Object-Oriented Programming: Inheritance - Java: How to Program Early Objects - page 379

For example, line 46 could have called getCommissionRate and getGrossSales to access
CommissionEmployee 's private instance variables commissionRate and grossSales , re-
spectively. Lines 56-58 also could have used appropriate get methods to retrieve the values
of the superclass's instance variables.

9.4.4 CommissionEmployee - BasePlusCommissionEmployee
Inheritance Hierarchy Using protected Instance Variables
To enable class BasePlusCommissionEmployee to directly access superclass instance vari-
ables firstName , lastName , socialSecurityNumber , grossSales and commissionRate ,
we can declare those members as protected in the superclass. As we discussed in
Section 9.3, a superclass's protected members are accessible by all subclasses of that su-
perclass. In the new CommissionEmployee class, we modified only lines 6-10 of Fig. 9.4
to declare the instance variables with the protected access modifier as follows:

protected final String firstName;
protected final String lastName;
protected final String socialSecurityNumber;
protected double grossSales; // gross weekly sales
protected double commissionRate; // commission percentage

The rest of the class declaration (which is not shown here) is identical to that of Fig. 9.4.
We could have declared CommissionEmployee 's instance variables public to enable sub-
class BasePlusCommissionEmployee to access them. However, declaring public instance
variables is poor software engineering because it allows unrestricted access to the these vari-
ables from any class, greatly increasing the chance of errors. With protected instance vari-
ables, the subclass gets access to the instance variables, but classes that are not subclasses and
classes that are not in the same package cannot access these variables directly—recall that
protected class members are also visible to other classes in the same package.

Class BasePlusCommissionEmployee
Class BasePlusCommissionEmployee (Fig. 9.9) extends the new version of class Commis-
sionEmployee with protected instance variables. BasePlusCommissionEmployee objects
inherit CommissionEmployee 's protected instance variables firstName , lastName , so-
cialSecurityNumber , grossSales and commissionRate —all these variables are now pro-
tected members of BasePlusCommissionEmployee . As a result, the compiler does not
generate errors when compiling line 45 of method earnings and lines 54-56 of method
toString . If another class extends this version of class BasePlusCommissionEmployee , the
new subclass also can access the protected members.

1 // Fig. 9.9: BasePlusCommissionEmployee.java

2 // BasePlusCommissionEmployee inherits protected instance

3 // variables from CommissionEmployee.

4
5
6

public class BasePlusCommissionEmployee extends CommissionEmployee

{

7 private double baseSalary; // base salary per week

Fig. 9.9 | BasePlusCommissionEmployee inherits protected instance variables from
CommissionEmployee . (Part 1 of 2.)