object oriented programming (oop): inheritance...

Overview Bicycle inheritance Scope Shape inheritance Polymorphism

Object oriented programming (OOP):Inheritance and polymorphism

Prof. Dionne Aleman

MIE250: Fundamentals of object-oriented programmingUniversity of Toronto

MIE250: Fundamentals of object-oriented programming (Aleman) OOP: Inheritance and polymorphism 1 / 51


Inheritance1

When classes have a lot in common, we can modularize common featuresusing inheritance.

1Bicycle example from Oracle’s Java TutorialsMIE250: Fundamentals of object-oriented programming (Aleman) OOP: Inheritance and polymorphism 2 / 51

http://docs.oracle.com/javase/tutorial/java/concepts/inheritance.html


The superclass (nothing special)

1 public class Bicycle {2 private int numGears;3 private double price;45 public Bicycle () {6 this.numGears = 0;7 this.price = 0.0;8 }9

10 public double getPrice () { return this.price; }11 public void setPrice(double price) { this.price = price; }12 public int getGears () { return this.numGears; }13 public void setGears(int numGears) { this.numGears = numGears; }1415 public void rideTheMountain () {16 System.out.println("Popped a tire.");17 }18 }

src/BikeInheritance/Bicycle.java



The subclass MountainBike

Only need members and methods that are different from the superclass

1 public class MountainBike extends Bicycle {2 private String suspension;3 private double trailRating;4

5 public MountainBike () {6 super(); // calls superclass constructor7 this.suspension = "Downhill";8 this.trailRating = 2;9 }

10

11 public double getTrailRating () { return this.trailRating; }12 public void setTrailRating(double t) { this.trailRating = t; }13 public String getSuspension () { return this.suspension; }14 public void setSuspension(String s) { this.suspension = s; }15

16 @Override // indicates that this method overrides the superclass17 public void rideTheMountain () {18 System.out.println("All systems w/in normal operating parameters.");19 }20 }

src/BikeInheritance/MountainBike.java



The subclass RoadBike


1 public class RoadBike extends Bicycle {23 @Override4 public void rideTheMountain () {5 System.out.println("Severe damage sustained.");6 }78 }

src/BikeInheritance/RoadBike.java



The subclass TandemBike


1 public class TandemBike extends Bicycle {2 private int nRiders;34 public TandemBike () {5 super();6 this.nRiders = 2;7 }89 public TandemBike(int n) {

10 super();11 this.setNRiders(n);12 }1314 public int getNRiders () { return this.nRiders; }15 public void setNRiders(int n) { this.nRiders = n; }1617 @Override18 public void rideTheMountain () {19 System.out.println(this.nRiders + " people stuck in a ditch.");20 }21 }

src/BikeInheritance/TandemBike.javaMIE250: Fundamentals of object-oriented programming (Aleman) OOP: Inheritance and polymorphism 6 / 51


What exactly is passed from parent to child?

I A subclass inherits all of the public and protected members of itsparent (superclass), no matter what package the subclass is in.

I A class inherits fields and methods from all its superclasses, whetherdirect or indirect.

I A subclass can override methods that it inherits.



Particulars

I Each class can have one direct superclass only.

I Each superclass can have infinite subclasses.



A note on @Override

I Optional, though some IDEs will flag if not used

I Will definitely be flagged if used incorrectly

I Good to use to let you know that a method is supposed to overridea superclass method

I If the superclass method changes later, the compiler will let youknow that something isn’t right



Bicycle inheritance exampleWe’ve already seen the superclass and the subclass. Let’s use them.

1 public class myBikes {23 public static void main(String [] args) {4 Bicycle Bike = new Bicycle ();5 MountainBike MtnBike = new MountainBike ();6 RoadBike RdBike = new RoadBike ();7 TandemBike TdmBike = new TandemBike (4);89 Bike.setPrice (250);

10 MtnBike.setPrice (375);11 MtnBike.setSuspension("Hard tail");12 Bike.setSuspension("Hard tail");13 TdmBike.setSuspension("Hard tail");1415 Bike.rideTheMountain ();16 MtnBike.rideTheMountain ();17 RdBike.rideTheMountain ();18 TdmBike.rideTheMountain ();19 }2021 }

src/BikeInheritance/myBikes.javaMIE250: Fundamentals of object-oriented programming (Aleman) OOP: Inheritance and polymorphism 10 / 51


Using inherited classes

I Exactly the same as regular classes!

I The only thing different is in writing the subclasses.

I So, we can save ourselves a lot of time and effort by just inheritingfrom other classes that have a lot of overlap with what we want ournew classes to do.



Go through it line-by-line

1 public class myBikes {2

3 public static void main(String [] args) {4 Bicycle Bike = new Bicycle ();5 MountainBike MtnBike = new MountainBike ();6 RoadBike RdBike = new RoadBike ();7 TandemBike TdmBike = new TandemBike (4);8

9 Bike.setPrice (250);10 MtnBike.setPrice (375);11 MtnBike.setSuspension("Hard tail");12 Bike.setSuspension("Hard tail");13 TdmBike.setSuspension("Hard tail");14

15 Bike.rideTheMountain ();16 MtnBike.rideTheMountain ();17 RdBike.rideTheMountain ();18 TdmBike.rideTheMountain ();19 }20

21 }

src/BikeInheritance/myBikes.java



Go through it line-by-line





21 }


Go to Bicycle constructor→



Bicycle Bike in main → Bicycle constructor

1 public class Bicycle {2 private int numGears;3 private double price;4

5 public Bicycle () {6 this.numGears = 0;7 this.price = 0.0;8 }9

10 public double getPrice () { return this.price; }11 public void setPrice(double price) { this.price = price; }12 public int getGears () { return this.numGears; }13 public void setGears(int numGears) { this.numGears = numGears; }14

15 public void rideTheMountain () {16 System.out.println("Popped a tire.");17 }18 }




Bicycle Bike in main → Bicycle constructor






Go back to main←



Back from Bicycle constructor





21 }









21 }


Go to MountainBike constructor→



MountainBike MtnBike in main → MountainBikeconstructor



10









10







MountainBike MtnBike in main → MountainBikeconstructor → Bicycle constructor








MountainBike MtnBike in main → MountainBikeconstructor → Bicycle constructor






Go back to MountainBike constructor←






10









10




Go back to main←



Back from MountainBike constructor





21 }




Back from MountainBike constructor





21 }


Go to RoadBike constructor→



RoadBike RdBike in main → RoadBike constructor

No specific constructor, so use the parent’s constructor!

1 public class RoadBike extends Bicycle {2

3 @Override4 public void rideTheMountain () {5 System.out.println("Severe damage sustained.");6 }7

8 }




RoadBike RdBike in main → RoadBike constructor

No specific constructor, so use the parent’s constructor!



8 }





RoadBike RdBike in main → RoadBike → Bicycleconstructor








RoadBike RdBike in main → RoadBike → Bicycleconstructor






Go back to main←








21 }









21 }


Go to TandemBike constructor→



TandemBike TdmBike in main → TandemBike constructor1 public class TandemBike extends Bicycle {2 private int nRiders;3

4 public TandemBike () {5 super();6 this.nRiders = 2;7 }8

9 public TandemBike(int n) {10 super();11 this.setNRiders(n);12 }13

14 public int getNRiders () { return this.nRiders; }15 public void setNRiders(int n) { this.nRiders = n; }16

17 @Override18 public void rideTheMountain () {19 System.out.println(this.nRiders + " people stuck in a ditch.");20 }21 }

src/BikeInheritance/TandemBike.java









There are two constructors ...Which to use?









Not this constructor!The arguments don’t match!









This constructor,since it takes the same arguments



TandemBike TdmBike in main → TandemBike constructor→ Bicycle constructor








TandemBike TdmBike in main → TandemBike constructor→ Bicycle constructor






Go back to TandemBike constructor←









Go to function setNRiders()in this class









Set this.nRiders = 4









Go back to main←



Back from TandemBike constructor





21 }




Back from TandemBike constructor





21 }


Go to Bike’s setPrice() function→



Bike.setPrice(250) in main → Bicycle’s setPrice()function








Bike.setPrice(250) in main → Bicycle’s setPrice()function






Set this.price = 250 andgo back to main

←



Back from Bike.setPrice()





21 }




Back from Bike.setPrice()





21 }


Go to MtnBike’s setPrice() function→



MtnBike.setPrice(375) in main → MountainBike’ssetPrice() function



10






MtnBike.setPrice(375) in main → MountainBike’ssetPrice() function



10




No setPrice() function here,so go to parent’s setPrice()

→



MtnBike.setPrice(375) in main → MountainBike class→ Bicycle’s setPrice() function








MtnBike.setPrice(375) in main → MountainBike class→ Bicycle’s setPrice() function






Set this.price = 375 andgo back to main

←



Back from MtnBike.setPrice()





21 }




Back from MtnBike.setPrice()





21 }


Go to MtnBike’ssetSuspension() function

→



MtnBike.setSuspension("Hard tail") in main →MountainBike’s setSuspension() function



10






MtnBike.setSuspension("Hard tail") in main →MountainBike’s setSuspension() function



10




Set this.suspension = "Hard tail"and go back to main

→



Back from MtnBike.setSuspension()





21 }









21 }


No! There is no suchfunction in Bicycle!








21 }









21 }


No! There is no suchfunction in TandemBike!








21 }









21 }


Go to Bike’srideTheMountain() function

→



Bike.rideTheMountain() in main → Bicycle’srideTheMountain() function








Bike.rideTheMountain() in main → Bicycle’srideTheMountain() function






Print message andgo back to main

←



Back from Bike.rideTheMountain()





21 }




Back from Bike.rideTheMountain()





21 }


Go to MtnBike’srideTheMountain() function

→



MtnBike.rideTheMountain() in main →MountainBike’s rideTheMountain() function



10






MtnBike.rideTheMountain() in main →MountainBike’s rideTheMountain() function



10





←



Back from MtnBike.rideTheMountain()





21 }




Back from MtnBike.rideTheMountain()





21 }


Go to RdBike’srideTheMountain() function

→



RdBike.rideTheMountain() in main → RoadBike’srideTheMountain() function



8 }




RdBike.rideTheMountain() in main → RoadBike’srideTheMountain() function



8 }



←



Back from RdBike.rideTheMountain()





21 }




Back from RdBike.rideTheMountain()





21 }


Go to TdmBike’srideTheMountain() function

→



TdmBike.rideTheMountain() in main → TandemBike’srideTheMountain() function

1 public class TandemBike extends Bicycle {2 private int nRiders;3








TdmBike.rideTheMountain() in main → TandemBike’srideTheMountain() function

1 public class TandemBike extends Bicycle {2 private int nRiders;3







←



Back from TdmBike.rideTheMountain()





21 }




Who sees what in inheritance?

Modifier Within class Outside class, insame package

Outside class,outside package

public X X X

protected X X

private X

Since we won’t deal with packages in this class, for our purposes, publicand protected are the same. So, let’s just ignore protected.2

2In most other OOP languages, protected is heavily used in inheritance situations.MIE250: Fundamentals of object-oriented programming (Aleman) OOP: Inheritance and polymorphism 41 / 51



I We know that if a member variable or method is private, it can’tbe seen in the main or elsewhere.

I Can a private member variable or method in a superclass be seenby its subclasses?




I We know that if a member variable or method is private, it can’tbe seen in the main or elsewhere.

I Can a private member variable or method in a superclass be seenby its subclasses?

No!



A scope example

1 public class valueBikes {2

3 public static void main(String [] args) {4 MountainBike MtnBike = new MountainBike ();5 MtnBike.setPrice (405);6 MtnBike.setTrailRating (2.5);7

8 if (MtnBike.isGoodValue ())9 System.out.println("Good value!");

10 else11 System.out.println("Bad value!");12 }13

14 }

src/BikeInheritance_scope/valueBikes.java



The same Bicycle class





src/BikeInheritance_scope/Bicycle.java



The new MountainBike class



10


16 public boolean isGoodValue () {17 if (this.trailRating > 2 && this.price < 400)18 return true;19 return false;20 }21 }

src/BikeInheritance_scope/MountainBike.java






10




The new function






10









10




Allowable?






10




No!price is private

to Bicycle!



So what should we do instead?

Keep in mind we don’t want to just make price public.



What if we had a geometry program that managed a lot ofshapes?

I What are common features different shapes have?

I Number of sidesI Number of dimensions (2D or 3D)I Name

I What are common things we would do with a shape?I Calculate areaI Calculate perimeter (only if 2D)I Calculate volume (only if 3D)I Print out name




I What are common features different shapes have?I Number of sides

I Number of dimensions (2D or 3D)I Name





I What are common features different shapes have?I Number of sidesI Number of dimensions (2D or 3D)

I Name





I What are common features different shapes have?I Number of sidesI Number of dimensions (2D or 3D)I Name






I What are common things we would do with a shape?

I Calculate areaI Calculate perimeter (only if 2D)I Calculate volume (only if 3D)I Print out name





I What are common things we would do with a shape?I Calculate area

I Calculate perimeter (only if 2D)I Calculate volume (only if 3D)I Print out name





I What are common things we would do with a shape?I Calculate areaI Calculate perimeter (only if 2D)

I Calculate volume (only if 3D)I Print out name





I What are common things we would do with a shape?I Calculate areaI Calculate perimeter (only if 2D)I Calculate volume (only if 3D)

I Print out name



Perfect time for inheritance

Let’s build a superclass and a few subclasses for our shape managementprogram.



Polymorphism

I Polymorphism is a lot like method overloading, but with inheritedmethods.

I Instead of having a different method for different inputs, we canhave a different method for different calling objects. In other words,many related classes with the same method name.

I We’ve already been doing it!



Polymorphism in the Bicycle example1 public class Bicycle {2 ...3 public void rideTheMountain () {4 System.out.println("Popped a tire.");5 }6 }789 public class MountainBike extends Bicycle {

10 ...11 public void rideTheMountain () {12 System.out.println("All systems w/in normal operating parameters.");13 }14 }151617 public class RoadBike extends Bicycle {18 ...19 public void rideTheMountain () {20 System.out.println("Severe damage sustained.");21 }22 }



Polymorphism in the Bicycle example

I Even though the superclass Bicycle has a rideTheMountain()method, each subclass has its own version.

I So, the actual method run depends on whether the method is calledby a Bicycle object, a MountainBike object, a RoadBike object,etc.

I It’s not the input arguments that determine which method is run (asin regular overloading), but the calling object.


object oriented programming (oop): inheritance...

Documents