inheritance examples by lesh

8/18/2019 Inheritance Examples by lesh

http://slidepdf.com/reader/full/inheritance-examples-by-lesh 1/25

Inheritance

CS 102 - Algorithms & Programming II

September 29, 2010

c2010 Aybar C. Acar

Some examples from Savitch, “Absolute Java 3Ed”, 2008

Inheritance



A Puzzle...

All Kobbles are also Popples. Some Popples are Snapples.Zibbles are always Snapples. Kobbles are never Snapples.

Dabbles are Snapples that are not Zibbles.Some Kobbles are Fizzles.Dabbles are either Gorks, Zorks or Jorks.Kibbles are Kobbles but not Fizzles.

Popple

Kobble Snapple

DabbleKibble ZibbleFizzle

Gork Zork Jork

Inheritance



Same Puzzle...

All HourlyEmployees are also Employees.Some Employees are Salaried.

Executives are always Salaried.HourlyEmployees are never Salaried.Technical staff are Salaried Emploees that are not Executives.Some Hourly employees are Part-Time.Technical staff are either Engineers, Technicians or Clerks.

FullTime hourly employees are Hourly Employees but notPart-Time.

Employee

HourlyEmployee SalariedEmployee

TechnicalFullTime ExecutivePartTime

Engineer Technician Clerical

Inheritance



Terminology of Objects and Classes

Classes are types of objects. A class is an hypothetical thinguntil an object of that class is created.

Objects are real constructs in memory. All objects mustbelong to a type (e.g. a class)

References are “names” for objects of a given type.

So,

Objects are created (or constructed, or instantiated).Classes are defined.

References are declared, followed and assigned.

Therefore:You never create a class, you create/instantiate/construct anobject of that class .

Creation always involves a new somewhere...

A reference can only point to an object of its declared type(class) or to nothing at all (null).

You never have a reference to a class .Inheritance



Inheritance

Class Inheritance allows us to:

Create simple parent objects and to make them more

specialized.

Have specializations share the variables & methods of theircommon ancestors.

Write the code once, and re-use in many children! (Laziness)

Inheritance



Deriving Classes in Java

Derived class, subclass, child class all mean the same thing.

Base class, superclass, parent class all mean the same thing.

Base Class:

1 p ub l ic c la ss E m pl o ye e

2 {

3 p ri va te S tr in g n am e ;

4 p ri va te l on g i dN um be r ;

5 }

Child (derived class):

1 p ub l ic c la ss H o u rl y Em p l oy e e e xt e nd s E m pl o ye e

2 {

3 p r iv at e f lo at h o ur l yR a te ;4 p r iv at e i nt h o ur s Wo r ke d ;

5 }

The HourlyEmployee objects have name and idNumber inherited from

Employee, but they also include new variables: hourlyRate and

hoursWorked.Inheritance



Structure of a Derived Class

this super

Employee

HourlyEmployee

55.45

765

float hourlyRate

int hoursWorked3482942932

"Mehmet Sarıcizmeli"

int idNumber

String name

Every child class isit’s own type and alsoof it’s parent’stype(s).

Creation of a derivedclass involves thecreation of its parent

as well.The derived classholds its parentwithin itself.

e.g. you have your

parents’ DNA.

Each derived objectrefers to its parentversion using the

reference superInheritance



Accessibility in Inheritance

There are three main classes of accessibility, in order of permissiveness:

public : Public variables and methods are accessible byeveryone.

protected : Protected variables and methods are only accesible

by the class and its derived classes.private : Private variables and methods are accessible only

by the class itself.

super and this are always private.

super is even more restrictive than private as itcannot be returned in a method or assigned toanother reference. It can only be used directly.

Inheritance

S



Sandbox:

Assume the following case:

1 c l a s s A {

2 p r i v a t e i n t vA1 ;3 p r o te c t e d f l o a t vA2 ;4 p u b l i c S t r i n g vA3 ;56 p u b l i c v o i d fA1 ( ) { S y st e m . o u t . p r i n t l n ( v A1 ) } ;7 p r ot e ct e d i n t f A 2 ( i n t m) { r e t u r n m ∗ ( i n t ) vA2 } ;8 p r i v a t e b oo le an fA3 ( ) { r e t u r n (vA1 < vA2) } ;

9 }1011 c l a s s B e x t e n d s A {12 p r i v a t e i n t v B 1 = 3 ;13 p u b l i c f l o a t vB2 ;14

15 }1617 c l a s s C {18 p r i v a t e A vC1 ;19 p r i v a t e B vC2 ;20 }

Inheritance

Cl Di



Class Diagrams

Another way of viewing the previous example is with a classdiagram:

+ fA1() : void

# fA2(int) : int- fA3() : bool

- vA1 : int# vA2 : float+ vA3 : String

A

- vB1 : int+ vB2 : float

B

- vC1 : A- vC2 : B

C

Inheritance

I A d H A



Is-A and Has-A

+ fA1() : void# fA2(int) : int- fA3() : bool


A


B

- vC1 : A- vC2 : B

C

A class is said to have an“Is-A” relationship withanother class if it is derivedfrom it.

e.g. Every B is an A

A class is said to have a“Has-A” relationship withanother class if objects of that class hold a referencesto objects of the other class.

e.g. Every C has an Aor, every C has a B.

Inheritance

E i



Exercise

+ fA1() : void# fA2(int) : int

- fA3() : bool


A


B

- vC1 : A- vC2 : B

C

Example

Can we add the following methodto the definition of class C?

1 p u b l i c A f C1 ( )2 {

3 r e t u r n vC1 ;4 }

Yes. The variable vC1 is areference to an object of type

(class) A. An object of class Chas access to its private variablesso it can read them (and returnthem in methods).

Inheritance

E i



Exercise



A


B

- vC1 : A- vC2 : B

C

ExampleCan we add the following methodto the definition of class C?

1 p u b l i c S t r i n g fC2 ( )

2 {3 r e t u r n vC1 . vA3 ;4 }

Yes. The variable vA1 is a publicvariable of class A. An object of class C has access to the publicvariables of objects of class A.

Inheritance

E is



Exercise



A


B

- vC1 : A- vC2 : B

C

Example


1 p u b l i c i n t fC3 ( )

2 {3 r e t u r n vC1 . vA1 ;4 }

NO. Class C does not have

access to the private variable vA1of class A.

Inheritance

Exercise



Exercise



A


B

- vC1 : A- vC2 : B

C

Example


1 p u b l i c f l o a t fC4 ( )

2 {3 r e t u r n vC1 . vA2 ;4 }

NO. Class C is not a child of

class A. Therefore it cannotaccess protected variables.

Inheritance

Exercise



Exercise



A


B

- vC1 : A- vC2 : B

C

Example

Can we add the following methodto the definition of class B?

1 p u b l i c S t r i n g fB1 ( )

2 {3 r e t u r n vA3 ;4 }

YES. vA3 is an instance variable

of B inherited from parent classA. It is public so B can access it.

Inheritance



Exercise



Exercise



A


B

- vC1 : A- vC2 : B

C

Example


1 p u b l i c i n t fB3 ( )2 {3 r e t u r n vA1 ;4 }

NO. vA1 is an instance variableof B inherited from parent class

A. But, it is private and even if B is a child of A, B can notaccess it.

Inheritance

Exercise



Exercise



A


B

- vC1 : A- vC2 : B

C


1 p u b l i c v oi d fC4 ( )

2 {3 vC2 . fA1 ( ) ;4 }

YES. fA1 is a method of Binherited from parent class A. Itis public so it can be accessed byobjects of class C.

Inheritance

Exercise



Exercise



A


B

- vC1 : A- vC2 : B

C


1 p u b l i c i n t f C 5 ( i n t a )

2 {3 r e t u r n vC2 . fA2 ( a ) ;4 }

NO. fA2 is a protected methodof B inherited from parent classA. Since C is not a child of A, itcannot access fA2.

Inheritance

Exercise



Exercise



A


B

- vC1 : A- vC2 : B

C

ExampleCan we add the following methodto the definition of class B?

1 p u b l i c i n t f B 4 ( i n t a )

2 {3 r e t u r n f A2 ( a ) ;4 }

YES. fA2 is a protected methodof B inherited from parent classA. Since B is a child of A, it canaccess fA2.

Inheritance

Exercise



Exercise



A


B

- vC1 : A- vC2 : B

C

Example


1 p u b l i c b oo lea n fB5 ( )

2 {3 r e t u r n f A3 ( ) ;4 }

NO. fA3 is a private method of

A. Although B is a child of A, itcan not access fA3.

Inheritance

Exercise



Exercise

+ fA1() : void# fA2(int) : int

- fA3() : bool


A


B

- vC1 : A- vC2 : B

C

Example


1 p u b l i c B f B6 ( )2 {3 r et ur n t h i s ;4 }

YES. this is a private variableof B. B can access and return itsown address.

However, this is sort of useless...If you can call fB6() on someobject, it means you already haveits reference!

Inheritance

Exercise





A


B

- vC1 : A- vC2 : B

C

Example


1 p u b l i c A f B6 ( )2 {

3 r e t u r n s u p e r ;4 }

NO. super is a private variableof B and normally this would be

fine. But super is an exceptionalcase in that B can use super butcan never give it away.

Inheritance

Overriding Methods



g

A method is uniquely defined by it’s name and parameter list.This is called the methods signature (or fingerprint )

When a class defines two methods with the same name butdifferent parameters, this is called overloading

e.g.

1 i n t f o o ( S t r i n g s )2 i n t fo o ( f l o a t f )

Java will not allow two different methods with the samesignature in the same class.How ever a child class can define a new method with thesame signature as that of its parent class.

Assume class A has:

1 p u b l ic v oi d fA1 ( ) { S y st e m . o u t . p r i n t l n ( v A1 ) ; }

Child class B can define:

1 p u b l ic v oi d fA1 ( ) { S y st e m . o u t . p r i n t l n ( v A2 ∗2 ) ; }

This is called overriding.

Inheritance

inheritance examples by lesh

Documents