Object-Oriented Programming: Inheritance - Java: How to Program Early Objects - page 376

9.4.3 Creating a CommissionEmployee -
BasePlusCommissionEmployee Inheritance Hierarchy
Now we declare class BasePlusCommissionEmployee (Fig. 9.8) to extend class Commis-
sionEmployee (Fig. 9.4). A BasePlusCommissionEmployee object is a CommissionEm-
ployee , because inheritance passes on class CommissionEmployee 's capabilities. Class
BasePlusCommissionEmployee also has instance variable baseSalary (Fig. 9.8, line 6).
Keyword extends (line 4) indicates inheritance. BasePlusCommissionEmployee inherits
CommissionEmployee 's instance variables and methods.

Software Engineering Observation 9.4
At the design stage in an object-oriented system, you'll often find that certain classes are
closely related. You should “factor out” common instance variables and methods and place
them in a superclass. Then use inheritance to develop subclasses, specializing them with
capabilities beyond those inherited from the superclass.

Software Engineering Observation 9.5
Declaring a subclass does not affect its superclass's source code. Inheritance preserves the
integrity of the superclass.

Only CommissionEmployee 's public and protected members are directly accessible
in the subclass. The CommissionEmployee constructor is not inherited. So, the public
BasePlusCommissionEmployee services include its constructor (lines 9-23), public
methods inherited from CommissionEmployee , and methods setBaseSalary (lines 26-
33), getBaseSalary (lines 36-39), earnings (lines 42-47) and toString (lines 50-60).
Methods earnings and toString override the corresponding methods in class Commis-
sionEmployee because their superclass versions do not properly calculate a BasePlusCom-
missionEmployee 's earnings or return an appropriate String representation, respectively.

1 // Fig. 9.8: BasePlusCommissionEmployee.java

2 // private superclass members cannot be accessed in a subclass.

3
4
5

public class BasePlusCommissionEmployee extends CommissionEmployee

{

6 private double baseSalary; // base salary per week

7
8 // six-argument constructor

9 public BasePlusCommissionEmployee(String firstName, String lastName,

10 String socialSecurityNumber, double grossSales,

11 double commissionRate, double baseSalary)

12 {

13 // explicit call to superclass CommissionEmployee constructor

super (firstName, lastName, socialSecurityNumber,
grossSales, commissionRate);

14

15

16
17 // if baseSalary is invalid throw exception

18 if (baseSalary < 0.0 )

19 throw new IllegalArgumentException(

20 "Base salary must be >= 0.0" );

21

Fig. 9.8 | private superclass members cannot be accessed in a subclass. (Part 1 of 3.)