SlideShare a Scribd company logo
CSSE221: Software Dev. Honors Day 6
 Announcements
 Questions?
 Hint on BallWorlds’ changing the x and y coords in
2nd half of lecture today.
This week: BallWorlds assignment
 Monday:
 Intro to UML as a communication tool
 Writing methods you don't call
 Using this
 Tuesday:
 Inheritance
 Polymorphism
 Thursday:
 Introducing next week’s assignment
 Arrays and ArrayLists
 (Using the debugger)
Reminder
 For the inheritance, polymorphism, and array
groups, you owe me 3 things:
1. Summary
2. Quiz
3. Answer key
Inheritance slides
 Some material from those produced by Fall
2006-2007 CSSE221 students:
 Michael Auchter
 Michael Boland
 Andrew Hettlinger
Inheritance
 Objects of different kinds (classes) have their
own unique behavior.
 Objects of different kinds often share similar
behavior too.
 For example:
 Student, Professor, Software Engineer, Chemical
Engineer, Physicist, Guitarist, Drummer
 Each has distinct actions to perform, but they also
have many features and behavior in common
Why not just copy-and-paste?
 Say I have an Employee class and want to
create an HourlyEmployee class that adds
info about wages. Why not copy-and-paste,
then modify?
1. Fixing bugs: what if one were wrong?
2. Maintenance: what if Employee changes?
3. Code-reuse: would code that takes an Employee
as a parameter also take an HourlyEmployee?
The Basics of Inheritance
 Inheritance allows you to reuse methods that
you’ve already written to create more specialized
versions of a class.
 Java keyword: extends.
 public class HourlyEmployee extends Employee.
 We say that an HourlyEmployee IS-A Employee
 Employee is said to be the parent class (or
superclass), and HourlyEmployee is called a child
class (or subclass).
 HourlyEmployee receives copies of all of the non-
private methods and variables present in Employee.
Your turn
 Quiz question: What is the relationship between
a parrot and a bird?
Some Key Ideas in Inheritance
 Code reuse
 Overriding methods
 Protected visibility
 The “super” keyword
Code re-use
 The subclass inherits all the public and
protected methods and fields of the superclass.
 Constructors are not inherited
 Constructors can be invoked by the subclass
 Subclass can add new methods and fields.
Overriding Methods
 DudThatMoves will “extend” Dud
 If it defines an act() method with the same
signature that overrides Dud’s method
 What do you think happens if our child class
doesn’t override a method in the parent class?
It’s exactly the
same as in the
parent class!
Visibility Modifiers
• Public – Accessible by any other class in any package.
• Private – Accessible only within the class.
• Protected – Accessible only by classes within the same
package and any subclasses in other packages.
• (For this reason, some choose not to use protected, but use
private with accessors)
• Default (No Modifier) – Accessible by classes in the
same package but not by classes in other packages.
• Use sparingly!
Protected Visibility
 Suppose in Dud you defined:
protected int xPos;
 Then:
 Instances of children inherit this field
 (one for each instance)
 Children can access/modify the fields
 this.xPos in Dud (parent) and this.xPos in
DudThatMoves (child) refer to same value
The “super” Keyword
 It’s like the word “this,” only “super”:
 In a child class, “super” refers to its parent.
 Two uses:
1. To call a parent’s method, use super.methodName(…)
2. To call a parent’s constructor, use super(some parameter)
from the child class’ constructor
 Reminder, still use this (super not needed) to
access parent’s fields: this.xPos
The “super” Keyword
 Methods can call super.methodName(…)
 To do the work of the parent class method, plus…
 Additional work for the child class
public class Workaholic extends Worker {
public void doWork() {
super.doWork();
drinkCoffee();
super.doWork();
}
}
The “super” Keyword
 Methods can call super.methodName(…)
 To do the work of the parent class method, plus…
 Additional work for the child class
public class RoseStudent extends Worker {
public void doWork() {
while (!isCollapsed) {
super.doWork();
drinkCoffee();
}
super.doWork();
}
}
Rules of using super in constructors
 A super(…) call must be the first line of the
code of an object’s constructor if it is to be
used.
 Instance variables cannot be passed along with
the super(…) call. Only variables that are
passed to the constructor that calls super may
be passed to super.
The this Keyword
1. this.someField and this.someMethod(): nice style
2. this alone is used to represent the whole object: env.addBall(this)
3. this is used to call another constructor inside of a method with
multiple constructors.
public class foo {
private String message;
public foo(){
this(“This foo is saaaaaad.”);
}
public foo(String s){
message = s;
}
}
 this has the same restrictions on it as super – that is, it must be the
first thing called in a constructor and it cannot be passed instance
variables.
 Therefore, super(…) and this(…) cannot be used in the same
constructor.
Final notes
 Every object in Java extends java.lang.Object
 Don’t have to say it explicitly
 This is why every class has a basic toString() method.
 What does it mean for a field to be declared final?
 Final fields can’t be assigned a new value
 Final methods cannot be overridden
 Final classes cannot be extended
 There is only single inheritance in Java.
 Subclass can be derived only from one superclass.
Inheritance Basics
(a checklist to which to refer)
Classes extending other classes:
 Only have to describe how they are different from parents
 Don’t have to repeat methods which are “good enough” in the
parent
 Can replace or “extend” any method which isn’t “good enough”
 Can access parent’s methods and constrcutors via super.
 Can share fields with a parent in a “protected” way (say,
this.xPos)
 Only need to declare imports they use (like java.awt.Color)
 Don’t have to redeclare interfaces of the parent (like
Drawable)
Inheritance Demo
 Be sure you finish the quiz
 Then take a break
Polymorphism
 Polymorphism is the concept of allowing a
reference to a subclass to be used in a place where a
reference to its parent’s class would be acceptable.
 An easy and generic example is as follows:
 Object o = new Foo();
 Since every class extends Object, any object can be
stored in an Object variable.
 They just have extra info that can’t be accessed.
Superclass Subclass
Example
 In the bird and parrot example, consider a bird method:
static void printCall(Bird bird) {
System.out.println(bird.call);
}
 Generic: printBirdCall expects a Bird, but any type of
bird is OK.
 Cannot write Parrot p = new Bird(); //there’s not
enough info!
 However, without casting, b can only use bird methods.
Bird b = new Parrot();
printBirdCall(b)
Parrot p = new Parrot();
printBirdCall(p)
Casting and instanceof
 If we know that b is a Parrot, we can cast it and use Parrot
methods:
((Parrot)b).speak()
 Side note: Ellipse2D.Doubles have x coordinates, Shapes do not:
((Ellipse2D.Double)shape).x += xVelocity;
 At runtime, if b is just a Bird, the JVM will throw a
ClassCastException.
 To test this, use instanceof:
if (b instanceof Parrot) { ((Parrot)b).speak()) }
Late Binding: The Power of
Polymorphism
Hourly Employee h = new HourlyEmployee("Wilma Worker", new
Date("October", 16, 2005), 12.50, 170);
SalariedEmployee s = new SalariedEmployee("Mark Manager",
new Date("June", 4, 2006), 40000);
Employee e = null;
if (getWeekDay().equals(“Saturday”)
e = h;
else
e = s;
System.out.println(e);
When can I tell which value e will have, at compile time or run time?
So Java defers the decision about which version of toString() will be used until
then: it binds the actual method call used as late as possible.
Late Binding is also called dynamic dispatch or dynamic binding.
Note: it uses the most specific version of the method it can.
Overriding vs. Overloading
 Recall: overriding a method is when a subclass
has method with the same signature (name and
parameter list) as its superclass
 Mover’s act() and Bouncer’s act()
 Overloading a method is when two methods
have the same name, but different parameter
lists
 Arrays.sort(array, begin, end) and Arrays.sort(array)
Quiz #2
 What do you think?
Back to the demo
 This part is much shorter.
 Make sure you turn in the second quiz.

More Related Content

PPTX
Ruby object model
Chamnap Chhorn
 
PPTX
L03 Software Design
Ólafur Andri Ragnarsson
 
PDF
Java Inheritance
Rosie Jane Enomar
 
PDF
Java inheritance
Hamid Ghorbani
 
PPTX
Only oop
anitarooge
 
PPTX
OCA Java SE 8 Exam Chapter 5 Class Design
İbrahim Kürce
 
PPTX
Inheritance in Java beginner to advance with examples.pptx
naeemcse
 
PPTX
PPT Lecture-1.4.pptx
HimanshuPandey957216
 
Ruby object model
Chamnap Chhorn
 
L03 Software Design
Ólafur Andri Ragnarsson
 
Java Inheritance
Rosie Jane Enomar
 
Java inheritance
Hamid Ghorbani
 
Only oop
anitarooge
 
OCA Java SE 8 Exam Chapter 5 Class Design
İbrahim Kürce
 
Inheritance in Java beginner to advance with examples.pptx
naeemcse
 
PPT Lecture-1.4.pptx
HimanshuPandey957216
 

Similar to 06 InheritanceAndPolymorphism.ppt (20)

PPTX
OOPS in java | Super and this Keyword | Memory Management in java | pacakages...
Sagar Verma
 
PPTX
Pi j3.1 inheritance
mcollison
 
PPT
38 object-concepts
raahulwasule
 
PPTX
L02 Software Design
Ólafur Andri Ragnarsson
 
PPTX
03 object-classes-pbl-4-slots
mha4
 
PPTX
03 object-classes-pbl-4-slots
mha4
 
PPT
Chap11
Terry Yoast
 
PPT
38-object-concepts.ppt
Ravi Kumar
 
PPTX
Modules 333333333³3444444444444444444.pptx
radhikacordise
 
PPT
oops -concepts
sinhacp
 
PPT
38 object-concepts (1)
Shambhavi Vats
 
PPTX
Chapter5.pptxfghwryhYETHYETH67IOIKUTJJUILOUI
berihun18
 
PPTX
Core java oop
Parth Shah
 
DOC
Delphi qa
sandy14234
 
PDF
Chapter- 3 Inheritance and Polymorphism-1x4.pdf
joshua211619
 
PDF
Basic object oriented concepts (1)
Infocampus Logics Pvt.Ltd.
 
PDF
E3
lksoo
 
PPTX
Object oreinted php | OOPs
Ravi Bhadauria
 
PPT
Inheritance in java.ppt
SeethaDinesh
 
OOPS in java | Super and this Keyword | Memory Management in java | pacakages...
Sagar Verma
 
Pi j3.1 inheritance
mcollison
 
38 object-concepts
raahulwasule
 
L02 Software Design
Ólafur Andri Ragnarsson
 
03 object-classes-pbl-4-slots
mha4
 
03 object-classes-pbl-4-slots
mha4
 
Chap11
Terry Yoast
 
38-object-concepts.ppt
Ravi Kumar
 
Modules 333333333³3444444444444444444.pptx
radhikacordise
 
oops -concepts
sinhacp
 
38 object-concepts (1)
Shambhavi Vats
 
Chapter5.pptxfghwryhYETHYETH67IOIKUTJJUILOUI
berihun18
 
Core java oop
Parth Shah
 
Delphi qa
sandy14234
 
Chapter- 3 Inheritance and Polymorphism-1x4.pdf
joshua211619
 
Basic object oriented concepts (1)
Infocampus Logics Pvt.Ltd.
 
E3
lksoo
 
Object oreinted php | OOPs
Ravi Bhadauria
 
Inheritance in java.ppt
SeethaDinesh
 

Recently uploaded (20)

PPTX
ASSIGNMENT_1[1][1][1][1][1] (1) variables.pptx
kr2589474
 
PDF
Wondershare Filmora 14.5.20.12999 Crack Full New Version 2025
gsgssg2211
 
PDF
Exploring AI Agents in Process Industries
amoreira6
 
PPTX
AI-Ready Handoff: Auto-Summaries & Draft Emails from MQL to Slack in One Flow
bbedford2
 
PDF
Multi-factor Authentication (MFA) requirement for Microsoft 365 Admin Center_...
Q-Advise
 
PDF
Teaching Reproducibility and Embracing Variability: From Floating-Point Exper...
University of Rennes, INSA Rennes, Inria/IRISA, CNRS
 
PDF
Key Features to Look for in Arizona App Development Services
Net-Craft.com
 
PDF
Jenkins: An open-source automation server powering CI/CD Automation
SaikatBasu37
 
PDF
Why Use Open Source Reporting Tools for Business Intelligence.pdf
Varsha Nayak
 
PDF
How to Seamlessly Integrate Salesforce Data Cloud with Marketing Cloud.pdf
NSIQINFOTECH
 
PPTX
Explanation about Structures in C language.pptx
Veeral Rathod
 
PPTX
Why Use Open Source Reporting Tools for Business Intelligence.pptx
Varsha Nayak
 
PPT
Activate_Methodology_Summary presentatio
annapureddyn
 
PDF
49785682629390197565_LRN3014_Migrating_the_Beast.pdf
Abilash868456
 
PPTX
PFAS Reporting Requirements 2026 Are You Submission Ready Certivo.pptx
Certivo Inc
 
PPTX
Role Of Python In Programing Language.pptx
jaykoshti048
 
PPTX
The-Dawn-of-AI-Reshaping-Our-World.pptxx
parthbhanushali307
 
PDF
QAware_Mario-Leander_Reimer_Architecting and Building a K8s-based AI Platform...
QAware GmbH
 
PPTX
Services offered by Dynamic Solutions in Pakistan
DaniyaalAdeemShibli1
 
PPTX
TestNG for Java Testing and Automation testing
ssuser0213cb
 
ASSIGNMENT_1[1][1][1][1][1] (1) variables.pptx
kr2589474
 
Wondershare Filmora 14.5.20.12999 Crack Full New Version 2025
gsgssg2211
 
Exploring AI Agents in Process Industries
amoreira6
 
AI-Ready Handoff: Auto-Summaries & Draft Emails from MQL to Slack in One Flow
bbedford2
 
Multi-factor Authentication (MFA) requirement for Microsoft 365 Admin Center_...
Q-Advise
 
Teaching Reproducibility and Embracing Variability: From Floating-Point Exper...
University of Rennes, INSA Rennes, Inria/IRISA, CNRS
 
Key Features to Look for in Arizona App Development Services
Net-Craft.com
 
Jenkins: An open-source automation server powering CI/CD Automation
SaikatBasu37
 
Why Use Open Source Reporting Tools for Business Intelligence.pdf
Varsha Nayak
 
How to Seamlessly Integrate Salesforce Data Cloud with Marketing Cloud.pdf
NSIQINFOTECH
 
Explanation about Structures in C language.pptx
Veeral Rathod
 
Why Use Open Source Reporting Tools for Business Intelligence.pptx
Varsha Nayak
 
Activate_Methodology_Summary presentatio
annapureddyn
 
49785682629390197565_LRN3014_Migrating_the_Beast.pdf
Abilash868456
 
PFAS Reporting Requirements 2026 Are You Submission Ready Certivo.pptx
Certivo Inc
 
Role Of Python In Programing Language.pptx
jaykoshti048
 
The-Dawn-of-AI-Reshaping-Our-World.pptxx
parthbhanushali307
 
QAware_Mario-Leander_Reimer_Architecting and Building a K8s-based AI Platform...
QAware GmbH
 
Services offered by Dynamic Solutions in Pakistan
DaniyaalAdeemShibli1
 
TestNG for Java Testing and Automation testing
ssuser0213cb
 

06 InheritanceAndPolymorphism.ppt

  • 1. CSSE221: Software Dev. Honors Day 6  Announcements  Questions?  Hint on BallWorlds’ changing the x and y coords in 2nd half of lecture today.
  • 2. This week: BallWorlds assignment  Monday:  Intro to UML as a communication tool  Writing methods you don't call  Using this  Tuesday:  Inheritance  Polymorphism  Thursday:  Introducing next week’s assignment  Arrays and ArrayLists  (Using the debugger)
  • 3. Reminder  For the inheritance, polymorphism, and array groups, you owe me 3 things: 1. Summary 2. Quiz 3. Answer key
  • 4. Inheritance slides  Some material from those produced by Fall 2006-2007 CSSE221 students:  Michael Auchter  Michael Boland  Andrew Hettlinger
  • 5. Inheritance  Objects of different kinds (classes) have their own unique behavior.  Objects of different kinds often share similar behavior too.  For example:  Student, Professor, Software Engineer, Chemical Engineer, Physicist, Guitarist, Drummer  Each has distinct actions to perform, but they also have many features and behavior in common
  • 6. Why not just copy-and-paste?  Say I have an Employee class and want to create an HourlyEmployee class that adds info about wages. Why not copy-and-paste, then modify? 1. Fixing bugs: what if one were wrong? 2. Maintenance: what if Employee changes? 3. Code-reuse: would code that takes an Employee as a parameter also take an HourlyEmployee?
  • 7. The Basics of Inheritance  Inheritance allows you to reuse methods that you’ve already written to create more specialized versions of a class.  Java keyword: extends.  public class HourlyEmployee extends Employee.  We say that an HourlyEmployee IS-A Employee  Employee is said to be the parent class (or superclass), and HourlyEmployee is called a child class (or subclass).  HourlyEmployee receives copies of all of the non- private methods and variables present in Employee.
  • 8. Your turn  Quiz question: What is the relationship between a parrot and a bird?
  • 9. Some Key Ideas in Inheritance  Code reuse  Overriding methods  Protected visibility  The “super” keyword
  • 10. Code re-use  The subclass inherits all the public and protected methods and fields of the superclass.  Constructors are not inherited  Constructors can be invoked by the subclass  Subclass can add new methods and fields.
  • 11. Overriding Methods  DudThatMoves will “extend” Dud  If it defines an act() method with the same signature that overrides Dud’s method  What do you think happens if our child class doesn’t override a method in the parent class? It’s exactly the same as in the parent class!
  • 12. Visibility Modifiers • Public – Accessible by any other class in any package. • Private – Accessible only within the class. • Protected – Accessible only by classes within the same package and any subclasses in other packages. • (For this reason, some choose not to use protected, but use private with accessors) • Default (No Modifier) – Accessible by classes in the same package but not by classes in other packages. • Use sparingly!
  • 13. Protected Visibility  Suppose in Dud you defined: protected int xPos;  Then:  Instances of children inherit this field  (one for each instance)  Children can access/modify the fields  this.xPos in Dud (parent) and this.xPos in DudThatMoves (child) refer to same value
  • 14. The “super” Keyword  It’s like the word “this,” only “super”:  In a child class, “super” refers to its parent.  Two uses: 1. To call a parent’s method, use super.methodName(…) 2. To call a parent’s constructor, use super(some parameter) from the child class’ constructor  Reminder, still use this (super not needed) to access parent’s fields: this.xPos
  • 15. The “super” Keyword  Methods can call super.methodName(…)  To do the work of the parent class method, plus…  Additional work for the child class public class Workaholic extends Worker { public void doWork() { super.doWork(); drinkCoffee(); super.doWork(); } }
  • 16. The “super” Keyword  Methods can call super.methodName(…)  To do the work of the parent class method, plus…  Additional work for the child class public class RoseStudent extends Worker { public void doWork() { while (!isCollapsed) { super.doWork(); drinkCoffee(); } super.doWork(); } }
  • 17. Rules of using super in constructors  A super(…) call must be the first line of the code of an object’s constructor if it is to be used.  Instance variables cannot be passed along with the super(…) call. Only variables that are passed to the constructor that calls super may be passed to super.
  • 18. The this Keyword 1. this.someField and this.someMethod(): nice style 2. this alone is used to represent the whole object: env.addBall(this) 3. this is used to call another constructor inside of a method with multiple constructors. public class foo { private String message; public foo(){ this(“This foo is saaaaaad.”); } public foo(String s){ message = s; } }  this has the same restrictions on it as super – that is, it must be the first thing called in a constructor and it cannot be passed instance variables.  Therefore, super(…) and this(…) cannot be used in the same constructor.
  • 19. Final notes  Every object in Java extends java.lang.Object  Don’t have to say it explicitly  This is why every class has a basic toString() method.  What does it mean for a field to be declared final?  Final fields can’t be assigned a new value  Final methods cannot be overridden  Final classes cannot be extended  There is only single inheritance in Java.  Subclass can be derived only from one superclass.
  • 20. Inheritance Basics (a checklist to which to refer) Classes extending other classes:  Only have to describe how they are different from parents  Don’t have to repeat methods which are “good enough” in the parent  Can replace or “extend” any method which isn’t “good enough”  Can access parent’s methods and constrcutors via super.  Can share fields with a parent in a “protected” way (say, this.xPos)  Only need to declare imports they use (like java.awt.Color)  Don’t have to redeclare interfaces of the parent (like Drawable)
  • 21. Inheritance Demo  Be sure you finish the quiz  Then take a break
  • 22. Polymorphism  Polymorphism is the concept of allowing a reference to a subclass to be used in a place where a reference to its parent’s class would be acceptable.  An easy and generic example is as follows:  Object o = new Foo();  Since every class extends Object, any object can be stored in an Object variable.  They just have extra info that can’t be accessed. Superclass Subclass
  • 23. Example  In the bird and parrot example, consider a bird method: static void printCall(Bird bird) { System.out.println(bird.call); }  Generic: printBirdCall expects a Bird, but any type of bird is OK.  Cannot write Parrot p = new Bird(); //there’s not enough info!  However, without casting, b can only use bird methods. Bird b = new Parrot(); printBirdCall(b) Parrot p = new Parrot(); printBirdCall(p)
  • 24. Casting and instanceof  If we know that b is a Parrot, we can cast it and use Parrot methods: ((Parrot)b).speak()  Side note: Ellipse2D.Doubles have x coordinates, Shapes do not: ((Ellipse2D.Double)shape).x += xVelocity;  At runtime, if b is just a Bird, the JVM will throw a ClassCastException.  To test this, use instanceof: if (b instanceof Parrot) { ((Parrot)b).speak()) }
  • 25. Late Binding: The Power of Polymorphism Hourly Employee h = new HourlyEmployee("Wilma Worker", new Date("October", 16, 2005), 12.50, 170); SalariedEmployee s = new SalariedEmployee("Mark Manager", new Date("June", 4, 2006), 40000); Employee e = null; if (getWeekDay().equals(“Saturday”) e = h; else e = s; System.out.println(e); When can I tell which value e will have, at compile time or run time? So Java defers the decision about which version of toString() will be used until then: it binds the actual method call used as late as possible. Late Binding is also called dynamic dispatch or dynamic binding. Note: it uses the most specific version of the method it can.
  • 26. Overriding vs. Overloading  Recall: overriding a method is when a subclass has method with the same signature (name and parameter list) as its superclass  Mover’s act() and Bouncer’s act()  Overloading a method is when two methods have the same name, but different parameter lists  Arrays.sort(array, begin, end) and Arrays.sort(array)
  • 27. Quiz #2  What do you think?
  • 28. Back to the demo  This part is much shorter.  Make sure you turn in the second quiz.