Object Oriented Object Oriented Programming: InheritanceProgramming: Inheritance

Chapter 9Chapter 9

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What You Will LearnWhat You Will Learn

Software reusabilitySoftware reusability(Recycling)(Recycling)

Inheriting data members and methods from Inheriting data members and methods from previously defined classespreviously defined classes

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IntroductionIntroduction

Software ReusabilitySoftware Reusability saves time in program developmentsaves time in program development encourages use of proven, debugged codeencourages use of proven, debugged code reduces problemsreduces problems

Write programs in general fashionWrite programs in general fashion Enables software designers to deal with Enables software designers to deal with

complexity of modern softwarecomplexity of modern software

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IntroductionIntroduction

When creating a new class …When creating a new class … designate that class to inherit data members, designate that class to inherit data members,

functions of previously defined functions of previously defined superclasssuperclass result is a result is a subclasssubclass

Subclass adds new data members and Subclass adds new data members and functionsfunctions

Replace and refine existing membersReplace and refine existing members

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Base Classes & Derived Base Classes & Derived ClassesClasses

Superclass is more generalSuperclass is more general student, shape, loanstudent, shape, loan

Subclass is more specificSubclass is more specific grad student, undergradgrad student, undergrad circle, triangle, rectanglecircle, triangle, rectangle carloan, home improvement, mortgagecarloan, home improvement, mortgage

Some languages talk ofSome languages talk of Base class (Superclass)Base class (Superclass) Derived class (Subclass)Derived class (Subclass)

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Superclass and SubclassSuperclass and Subclass

Inheritance produces tree like structuresInheritance produces tree like structures

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Superclass and SubclassSuperclass and Subclass

Inheritance produces tree like structuresInheritance produces tree like structures

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Design TipDesign Tip

Important link between subclass and Important link between subclass and superclasssuperclass The “IS-A” relationshipThe “IS-A” relationship

ExamplesExamples A checking account IS-A banking accountA checking account IS-A banking account A savings account IS NOT a checking accountA savings account IS NOT a checking account

If there is no IS-A relationship, do not If there is no IS-A relationship, do not use inheritanceuse inheritance

Sun comments on the concept of inheritance

Sun comments on the concept of inheritance

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protectedprotected Members Members

protectedprotected access access Intermediate level of protection between Intermediate level of protection between publicpublic

and and privateprivate protectedprotected members accessible by members accessible by

superclass memberssuperclass members subclass memberssubclass members Class members in the same packageClass members in the same package

Subclass access to superclass memberSubclass access to superclass member Keyword Keyword supersuper and a dot (.) and a dot (.)

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Comments on Private vs. ProtectedComments on Private vs. Protected

Use Use protectedprotected when when Superclass should provide a service only to its Superclass should provide a service only to its

subclassessubclasses Should not provide service to other clientsShould not provide service to other clients

Use Use privateprivate so that so that Superclass implementation can change without affecting Superclass implementation can change without affecting

subclass implementationssubclass implementations

Author advocates avoiding Author advocates avoiding protectedprotected Instead provide set and get methods to access Instead provide set and get methods to access privateprivate

data items (see data items (see Figures 9.12Figures 9.12, , 9.139.13 in text) in text)

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Relationship between Superclasses Relationship between Superclasses and Subclassesand Subclasses

Superclass and subclass relationshipSuperclass and subclass relationship Example: Example: CommissionEmployee/CommissionEmployee/BasePlusCommissionEmployeeBasePlusCommissionEmployee inheritance inheritance hierarchyhierarchy CommissionEmployeeCommissionEmployee

First name, last name, SSN, commission rate, gross sale First name, last name, SSN, commission rate, gross sale amountamount

BasePlusCommissionEmployeeBasePlusCommissionEmployee First name, last name, SSN, commission rate, gross sale First name, last name, SSN, commission rate, gross sale

amountamount Base salaryBase salary

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Creating and Using a Creating and Using a CommissionEmployeeCommissionEmployee Class Class

Class Class CommissionEmployeeCommissionEmployee Extends class Extends class ObjectObject

Keyword Keyword extendsextends Every class in Java extends an existing classEvery class in Java extends an existing class

Except Except ObjectObject

Every class inherits Every class inherits ObjectObject’s methods’s methods New class implicitly extends ObjectNew class implicitly extends Object

If it does not extend another classIf it does not extend another class

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Creating and Using a Creating and Using a CommissionEmployeeCommissionEmployee Class Class

Class Class CommissionEmployeeCommissionEmployee Extends class Extends class ObjectObject

Keyword Keyword extendsextends Every class in Java extends an existing classEvery class in Java extends an existing class

Except Except ObjectObject

Every class inherits Every class inherits ObjectObject’s methods’s methods New class implicitly extends ObjectNew class implicitly extends Object

If it does not extend another classIf it does not extend another class

View class View class Figure 9.4Figure 9.4View test program, View test program, Figure 9.5Figure 9.5

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Creating a Creating a BasePlusCommissionEmployeeBasePlusCommissionEmployee

Class Class withoutwithout Using Inheritance Using InheritanceClass Class BasePlusCommissionEmployeeBasePlusCommissionEmployee

Implicitly extends Implicitly extends ObjectObject Much of the code is similar to Much of the code is similar to CommissionEmployeeCommissionEmployee privateprivate instance variables instance variables publicpublic methods methods constructorconstructor

AdditionsAdditions privateprivate instance variable instance variable baseSalarybaseSalary Methods Methods setBaseSalarysetBaseSalary and and getBaseSalarygetBaseSalary

View classFigure 9.6View classFigure 9.6

View test program

Figure 9.7

View test program

Figure 9.7

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Creating a Creating a CommissionEmployee-CommissionEmployee-BasePlusCommiionEmployeeBasePlusCommiionEmployee Inheritance Inheritance

HierarchyHierarchy

Class Class BasePlusCommissionEmployee2BasePlusCommissionEmployee2 Extends class Extends class CommissionEmployeeCommissionEmployee Is a Is a CommissionEmployeeCommissionEmployee Has instance variable Has instance variable baseSalarybaseSalary Inherits Inherits publicpublic and and protectedprotected members members Constructor not inheritedConstructor not inherited

Note these points plus Note these points plus invalidinvalid references, references, Figure 9.8 Figure 9.8 – note compiler – note compiler error messageserror messages

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Using Using protectedprotected Instance Variables Instance Variables

Use Use protectedprotected instance variables instance variables Enable class Enable class BasePlusCommissionEmployeeBasePlusCommissionEmployee

to directly access superclass instance variablesto directly access superclass instance variables Superclass’s Superclass’s protectedprotected members are inherited members are inherited

by all subclases of that superclassby all subclases of that superclass

View version that now works, View version that now works, Figure 9.10Figure 9.10Test program, Test program, Figure 9.11Figure 9.11

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Using Using protectedprotected Instance Variables Instance VariablesAdvantagesAdvantages

SubclassesSubclasses can modify values directlycan modify values directlySlight increase in performanceSlight increase in performance

Avoid set/get methodAvoid set/get method call overhead call overhead

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Using Using protectedprotected Instance Variables Instance VariablesDisadvantagesDisadvantages

No validity checkingNo validity checking subclass can assign illegal valuesubclass can assign illegal value

Implementation dependentImplementation dependent subclass methods more likely dependent on subclass methods more likely dependent on

superclass implementationsuperclass implementation superclass implementation changes may result superclass implementation changes may result

in subclass modificationsin subclass modifications Fragile (brittle) softwareFragile (brittle) software

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Reexamine HierarchyReexamine Hierarchy

Use the best software engineering practiceUse the best software engineering practice Declare instance variables as Declare instance variables as privateprivate Provide public Provide public getget and and setset methods methods Use Use getget method to obtain values of instance method to obtain values of instance

variablesvariablesView new version of CommissionEmployee, View new version of CommissionEmployee,

Figure 9.12Figure 9.12New version of New version of

BasePlusCommissiionEmployee, Figure 9.13BasePlusCommissiionEmployee, Figure 9.13Test program, Figure 9.14Test program, Figure 9.14

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Instantiating Subclass ObjectInstantiating Subclass ObjectChain Of Constructor CallsChain Of Constructor Calls

Subclass constructor invokes superclass Subclass constructor invokes superclass constructorconstructor Implicitly or explicitlyImplicitly or explicitly

Base of inheritance hierarchyBase of inheritance hierarchy Last constructor called in chain is Last constructor called in chain is ObjectObject’s constructor’s constructor Original subclass constructor’s body finishes executing Original subclass constructor’s body finishes executing

lastlast

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Instantiating Subclass ObjectInstantiating Subclass ObjectChain Of Constructor CallsChain Of Constructor Calls

Example, Figure 9.15 : Example, Figure 9.15 : CommissionEmployee3-CommissionEmployee3-BasePlusCommissionEmployee4BasePlusCommissionEmployee4 hierarchy hierarchy CommissionEmployee3CommissionEmployee3 constructor called second last constructor called second last

(last is (last is Object Object constructor)constructor) CommissionEmployee3CommissionEmployee3 constructor’s body finishes constructor’s body finishes

execution second (first is execution second (first is ObjectObject constructor’s body) constructor’s body)

Figure 9.16, BasePlusCommissionEmploye5Figure 9.16, BasePlusCommissionEmploye5Figure 9.17, Test program, demonstrates Figure 9.17, Test program, demonstrates

constructor callsconstructor calls

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Software Engineering with Software Engineering with InheritanceInheritance

At the design stage, certain classes found to At the design stage, certain classes found to be closely relatedbe closely related Factor out common attributes, behaviorsFactor out common attributes, behaviors Place these in a superclassPlace these in a superclass Use inheritance to develop subclasses with Use inheritance to develop subclasses with

inherited capabilitiesinherited capabilities This avoids proliferation of classes, This avoids proliferation of classes,

"reinventing the wheel""reinventing the wheel" NoteNote

Declaring a subclass does not affect superclass Declaring a subclass does not affect superclass source codesource code