Lecture 5 OOP-Inheritance

7/31/2019 Lecture 5 OOP-Inheritance

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Inheritance


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Inheritance: Introduction

Inheritance, is a form of softwarereuse in which a new class is createdby absorbing an existing class's

members and embellishing themwith new or modified capabilities.

With inheritance, programmers save

time during program development byreusing proven and debugged high-quality software. This also increases

the likelihood that a system will be


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u c ass, recsuperclass and indirect

superclass A subclass normally adds its own fields and

methods.

is more specific than its superclass andrepresents a more specialized group ofobjects.

In short, it exhibits the behaviors of itssuperclass and other behaviors specificto the subclass.

The direct superclass is the


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Single and multipleinheritance

In single inheritance a class isderived from one direct superclass

Analogy: Cell division (asexualreproduction) one parent

Java supports only single inheritance

In multiple inheritance a class isderived from more than one directsuperclass.

Analogy: sexual reproduction twoparents (or multiple parents for


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Experience in buildingsoftware systems indicates

that significant amounts ofcode deal with closely-relatedspecial cases. When

programmers are pre-occupied with special cases,the details can obscure the

big picture.


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"is-a" versus "has-a"relationship

We distinguish between the "is-a"relationship and the "has-a"relationship.

"Is-a" represents inheritance whereby,an object of a subclass can also betreated as an object of its superclass.

For example, a car is a vehicle. "has-a" represents composition

whereby, an object contains one ormore object references as members. For

example, a car has a steering wheel


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Inheritance Examples: inGeometry

Often, an object of one class "is an"object of another class as well. Forexample, in geometry,

a rectangle is a quadrilateral

class Rectangle can be said to inherit fromclass Quadrilateral.

class Quadrilateral is a superclass and

class Rectangle is a subclass.

A rectangle is a specific type of

quadrilateral, but it is incorrect to claim thatevery quadrilateral is a rectangle. the


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Inheritance: OtherExamples

Superclass Subclasses

Student PhDStudent, GraduateStudent,UndergraduateStudent

Shape Circle, Triangle, Rectangle

Loan CarLoan, HomeImprovementLoan,

MortgageLoan

Employee Faculty, Staff

BankAccount CheckingAccount, SavingsAccount


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Because every subclass object "isan" object of its superclass, and onesuperclass can have many

subclasses, the set of objectsrepresented by a superclass istypically larger than the set of

objects represented by any of itssubclasses.

For example, the superclass Vehiclerepresents all vehicles, including cars,trucks, boats, bicycles and so on.


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Inheritance hierarchy: universityCommunityMembers

A university community hasthousands of members, includingemployees, students and alumni.

Employees are either faculty membersor staff members.

Faculty members are either

administrators (such as deans anddepartment chairpersons) or teachers.Note that the hierarchy could containmany other classes. For example,

students can be graduate or


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Each arrow in the hierarchyrepresents an "is-a" relationship. Aswe follow the arrows in this class

hierarchy, we can state, for instance,that

"an Employee is a CommunityMember"

and "a Teacher is a Faculty member."

CommunityMember is the direct

superclass of Employee, Student and


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Inheritance hierarchy:for Shapes


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Has a Relationship

In "has-a" relationship, classes havemembers that are references toobjects of other classes. Such

relationships create classes bycomposition of existing classes. Forexample, given the classes

Employee, BirthDate andTelephoneNumber,

it is improper to say that an Employee isa BirthDate or that an Employee is a

TelephoneNumber.


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Problems of Inheritance

A subclass can inherit methods thatit does not need or should not have.

Even when a superclass method isappropriate for a subclass, thatsubclass often needs a customizedversion of the method. In such cases,

the subclass can override(redefine) the superclass methodwith an appropriate implementation.


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Public versus privatemembers

A class's public members areaccessible wherever the program hasa reference to an object of that class

or one of its subclasses. A class's private members are

accessible only from within the class

itself. A superclass's private members are not

inherited by its subclasses.


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protected Members

Using protected access offers anintermediate level of access betweenpublic and private.

A superclass's protected members can beaccessed by members of thatsuperclass, by members of its

subclasses and by members ofotherclasses in the same package (i.e.,protected members also have packageaccess).

All public and protected superclass


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Software Engineering Observation

Methods of a subclass cannot directlyaccess private members of their

superclass. A subclass can change thestate of private superclass instancevariables only through non-private(public, protected) methods providedin the superclass and inherited by thesubclass.

Declaring private instance variables

helps programmers test, debug and


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e a ons p e weenSuperclasses and

Subclasses Consider types of employees in acompany's payroll application todiscuss the relationship between a

superclass and its subclass. commission employees (who will be

represented as objects of a superclass)

are paid a percentage of their sales base-salaried commission

employees (who will be represented asobjects of a subclass) receive a basesalary plus a percentage of their sales.


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ClassCommissionEmployee

1st example: class CommissionEmployee directly inherits fromclass Object and declares as private instance variables a first name,last name, social security number, commission rate and gross (i.e.,total) sales amount.


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Creating aBasePlusCommissionEmployee Class

without Using Inheritance 2nd example: class BasePlusCommissionEmployee directly

inherits from class Object and declares as private instance variablesa first name, last name, social security number, commission rate,gross sales amount and base salary.

This class BasePlusCommissionEmployee is created by writing every line of

code the class requires.

it is more efficient to create this class by inheriting from classCommissionEmployee.

This example declares and tests (a completely new andindependent) class BasePlusCommissionEmployee, which contains a

first name, last name, social security number, gross sales amount,commission rate and base salary. ClassBasePlusCommissionEmployee's public services include aBasePlusCommissionEmployee constructor and methods earningsand toString. declare public get and set methods for the class'sprivate instance variables firstName, lastName,

socialSecurityNumber, grossSales, commissionRate and baseSalary.These variables and methods encapsulate all the necessary


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Copying and pasting code from oneclass to another can spread errors

across multiple source code files. Toavoid duplicating code (and possiblyerrors), use inheritance, rather than the"copy-and-paste" approach, in situations

where you want one class to "absorb"the instance variables and methods ofanother class.

With inheritance, the common instance


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Creating a CommissionEmployeeBasePlusCommissionEmployee

Inheritance Hierarchy

3rd example: declares a separateBasePlusCommissionEmployee2class that extends class

CommissionEmployee (i.e., aBasePlusCommissionEmployee2 is aCommissionEmployee who also has abase salary) and attempts to access

class CommissionEmployee's privatemembers. This results in compilationerrors, because the subclass cannotaccess the superclass's private instance

variables.


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Each subclass constructor mustimplicitly or explicitly call its

superclass constructor to ensure thatthe instance variables inherited fromthe superclass are initialized properly.

BasePlusCommissionEmployee2's six-argument constructor explicitly callsclass CommissionEmployee's five-argument constructor to initialize the

superclass portion of aBasePlusCommissionEmployee2 object.

BasePlusCommissionEmployee2's six-argument constructor invokes theCommissionEmployee's five-argument


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CommissionEmployeeBasePlusCommissionEmployee Inheritance HierarchyUsing protected Instance Variables

4th example: we show that ifCommissionEmployee's instancevariables are declared as protected,

aBasePlusCommissionEmployee3class that extends class

CommissionEmployee2 can accessthat data directly.

declare class CommissionEmployee2with protected instance variables.

omm ss on mp oyee ase us o


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omm ss on mp oyee ase us ommissionEmployee Inheritance

Hierarchy Using protected

Instance Variables A superclass's protected members areinherited by all subclasses of thatsuperclass.

Class CommissionEmployee2 is amodification of classCommissionEmployee that declaresinstance variables firstName, lastName,

socialSecurityNumber, grossSales andcommissionRate as protected ratherthan private.


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ClassBasePlusCommissionEmployee3 doesnot inherit class

CommissionEmployee2's constructor. However, class

BasePlusCommissionEmployee3's

six-argument constructor calls classCommissionEmployee2's five-argument constructor explicitly.

BasePlusCommissionEmployee3's six-


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STOP HERE


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In this example, we declaredsuperclass instance variables asprotected so that subclasses could

inherit them. Inheriting protected instance

variables slightly increases

performance, because we candirectly access the variables in thesubclass without incurring the

overhead of a set or get method call.


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The Java compiler sets the superclass ofa class to Object when the class

declaration does not explicitly extend asuperclass.

Common Programming Error

It is a syntax error to override a methodwith a more restricted access modifier.

A public method of the superclass

cannot become a protected or private


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Programming Tips

Constructors are not inherited,

so class CommissionEmployee does notinherit class Object's constructor.

However, class CommissionEmployee'sconstructor calls class Object'sconstructor implicitly.

the first task of any subclass constructoris to call its direct superclass'sconstructor, either explicitly or implicitly(if no constructor call is specified), to

ensure that the instance variables


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Problems with protected instancevariables

Using protected instance variablescreates several potential problems.

First, the subclass object can set an

inherited variable's value directlywithout using a set method. Therefore, asubclass object can assign an invalidvalue to the variable, thus leaving theobject in an inconsistent state. Forexample, if we were to declareCommissionEmployee3's instancevariable grossSales as protected, asubclass ob ect e. .,


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Use the protected access modifier whena superclass should provide a method

only to its subclasses and other classesin the same package, but not to otherclients.

Software Engineering Observation Declaring superclass instance variables

private (as opposed to protected)enables the superclass implementationof these instance variables to change


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C i i E l B Pl C i


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CommissionEmployeeBasePlusCommissionEmployee Inheritance Hierarchy

Using private Instance Variables

Lets now use the best softwareengineering practices.

Class CommissionEmployee3 declares

all instance variables as private andprovides some public methods (seemethod list below) for manipulatingthese values.

methods earnings and toString usethe class's get methods to obtain thevalues of its instance variables.

If we decide to change the instance variable

instance variables: firstName, lastName, socialSecurityNumber,grossSales and commissionRate

methods: setFirstName, getFirstName, setLastName,

getLastName, setSocialSecurityNumber, getSocialSecurityNumber,setGrossSales, getGrossSales, setCommissionRate, getCommissionRate,


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ClassBasePlusCommissionEmployee4 hasseveral changes to its method

implementations that distinguish itfrom classBasePlusCommissionEmployee3.

Methods earnings and toString eachinvoke method getBaseSalary to obtainthe base salary value, rather thanaccessing baseSalary directly. If we

decide to rename instance variable


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BasePlusCommissionEmployee4'stoString method overrides classCommissionEmployee3's toStringmethod to return a string representationthat is appropriate for a base-salariedcommission employee. The new versioncreates part of a

BasePlusCommissionEmployee4 object'sstring representation (i.e., the string"commission employee" and the valuesof class CommissionEmployee3's private

instance variables) by calling'


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Software Engineering Observation:

if a method performs all or some of the actionsneeded by another method, call that method ratherthan duplicate its code. By having

BasePlusCommissionEmployee4's earnings methodinvoke CommissionEmployee3's earnings method tocalculate part of a BasePlusCommissionEmployee4object's earnings, we avoid duplicating the code andreduce code-maintenance problems.

Common Programming Error

When a superclass method is overridden in asubclass, the subclass version often calls the

superclass version to do a portion of the work.


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Conclusion

In this section, we have shown youan evolutionary set of examples thatwas carefully designed to teach key

capabilities for good softwareengineering with inheritance.

You learned

how to use the keyword extends tocreate a subclass using inheritance

how to use protected superclass

members to enable a subclass to access

C t t i


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Constructors inSubclasses

Instantiating a subclass object beginsa chain of constructor calls in whichthe subclass constructor, before

performing its own tasks, invokes itsdirect superclass's constructor eitherexplicitly (via the super reference) or

implicitly (calling the superclass'sdefault constructor or no-argumentconstructor).

Similarly, if the superclass is derived


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When a program creates a subclassobject, the subclass constructor

immediately calls the superclassconstructor (explicitly, via super, orimplicitly). The superclass constructor'sbody executes to initialize the

superclass's instance variables that arepart of the subclass object, then thesubclass constructor's body executes toinitialize the subclass-only instance

variables. Java ensures that even if a


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Our next example declares a

CommissionEmployee4 classcontaining the same features as

CommissionEmployee.

The constructor is modified to output textupon its invocation.

BasePlusCommissionEmployee5class is almost identical to

BasePlusCommissionEmployee4, except'

O d f C t t


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Orders of ConstructorExecution

O d f C t t


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Orders of ConstructorExecution

Class ConstructorTest demonstratesthe order in which constructors arecalled for objects of classes that are

part of an inheritance hierarchy. Method main begins by instantiating

CommissionEmployee4 object

employee1. instantiate

BasePlusCommissionEmployee5 objectemployee2. This invokes the

CommissionEmployee4 constructor,

S ft E i i


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Software Engineeringwith Inheritance

When a new class extends anexisting class, the new class inheritsthe non-private members of the

existing class. We can customize the new class to

meet our needs by

including additional members and by

overriding superclass members. Doingthis does not require the subclass

programmer to change the superclass's


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Sometimes, students have difficultyappreciating the scope of theproblems faced by designers who

work on large-scale software projectsin industry. People experienced withsuch projects say that effective

software reuse improves the softwaredevelopment process. Object-oriented programming facilitatessoftware reuse, potentially

shortening development time.


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Object Class

All classes in Java inherit directly orindirectly from the Object class(package java.lang), so its 11

methods are inherited by all otherclasses. Figure 9.18 summarizesObject's methods.

ec me o s a are


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ec me o s a areinherited directly or

indirectly by all classes clone This protected method, which takes no

arguments and returns an Object

reference, makes a copy of the object onwhich it is called. When cloning isrequired for objects of a class, the classshould override method clone as a

public method and should implementinterface Cloneable (package java.lang).

The default implementation of thismethod performs a so-called shallow

copyinstance variable values in one


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Equals

This method compares two objects forequality and returns TRue if they are

equal and false otherwise. The methodtakes any Object as an argument. Whenobjects of a particular class must becompared for equality, the class should

override method equals to compare thecontents of the two objects.

The method's implementation shouldmeet the following requirements:


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finalize

This protected method is called by thegarbage collector to perform termination

housekeeping on an object just beforethe garbage collector reclaims theobject's memory. It is not guaranteedthat the garbage collector will reclaim

an object, so it cannot be guaranteedthat the object's finalize method willexecute. The method must specify anempty parameter list and must return

void. The default implementation of this


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hashCode

A hashtable is a data structure(discussed in Section 19.10) that relates

one object, called the key, to anotherobject, called the value. When initiallyinserting a value into a hashtable, thekey's hashCode method is called. The

hashcode value returned is used by thehashtable to determine the location atwhich to insert the corresponding value.

The key's hashcode is also used by the

hashtable to locate the key's


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toString

This method returns a Stringrepresentation of an object. The default

implementation of this method returnsthe package name and class name ofthe object's class followed by ahexadecimal representation of the value

returned by the object's hashCodemethod.


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Software Engineering Observation9.10

At the design stage in an object-oriented

system, the designer often finds thatcertain classes are closely related. Thedesigner should "factor out" commoninstance variables and methods and

place them in a superclass. Then thedesigner should use inheritance todevelop subclasses, specializing themwith capabilities beyond those inherited

Lecture 5 OOP-Inheritance

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