Chapter 13: Inheritance and Polymorphism

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Chapter 13 Inheritance and Polymorphism

• CS180 Fall 2006

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Announcements

• Project 5 grading is wait for it wait for
  itDONE!
• Grades will be released by early next week
• Gradesheets handed back in your labs
• Hand back exam 2
• Mean 67.8
• Median 71
• High 97
• Next week generics
• Online chapter
• You should be able to access it with the password
  that came with your book (if you bought it in the
  bookstore)
• Or go here http//www.cs.purdue.edu/mrcarlso/c
  s180
• We will provide a user/password for you

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Definitions

• Inheritance object oriented way to form new
  classes from pre-existing ones
• Superclass
• The parent class
• If class is final, cannot inherit from this class
• Subclass
• Class which inherits all non-private properties
  from the superclass
• Generally a specific instance of a superclass
  (Animal-gtFerret)
• Polymorphism a object can take many forms
• Not all animals are ferrets, could be platypuss
  as well
• A makeSound() method may be different for a
  ferret and a platypus
• Combining the two, can create subclasses with
  different properties
• Your platypus class may have a swim() method, but
  ferrets cannot swim!

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Inheriting From the Superclass

• Two ways
• Use the protected keyword instead of the private
  keyword
• Variables and methods which are protected are
  accessible by the subclass, but not the world
• Use public for methods to inherit, private for
  methods to not inherit and all variables
• Variables no longer accessible directly in
  subclass
• Use accessors and mutators to modify them
• Overwrite public (or protected) methods as
  necessary
• If a method is declared final, then it cannot be
  overwritten in a subclass
• Can also overload methods if desired
• Constructors
• Not inherited! Must rewrite them
• Use super keyword to call superconstructor from
  parent class
• Can only use super as first line of a constructor
• If no superconstructor is explicitly called, then
  super() (the default superconstructor) is
  automatically called as the first line
• super keyword can also be used to call instances
  of methods from superclasses

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Syntax

• To extend a class, use the extends keyword
• The class will now inherit all non-private
  characteristics of the superclass
• Do not have to re-write methods which are the
  same
• Can introduce new methods and/or variables

public class A private int x public A()
set(0) public A(int x) set(x) public
void set(int x) this.x x public int get()
return x public void add1() bump()
private void bump() x public class B
extends A public B() super() public
B(int x) super(x) public void add1()
set(get()1)
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Inheriting From the Superclass
public class A protected int x public A()
set(0) public A(int x) set(x) public
void set(int x) this.x x public int get()
return x public void add1() bump()
private void bump() x public class B
extends A public B() super() public
B(int x) super(x)
public class A private int x public A()
set(0) public A(int x) set(x) public
void set(int x) this.x x public int get()
return x public void add1() bump()
private void bump() x public class B
extends A public B() super() public
B(int x) super(x) public void add1()
set(get()1)
The add1() method is overwritten here!
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Inheriting From the Superclass
On the superclass constructor calls, constructors
are executed top-down on the hierarchy.
public class A A() SOP(A) public
class B extends A public B() SOP(B)
public class C extends A public C()
SOP(C) public class D extends B public
D() SOP(D)
A new A() Output A B new B() Output
AB C new C() Output AC D new
D() Output ABD
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Object Class

• Class Object is the superclass of all
  superclasses in Java
• All classes defined implicitly inherit from
  Object if not explicitly extending from another
  class
• Even if explicitly extending another class, that
  other class either extends from another class or
  implicitly extends from Object
• Some useful methods which are commonly
  overwritten
• equals for comparisons (defaults to reference
  address comparision)
• toString translates your object into an
  appropriate String (defaults to the reference
  address)

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Polymorphism
public class Animal protected int
age public void makeSound() SOP(Make
sound!) public class Ferret extends Animal
public Ferret() super() public void
eatSweets() SOP(Yummy!) public void
makeSound() SOP(Squeal!) public class
Platypus extends Animal public Platypus()
super() public void swim() SOP(Swim!)
public void makeSound() SOP(Quack!)
_____________________________________________
Animal a new Ferret() a.swim() //
illegal! Not all animals know how to
swim! a.makeSound() // legal! All animals make
sounds! a.eatSweets() // illegal! Only
ferrets like candies and cookies!

• Allows a variable to take on many forms
• A variable of type Animal may point to may
  different types of animals
• Only methods scoped within the variable type are
  available
• Not all animals swim, so even though an Animal
  object may point to a platypus, you cannot call
  the swim() method

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Dynamic Binding

• On the call the makeSound(), what happens?
• If Animal object a is a Platypus, then Quack!
• If Animal object a is a Ferret, then Squeal!
• Why not Make sound!? ? dynamic binding!
• Objects have two types
• Static type the type of the variable name
• Dynamic type the actual type of the variable
  in memory
• Dynamic binding
• Used for method lookup
• Look up most specific instance of method call
  (look up the method in the dynamic type if it
  exists)
• Most specific instance of a method may be in a
  parent class

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Dynamic Binding
public class A protected int x public A()
set(0) public A(int x) set(x) public
void set(int x) this.x x public int get()
return x public void bump() x2
public class B extends A public B()
super() public B(int x) super(x)
public void bump() x
static type
dynamic type
A a new B() a.bump() SOP(aa.get())
Object a is of static type A, dynamic type B. On
a method call, use dynamic type first!
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Determining Class Type

• In order to access a method in a subclass from an
  object with a static type of the superclass, need
  to downcast
• Downcasting is unsafe! Need to know that
  downcast is legal.
• Use instanceof keyword or other methods
• x instanceof A true if x is an instance of
  class A or a subclass of A
• x.getClass().equals(A.class) true if x is an
  instance of class A
• A.class.isAssignableFrom(x.getClass()) is true if
  x is an instance of class A or a subclass of A

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Determining Class Type
public class Animal protected int
age public void makeSound() SOP(Make
sound!) public class Ferret extends Animal
public Ferret() super() public void
eatSweets() SOP(Yummy!) public void
makeSound() SOP(Squeal!) public class
Platypus extends Animal public Platypus()
super() public void swim() SOP(Swim!)
public void makeSound() SOP(Quack!)
Animal a new Ferret() if (a instanceof
Ferret) ((Ferret)a).eatSweets() a new
Platypus() if (a.getClass()
Platypus.class) ((Platypus)a).swim() if
(Platypus.class.isAssignableFrom(a)) ((Platypus)a
).swim() ________________________________ Animal
a new Ferret() ((Platypus)a).swim() //
compiles, but runtime error!
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Abstract Classes

• Sometimes a superclass never needs to be
  instantiated you only make instances of the
  subclasses
• Make class abstract
• Cannot be instantiated
• May have some methods be abstract
• Is like a blueprint for subclasses, with methods
  that need to be filled in

public abstract class Animal protected int
age public abstract void makeSound() public
class Ferret extends Animal public Ferret()
super() public void eatSweets()
SOP(Yummy!) public void makeSound()
SOP(Squeal!) public class Platypus
extends Animal public Platypus() super()
public void swim() SOP(Swim!) public
void makeSound() SOP(Quack!)
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Interfaces

• Contains method headers
• Cannot instantiate interface
• Cannot have any non-final variables
• A class can implement many interfaces
• Interfaces can extend from each other
• Example Comparable
• Contains one method, compareTo, which takes two
  objects and compares them
• Returns a negative number if less than
• Returns 0 if equal to
• Returns a positive number if greater than
• Comparable is an interface and not an abstract
  class because not all things comparable are
  related
• Numbers and people are both comparable, but have
  no reasonable relationship between them

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Interfaces
public interface I public void
doStuff() public interface J extends I
public void doMoreStuff() public interface
K public void doEvenMoreStuff() public
class A implements J, K public void doStuff()
public void doMoreStuff() public void
doEvenMoreStuff
J extends I, so interface J actually has 2
methods doStuff() and doMoreStuff().
A implements J and K, so A must have a method for
each method in the interfaces it implements.
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Putting It All Together
public abstract class Animal implements
SoundMaker protected int age public
abstract void makeSound() public class Ferret
extends Animal public Ferret() super()
public void eatSweets() SOP(Yummy!)
public void makeSound() SOP(Squeal!)
public class Platypus extends Animal
implements EggLayer, Swimmer public Platypus()
super() public void swim() SOP(Swim!)
public void makeSound() SOP(Quack!)
public void layEgg() SOP(Egg layed!)
public class Trumpet extends SoundMaker
public void makeSound() SOP(Toot!)
public interface SoundMaker public void
makeSound() public interface EggLayer
public void layEgg() public interface
Swimmer public void swim()
Images taken from Wikipedia.
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Abstract Class vs. Interface

• Abstract classes are blueprints, interfaces are
  contracts
• Interface Heres some methods. If your class
  implements this interface, you must provide an
  implementation of each method in the interface in
  the class.
• Method headers
• final variables
• A class can implement multiple interfaces
• Cannot instantiate
• Abstract class Heres a blueprint for a class.
  If your class extends this abstract class, you
  can use any functionality here and add onto it,
  but you must fill in what I have not.
• Abstract method headers
• Methods
• Variables
• A class can extend only one class
• Cannot instantiate

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Next Week, in CS180(think TV show voiceover)

• Recall that arrays are fixed in size
• Dynamic data structures allow us to create
  collections of objects which vary in size
• Many found in java.util
• Standard collections
• Vector (a resizable array)
• LinkedList (a chain of objects)
• ArrayList (a hybrid of vectors and linked lists)
• Special collections
• Queue (interface first in first out structure)
• Stack (last in first out structure)
• We will talk about these structures in much more
  detail next week with generics (stay tuned)

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Static Binding (optional)

• The keyword static means that a variable or
  method lookup is resolved at compile time
• In Java
• Methods in a class are dynamically bound (unless
  said to be static)
• Lookup resolved at runtime with dynamic binding
• Variables in a class are statically bound
• Use static binding to lookup values
• This rarely occurs unless you make your variables
  public (which you shouldnt)

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Static Binding (optional)
public class A public int x public A()
set(0) public A(int x) set(x) public
void set(int x) this.x x public int get()
return x public void bump() x2
public class B extends A public B()
super() public B(int x) super(x)
public void bump() x
A a new B() a.bump() SOP(aa.x) B b
new B(4) b.set(15) SOP(bb.x)
bs static and dynamic type are both B. On the
call to set, dynamic binding dictates that the
set method in class A is called. Since we are in
class A, the variable x, statically bound, is
used.
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Static Binding (optional)
public class A public int x public A()
set(0) public A(int x) set(x) public
void set(int x) this.x x public int get()
return x public void bump() x2
public class B extends A public int x
-1 public B() super() public B(int x)
super(x) public void bump() x
A a new B() a.bump() SOP(aa.x) B b
new B(4) b.set(15) SOP(bb.x)
bs static and dynamic type are both B. On the
call to set, dynamic binding dictates that the
set method in class A is called. Since we are in
class A, the variable x in class A, statically
bound, is used. However, in the print out, x is
statically retrieved from a B static type object,
and thus -1 is printed out.
Moral None of this happens if 1) your
variables are private, and 2) you dont
re-declare the same objects in a subclass!