this method
abstract , we indicate that each concrete subclass must provide an appropriate earnings
implementation and that a program will be able to use superclass Employee variables to
invoke method earnings polymorphically for any type of Employee .

10.5.2 Concrete Subclass SalariedEmployee
Class SalariedEmployee (Fig. 10.5) extends class Employee (line 4) and overrides abstract
method earnings (lines 38-42), which makes SalariedEmployee a concrete class. The class
includes a constructor (lines 9-19) that receives a first name, a last name, a social security
number and a weekly salary; a set method to assign a new nonnegative value to instance vari-
able weeklySalary (lines 22-29); a get method to return weeklySalary 's value (lines 32-
35); a method earnings (lines 38-42) to calculate a SalariedEmployee 's earnings; and a
method toString (lines 45-50), which returns a String including "salaried employee: "
followed by employee-specific information produced by superclass Employee 's toString
method and Salaried-Employee 's getWeeklySalary method. Class SalariedEmployee 's
constructor passes the first name, last name and social security number to the Employee con-
structor (line 12) to initialize the private instance variables of the superclass. Once again,
we've duplicated the weeklySalary validation code in the constructor and the setWeekly-
Salary method. Recall that more complex validation could be placed in a static class meth-
od that's called from the constructor and the set method.

Error-Prevention Tip 10.1
We've said that you should not call a class's instance methods from its constructors—you
can call static class methods and make the required call to one of the superclass's con-
structors. If you follow this advice, you'll avoid the problem of calling the class's overrid-
able methods either directly or indirectly, which can lead to runtime errors.

Method earnings overrides Employee 's abstract method earnings to provide a con-
crete implementation that returns the SalariedEmployee 's weekly salary. If we do not