Lecture 10: Polymorphism

8/20/2019 Lecture 10: Polymorphism

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Chapter 10: Polymorphism

!


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Polymorphism, Sorting and Searching

Polymorphism

•

Polymorphism is an OO concept that allows us to create

versatile software designs

• Chapter 10 focuses on:!

defining polymorphism and its benefits

! using inheritance to create polymorphic references

!

using interfaces to create polymorphic references

! using polymorphism to implement sorting and searching

algorithms

!

additional GUI components

#$% !&' '


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Binding

• At some point, this call is bound to the definition of themethod that it calls

!

but when?

! If at compile time, it would call the same method every time

!

If at runtime, it could be bound to a method "on the fly"

•

Java chooses the latter – it's called dynamic binding orlate binding

• Late binding is the basis for polymorphism ! providesflexibility in program design

#$% !&' (

student.computeGrade();

•

Consider this method call:


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Polymorphism

•

Polymorphism literally means "having many forms"

• A polymorphic reference is a variable that can refer todifferent types of objects at different times

• Which method is called through a polymorphic referencecan change from one call to the next

#$% !&' )

All object references in Javaare potentially polymorphic


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Polymorphism

•

Java allows job to point to:

!

an Occupation object, or

! an object of any compatible type

•

This compatibility can be based on inheritance orinterfaces! as we'll see!

#$% !&' &

Occupation job;

•

Given this declaration:


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References and Inheritance

•

Consider this example

#$% !&' *

Bicycle

RoadBike MountainBike

• Suppose Bicycle has a method called printDescription

•

And that RoadBike and MountainBike override this method

with customized versions


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•

Examine this code:

#$% !&' +

public class TestBikes

{

public static void main(String[] args)

{

Bicycle bike1, bike2, bike3;bike1 = new Bicycle(20, 10, 1);

bike2 = new MountainBike(20, 10, 5, "Dual");

bike3 = new RoadBike(40, 20, 8, 23);

bike1.printDescription();

bike2.printDescription();bike3.printDescription();

}

}

Who ya gonna call?


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#$% !&' ,

Bicycle bike3;

bike3 = new RoadBike (40, 20, 8, 23 );

MountainBike bike2 ;

bike2 = (MountainBike) new Bicycle (20,10,1);

Bicycle

RoadBike MountainBike

Wideningconversion

(no problem)

Narrowing

conversion

(needs cast)


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#$% !&' -

public class TestBikes

{

public static void main(String[] args)

{Bicycle bike1, bike2, bike3;

bike1 = new Bicycle(20, 10, 1);

bike2 = new MountainBike(20, 10, 5, "Dual");

bike3 = new RoadBike(40, 20, 8, 23);




}

}

•

It is the type of the object being referenced, not thereference type, that determines which method is invoked


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Polymorphism via Inheritance

•

Consider the task ofpaying employees:

#$% !&' !.

StaffMember

Executive Hourly

Volunteer Employee

(Abstract class)

See Firm example (pg. 490-501)


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Polymorphism via Interfaces

•

Suppose we have an interface called Speaker whichprescribes a method called speak

• The interface name can be used as a type:

#$% !&' !!

Speaker current;

• The variable current can point to any object of any classthat implements Speaker

current.speak();

Who ya gonna call?

• Which speak method is calleddepends on the type of object

that current is pointing to at that

time.


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Polymorphism via Interfaces

•

Suppose two classes, Philosopher and Dog, bothimplement the Speaker interface, and provide distinct

versions of the speak method:

#$% !&' !'

yada yada!

Speaker guest = new Philospher ( );

guest.speak ( );

guest = new Dog ( );

guest.speak ( );

Squirrel !!


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Sorting

•

Sorting arranges a list of items in a particular order

• The sorting process is based on specific value(s)! Sort test scores in ascending numeric order

!

Sort people alphabetically by last name

• Many sorting algorithms:! Vary in efficiency and complexity

•

Will look at two simple algorithms:!

Selection Sort

! Insertion Sort

#$% !&' !(


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Selection Sort

•

General approach:!

Select a value and put it in its final place into the list

! Repeat for all other values

#$% !&' !)

Selection Sort Algorithm

• find the smallest value in the list

• switch it with the value in the first position

•

find the next smallest value in the list

•

switch it with the value in the second position

• repeat until all values are in their proper places


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Selection Sort

•

An example:

#$% !&' !&

•

Each time, the smallest remaining value is found and

swapped with the element at the front of the unsorted part

of the list

3 9 6 1 2

1 9 6 3 2

1 2 6 3 9

1 2 3 6 9

1 2 3 6 9

select smallest:

swap with 1st, select smallest:

swap with 2nd, select smallest:

swap with 3rd, select smallest:

6 ok (don't swap), only one left:

select


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Swapping

•

By "swap", we mean exchange the contents of two

variables (or array elements):

#$% !&' !*

temp = blueBucket;blueBucket = redBucket;

redBucket = temp;

temp

1

2

3


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END OF THIS LECTURE!

Have a Coke and a smile – or whatever . . .

#$% !&' !+

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Lecture 10: Polymorphism