CS 100 Lecture 24 1

CS100J Lecture 24CS100J Lecture 24

Previous LecturePrevious Lecture

– MatLab demonstrationMatLab demonstration

This LectureThis Lecture

– InheritanceInheritance

– Method overridingMethod overriding

– PolymorphismPolymorphism

– Reading: Reading: Lewis and Loftus, Chapter 7Lewis and Loftus, Chapter 7

Savitch, Chapter 7Savitch, Chapter 7

CS 100 Lecture 24 2

Object-Oriented ProgrammingObject-Oriented Programming

Object-oriented programming aspires to model Object-oriented programming aspires to model

the real world as a collection of the real world as a collection of objectsobjects..

Objects are classified into categories called Objects are classified into categories called

classesclasses..

Objects in the real world and in mathematics can Objects in the real world and in mathematics can

be further classified by be further classified by hierarchicalhierarchical taxonomiestaxonomies. .

For the taxonomyFor the taxonomy

we say “every Y is-a X”. For example,we say “every Y is-a X”. For example,

““every kitten is a cat”.every kitten is a cat”.

X

Y

Cat

Kitten

CS 100 Lecture 24 3

Taxonomy of PolygonsTaxonomy of Polygons

Polygon

Triangle

Isosceles

Equilateral

Rhombus

Square

Trapezoid

Parallelogram

Quadrilateral

CS 100 Lecture 24 4

Two Taxonomies of PersonTwo Taxonomies of Person

Person

boy girl

OldYoung

man woman

Person

boy man

FemaleMale

girl woman

CS 100 Lecture 24 5

Not a TaxonomyNot a Taxonomy

Person

boy man

FemaleMale

girl woman

OldYoung

Taxonomies are strictly hierarchicalTaxonomies are strictly hierarchical

CS 100 Lecture 24 6

The Class HierarchyThe Class Hierarchy

In Java, all classes are organized into a In Java, all classes are organized into a

taxonomy known as the taxonomy known as the classclass hierarchyhierarchy, Class , Class

ObjectObject is at the top of the hierarchy. is at the top of the hierarchy.

If class s is below class c in the class hierarchy, If class s is below class c in the class hierarchy,

class s is called a class s is called a subclasssubclass of c, a c is a of c, a c is a

superclasssuperclass of s. of s.

The Java class definitionThe Java class definition

classclass class-name class-name

{ . . . }{ . . . }

implicitlyimplicitly defines defines class-nameclass-name to be a subclass of to be a subclass of

class class ObjectObject..

ExampleExample. Classes . Classes AccountAccount, , RoomRoom, , PersonPerson, and , and

MatrixMatrix are each subclasses of class are each subclasses of class ObjectObject

Person

Object

Room MatrixAccount

CS 100 Lecture 24 7

Defining a SubclassDefining a Subclass

The Java class definitionThe Java class definition

classclass class-nameclass-name11 extendsextends class-name class-name22

{ . . . }{ . . . }

explicitlyexplicitly defines defines class-nameclass-name11 to be a subclass to be a subclass

of of class-nameclass-name22..

ExampleExample. .

classclass Male Male extendsextends PersonPerson

{ . . . }{ . . . }

classclass Female Female extendsextends PersonPerson

{ . . . }{ . . . }

Person

Object

Room Matrix

FemaleMale

Account

CS 100 Lecture 24 8

InheritanceInheritance

Objects of a given class have all characteristics Objects of a given class have all characteristics

(fields and methods) of objects above them in (fields and methods) of objects above them in

the hierarchy.the hierarchy.

A subclass is said to A subclass is said to inheritinherit the fields and the fields and

methods of its superclass.methods of its superclass.

The class hierarchy is sometime called the The class hierarchy is sometime called the

inheritance hierarchyinheritance hierarchy..

CS 100 Lecture 24 9

Method ResolutionMethod Resolution

Let o be an object of type Let o be an object of type tt, i.e., o was , i.e., o was

constructed by a constructor constructed by a constructor tt..

QuestionQuestion. Suppose you invoke method m on . Suppose you invoke method m on

object o. Which definition of m is invoked? object o. Which definition of m is invoked?

AnswerAnswer. The first definition of . The first definition of mm found (at run found (at run

time) in the class hierarchy, starting at time) in the class hierarchy, starting at tt, ,

working up through its superclasses, to Object.working up through its superclasses, to Object.

CS 100 Lecture 24 10

Inheritance of MethodsInheritance of Methods

classclass Room Room

{{

int id;int id; // Id number of room// Id number of room

. . .. . .

publicpublic String toString() String toString()

{ { returnreturn ”Room: ” + id; } ”Room: ” + id; }

}}

classclass Bathroom Bathroom extendsextends Room Room

{{

boolean shower; // true if room has shower.boolean shower; // true if room has shower.

. . .. . .

}}

// Client code// Client code

Room r1 = Room r1 = newnew Room(); Room();

Bathroom r2 = Bathroom r2 = newnew Bathroom(); Bathroom();

/* Room’s toString method is available for both /* Room’s toString method is available for both Rooms and Bathrooms. */Rooms and Bathrooms. */

System.out.println(r1);System.out.println(r1);

System.out.println(r2); System.out.println(r2);

CS 100 Lecture 24 11

Method OverridingMethod Overriding

Recall that if you don’t define a Recall that if you don’t define a toStringtoString

methods in a class, a “default” methods in a class, a “default” toStringtoString

method is used.method is used.

Where does that default method come from? It is Where does that default method come from? It is

the the toStringtoString method of class method of class ObjectObject..

Redefining a method that is already defined in a Redefining a method that is already defined in a

superclass is called superclass is called method overridingmethod overriding (not to be (not to be

confused with method overloading).confused with method overloading).

Method overriding lets you have a method that Method overriding lets you have a method that

is specialized for a subclass.is specialized for a subclass.

Room

Object

Bathroom

CS 100 Lecture 24 12

Field SelectionField Selection

Let e be a reference expression of type t, where Let e be a reference expression of type t, where

t is some class. i.e., at run time expression e will t is some class. i.e., at run time expression e will

evaluate to a reference to some object of type t’, evaluate to a reference to some object of type t’,

where t’ is t or a subtype of t.where t’ is t or a subtype of t.

QuestionQuestion. Suppose you select field f of whatever . Suppose you select field f of whatever

object e refers to. Which field f is accessed? object e refers to. Which field f is accessed?

AnswerAnswer. The first definition of f found (at . The first definition of f found (at

compile time) in the class hierarchy, starting at compile time) in the class hierarchy, starting at

tt, working up through its superclasses, to , working up through its superclasses, to

Object. Object.

Important point to be clarified later in the Important point to be clarified later in the

discussion about polymorphismdiscussion about polymorphism: In resolving the : In resolving the

field reference f, the search up the inheritance field reference f, the search up the inheritance

hierarchy starts at hierarchy starts at type ttype t (the type of expression (the type of expression

e), and not e), and not type t’type t’ (the type of the object). (the type of the object).

CS 100 Lecture 24 13

Inheritance of FieldsInheritance of Fields

classclass Account Account{{

int balance = 0;int balance = 0; // current balance// current balance. . .. . .

}}

classclass SavingsAccount SavingsAccount extendsextends Account Account{{

double rate = 0; double rate = 0; // interest rate// interest rate. . .. . .

}}

// Client code// Client code

Account act1 = Account act1 = newnew Account(); Account();

SavingsAccount act2 = SavingsAccount act2 = newnew SavingsAccount(); SavingsAccount();

// A SavingsAccount has both balance and rate.// A SavingsAccount has both balance and rate.

System.out.println(act2.balance); // LEGAL!System.out.println(act2.balance); // LEGAL!

System.out.println(act2.rate);System.out.println(act2.rate);

// An account has only a balance.// An account has only a balance.

System.out.println(act1.balance);System.out.println(act1.balance);

// System.out.println(act1.rate); NOT LEGAL!// System.out.println(act1.rate); NOT LEGAL!

CS 100 Lecture 24 14

Types of VariablesTypes of Variables

Recall that the type of a variable determines the Recall that the type of a variable determines the types of values that can be stored in the types of values that can be stored in the variable.variable.

A type A type t t variable can contain typevariable can contain type t t objects. objects.

Room r = Room r = newnew Room(); Room(); // LEGAL! // LEGAL!

Account act = Account act = newnew Acount(); // LEGAL! Acount(); // LEGAL!

A typeA type t t variable may not contain objects of typevariable may not contain objects of type t’t’ if if t t and and t’ t’ are unrelated types.are unrelated types.

// Room r = // Room r = newnew Account(); NOT LEGAL! Account(); NOT LEGAL!

// Account act = // Account act = newnew Room(); NOT LEGAL! Room(); NOT LEGAL!

CS 100 Lecture 24 15

PolymorphismPolymorphism

A type A type tt variable can contain any object whose variable can contain any object whose type is type is t t or a subtype of or a subtype of tt

// Client Code. // Client Code.

/* Because every Bathroom is a Room, both of the /* Because every Bathroom is a Room, both of the following statements are legal. */following statements are legal. */

Room r1 = Room r1 = newnew Room(); Room(); // LEGAL!// LEGAL!

Room r2 = Room r2 = newnew Bathroom(); Bathroom(); // LEGAL!// LEGAL!

/* You can only access fields that are guaranteed /* You can only access fields that are guaranteed to exist based on the type of the object to exist based on the type of the object reference. */reference. */

// System.out.println(r2.shower); NOT LEGAL!// System.out.println(r2.shower); NOT LEGAL!

/* Because not every Room is a Bathroom, only the /* Because not every Room is a Bathroom, only the second of the following statements is legal. second of the following statements is legal. */*/

// Bathroom r3 = // Bathroom r3 = newnew Room(); Room(); NOT LEGAL!NOT LEGAL!

Bathroom r4 = Bathroom r4 = newnew Bathroom(); Bathroom(); // LEGAL!// LEGAL!

CS 100 Lecture 24 16

Polymorphism, continuedPolymorphism, continued

classclass Room Room

{{

int id;int id; // Id number of room// Id number of room

. . .. . .

publicpublic String toString() String toString()

{ { returnreturn ”Room: ” + id; } ”Room: ” + id; }

}}

classclass Bathroom Bathroom extendsextends Room Room

{{

. . .. . .

publicpublic String toString() String toString()

{ { returnreturn ”Bathroom: ” + id; } ”Bathroom: ” + id; }

}}

/* Client code. The class of the object, not the /* Client code. The class of the object, not the type of the variable, determines which method type of the variable, determines which method is invoked. */is invoked. */

Room r1 = Room r1 = newnew Room(); Room();

System.out.println(r1); // output: “Room: …”System.out.println(r1); // output: “Room: …”

Room r2 = Room r2 = newnew Bathroom(); Bathroom();

System.out.println(r2); // output: “Bathroom: …”System.out.println(r2); // output: “Bathroom: …”