1. Object Oriented Programming: Inheritance
Chapter 9

2. What You Will Learn Software reusability (Recycling)
Inheriting data members and methods from previously defined classes

3. Introduction Software Reusability Write programs in general fashion
saves time in program development
encourages use of proven, debugged code
reduces problems
Write programs in general fashion
Enables software designers to deal with complexity of modern software

4. Introduction When creating a new class …
designate that class to inherit data members, functions of previously defined superclass
result is a subclass
Subclass adds new data members and functions
Replace and refine existing members

5. Base Classes & Derived Classes
Superclass is more general
student, shape, loan
Subclass is more specific
grad student, undergrad
circle, triangle, rectangle
carloan, home improvement, mortgage
Some languages talk of
Base class (Superclass)
Derived class (Subclass)

6. Superclass and Subclass
Inheritance produces tree like structures

7. Superclass and Subclass
Inheritance produces tree like structures

8. Sun comments on the concept of inheritance
Design Tip
Important link between subclass and superclass
The “IS-A” relationship
Examples
A checking account IS-A banking account
A savings account IS NOT a checking account
If there is no IS-A relationship, do not use inheritance
Sun comments on the concept of inheritance

9. protected Members protected access
Intermediate level of protection between public and private
protected members accessible by
superclass members
subclass members
Class members in the same package
Subclass access to superclass member
Keyword super and a dot (.)

10. Comments on Private vs. Protected
Use protected when
Superclass should provide a service only to its subclasses
Should not provide service to other clients
Use private so that
Superclass implementation can change without affecting subclass implementations
Author advocates avoiding protected
Instead provide set and get methods to access private data items (see Figures 9.12, 9.13 in text)

11. Relationship between Superclasses and Subclasses
Superclass and subclass relationship
Example: CommissionEmployee/ BasePlusCommissionEmployee inheritance hierarchy
CommissionEmployee
First name, last name, SSN, commission rate, gross sale amount
BasePlusCommissionEmployee
Base salary

12. Creating and Using a CommissionEmployee Class
Class CommissionEmployee
Extends class Object
Keyword extends
Every class in Java extends an existing class
Except Object
Every class inherits Object’s methods
New class implicitly extends Object
If it does not extend another class

13. Creating and Using a CommissionEmployee Class
Class CommissionEmployee
Extends class Object
Keyword extends
Every class in Java extends an existing class
Except Object
Every class inherits Object’s methods
New class implicitly extends Object
If it does not extend another class
View class Figure 9.4
View test program, Figure 9.5

14. Creating a BasePlusCommissionEmployee Class without Using Inheritance
Class BasePlusCommissionEmployee
Implicitly extends Object
Much of the code is similar to CommissionEmployee
private instance variables
public methods
constructor
Additions
private instance variable baseSalary
Methods setBaseSalary and getBaseSalary
View class Figure 9.6
View test program Figure 9.7

15. Class BasePlusCommissionEmployee2
Creating a CommissionEmployee-BasePlusCommiionEmployee Inheritance Hierarchy
Class BasePlusCommissionEmployee2
Extends class CommissionEmployee
Is a CommissionEmployee
Has instance variable baseSalary
Inherits public and protected members
Constructor not inherited
Note these points plus invalid references, Figure 9.8 – note compiler error messages

16. Using protected Instance Variables
Use protected instance variables
Enable class BasePlusCommissionEmployee to directly access superclass instance variables
Superclass’s protected members are inherited by all subclases of that superclass
View version that now works, Figure 9.10
Test program, Figure 9.11

17. Using protected Instance Variables Advantages
Subclasses can modify values directly
Slight increase in performance
Avoid set/get method call overhead

18. Using protected Instance Variables Disadvantages
No validity checking
subclass can assign illegal value
Implementation dependent
subclass methods more likely dependent on superclass implementation
superclass implementation changes may result in subclass modifications
Fragile (brittle) software

19. Reexamine Hierarchy Use the best software engineering practice
Declare instance variables as private
Provide public get and set methods
Use get method to obtain values of instance variables
View new version of CommissionEmployee, Figure 9.12
New version of BasePlusCommissiionEmployee, Figure 9.13
Test program, Figure 9.14

20. Instantiating Subclass Object
Chain Of Constructor Calls
Subclass constructor invokes superclass constructor
Implicitly or explicitly
Base of inheritance hierarchy
Last constructor called in chain is Object’s constructor
Original subclass constructor’s body finishes executing last

21. Instantiating Subclass Object
Chain Of Constructor Calls
Example, Figure 9.15 : CommissionEmployee3-BasePlusCommissionEmployee4 hierarchy
CommissionEmployee3 constructor called second last (last is Object constructor)
CommissionEmployee3 constructor’s body finishes execution second (first is Object constructor’s body)
Figure 9.16, BasePlusCommissionEmploye5
Figure 9.17, Test program, demonstrates constructor calls

22. Software Engineering with Inheritance
At the design stage, certain classes found to be closely related
Factor out common attributes, behaviors
Place these in a superclass
Use inheritance to develop subclasses with inherited capabilities
This avoids proliferation of classes, "reinventing the wheel"
Note
Declaring a subclass does not affect superclass source code