Object Oriented Programming: Inheritance

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 • 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. 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 • First name, last name, SSN, commission rate, gross sale amount • 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 classFigure 9.6 View test programFigure 9.7

15. 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 VariablesAdvantages • Subclassescan modify values directly • Slight increase in performance • Avoid set/get method call overhead

18. Using protected Instance VariablesDisadvantages • 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