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Inheritance, Polymorphism, and Interfaces
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Advanced PlacementComputer Science
Inheritance and Polymorphism^ What’s past is prologue.Don’t write it twice — write it once and reuse it.
Bekki George
James E. Taylor HS, Katy
Inheritance, Polymorphism, and Interfaces
Main Tenants of OO Programming^ Encapsulation– abstraction, information hiding, responsibilitydriven programming^ Inheritance– code reuse, specialization "New code using oldcode."^ Polymorphism– do X for a collection of various types of objects,where X is
different
depending on the type of
object– "Old code using new code."
Inheritance, Polymorphism, and Interfaces
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Explanation of Inheritance 1 of the fundamental principles of OOP– allows code reuse Models the
IS-A^ relationship
– a student is-a person– an undergraduate is-a student– a rectangle is-a shape– a rook is-a piece Contrast with the
Has-A
relationship (or uses a)
– a student has-a name– a rook has-a position– a Stack uses a List Is-a relationships indicate inheritance, has-arelationships indicate composition (fields)
Inheritance, Polymorphism, and Interfaces
Nomenclature of Inheritance The^ extends
keyword is used to specify which
preexisting class a new class is inheriting from^ public class Student extends Person^ Person is said to be– the parent class of Student– the super class of Student– the base class of Student– an ancestor of Student^ Student is said to be– a child class of Person– a sub class of Person– a derived class of Person– a descendant of Person
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The Mechanics of Inheritance Java is a pure object oriented language all code is part of some class all classes, except one, must inherit fromexactly one other class The^ Object
class is the
cosmic super class
-^ The
Object
class does not inherit from any other class
-^ The
Object
class has several important methods:
toString
,^ equals
,^ hashCode
,^ clone
,^ getClass
implications:– all classes are descendants of
Object
-^ all classes, and thus all objects, have a
toString
equals
,^ hashCode
,^ clone
, and^ getClass
method
-^ toString
,^ equals
,^ hashCode
,^ clone^
normally overridden
Inheritance, Polymorphism, and Interfaces
Inheriting from a Class
If a class header does not include theextends clause the class extends the Object
class by default
public class Card– Object
is an ancestor to all classes
– it is the only class that does not extend someother class A class extends exactly one other class– extending two or more classes is
multiple
inheritance.
Java does not support this directly,
rather it uses
Interfaces
Inheritance, Polymorphism, and Interfaces
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Implications of Inheritance The sub class gains all of the behavior (methods) and dataregarding state (instance variables) of the super class
and
all ancestor classes Sub classes can:– add new fields– add new methods–^ override
existing methods (change behavior)
Sub classes may not– remove fields– remove methods Note, even though an object may have instance variablesfrom its parent they may not be accessible by the code ofthe child class if the fields are private
Inheritance, Polymorphism, and Interfaces
The Real PictureFields from Object classInstance variablesdeclared in ObjectFields from String classInstance Variablesdeclared in StringBehaviors (methods) from String classand Object class.
A Stringobject
Inheritance, Polymorphism, and Interfaces
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A ClosedShape class
public
class^
ClosedShape {^ private
int^ iMyX; private
int^ tMyY; public
ClosedShape() {^ this(0,0);
public
ClosedShape
(int^
x,^ int
y)
{^ iMyX
=^ x;iMyY =^ y; } public
String
toString() {^ return
"x:^ "^
+^ iMyX
+^ "^ y:
"^ +^ iMyY;
public
int^ getX(){
return
iMyX;
public
int^ getY(){
return
iMyY;
}//^ Other
methods
not^ shown
Inheritance, Polymorphism, and Interfaces
Constructors
Constructors handle initialization of objects When creating an object with one or more ancestors (everytype except Object) a chain of constructor calls takes place The reserved word
super
may be used in a constructor to
specify which of the parent's constructors to call– must be first line of constructor if no parent constructor is explicitly called the default, 0parameter constructor of the parent is called– if no default constructor exists a syntax error results If a parent constructor is called another constructor in thesame class may no be called– no^ super();this();
allowed. One or the other, not both
- good place for an initialization method
Inheritance, Polymorphism, and Interfaces
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A Rectangle Constructor
public class Rectangle extends ClosedShape{^ private int iMyWidth;private int iMyHeight;public Rectangle(int width, int height,
int x, int y)
{^ super(x,y);// calls the 2 int constructor in// ClosedShapeiMyWidth = width;iMyHeight = height;} // other methods not shown}
Inheritance, Polymorphism, and Interfaces
A Rectangle Class public class^ Rectangle
extends
ClosedShape
{^ private
int^ iMyWidth;private int^ iMyHeight;public Rectangle(){ this(0,^ 0);} public Rectangle(int
width,^
int^ height)
{^ this(width,
height,
0,^ 0); } public^ Rectangle(int
width,^
int^ height, int^ x,^ int
y) {^ super(x,
y); iMyWidth
=^ width; iMyHeight
=^ height; } public^ String
toString() {^ return
super.toString()
+^ "^ width:
"^ +^ iMyWidth
+^ "^ height:
"^ +^ iMyHeight; }}
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Initialization method public class Rectangle
extends ClosedShape
{^ private int iMyWidth;private int iMyHeight;public Rectangle(){^ init(0, 0);} public Rectangle(int width, int height){^ init(width, height);} public Rectangle(int width, int height,
int x, int y){ super(x, y);init(width, height);} private void init(int width, int height){ iMyWidth = width;iMyHeight = height;}
Inheritance, Polymorphism, and Interfaces
Overriding methods
any method that is not
final
may be
overridden by a descendant class– overriding is a replacement of a behavior– overloading is the addition of a behavior^ same signature as method in ancestor^ may not reduce visibility^ may use the original method if simply want toadd more behavior to existing– also called
partial overriding
The Rectangle class– adds data, partially overrides
toString
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The Keyword
super
super
is used to access something (any protected or
public field or method) from the super class that hasbeen overridden Rectangle's
toString
makes use of the
toString
in
ClosedShape my calling
super.toString()
without the super calling
toString
would result in
infinite recursive calls Java does not allow nested supers^ super.super.toString() results in a syntax error even though technically thisrefers to a valid method, Object's
toString
Rectangle
partially
overrides ClosedShape's
toString
Inheritance, Polymorphism, and Interfaces
What Can Rectangles Do? Rectangle
r1^ =^
new^ Rectangle(); Rectangle
r2^ =^
new^ Rectangle(10,
Rectangle
r3^ =^
new^ Rectangle(10,
15,^ 2,
System.out.println(
r1^ );
System.out.println(
r2^ );
System.out.println(
r3^ );
int^ a^
=^ r1.getX()
+^ r1.getY(); ClosedShape
s^ =^ new
Rectangle(5,
10,^ 3,
System.out.println(
s.toString()
a^ +=^ s.getX();ClosedShape[]
sList
=^ new
ClosedShape[3];
sList[0]
=^ new
ClosedShape(5,
sList[1]
=^ new
Rectangle(10,
25,^ 10,
sList[2]
=^ r2; for(int
i^ =^ 0;
i^ <^ sList.length;
i++)
System.out.println(
sList[i].toString()
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A^ Circle Class
public
class
Circle
extends
ClosedShape
{^ int
iMyRadius;public^ Circle(){ this(1);
public
Circle(int
radius) {^ iMyRadius
=^ radius; } public
Circle(int
radius,
int^ x,
int^ y)
{^ super(x,y);iMyRadius
=^ radius; } public
int^ getArea() {^ return
Math.PI
*^ iMyRadius
*^ iMyRadius;
public
String
toString() {^ return
super.toString()
+^ "^ radius:
"^ +^ iMyRadius;
Inheritance, Polymorphism, and Interfaces
Polymorphism in Action public class UsesShapes{ public static void go(){ ClosedShape[] sList = new ClosedShape[10];int a, b, c, d;int x;for(int i = 0; i < 10; i++){^ a = (int)(Math.random() * 100);b = (int)(Math.random() * 100);c = (int)(Math.random() * 100);d = (int)(Math.random() * 100);x = (int)(Math.random() * 3 );if( x == 0 )
sList[i] = new Rectangle(a,b,c,d);else if(x == 1)sList[i] = new Square(a,c,d);elsesList[i] = new Circle(a,c,d); }int total =0;for(int i = 0; i < 10; i++){^ total += sList[i].getArea();System.out.println( sList[i] );}} }
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The Kicker
We want to expand our pallet of shapes Triangle could also be a sub class ofClosedShape.– it would
inherit
from ClosedShape
public int getArea(){ return 0.5 * iMyWidth * iMyHeight;} What changes do we have to make to thecode on the previous slide for totaling areaso it will now handle Triangles as well? Power.
Inheritance, Polymorphism, and Interfaces
Object Variables
The above code works if Rectangle extendsClosedShape An object variable may point to an object of itsbase type or a descendant in the inheritance chain– The is-a relationship is met. A Rectangle object is-ashape so
s^ may point to it
This is a form of
polymorphism
and is used
extensively in the Java
Collection
classes
Rectangle r = new Rectangle(10, 20);ClosedShape s = r;System.out.println("Area is " + s.getArea());– Vector, ArrayList are lists of Objects
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polymorphism allows s to point at a Rect object butthere are limitations The above code will not compile Statically,
s^ is declared to be a shape
- no changeWidth method in Shape class– must
cast^ s^
Type Compatibilityto a^ Rectangle
Rectangle r = new Rectangle(5, 10);ClosedShape s = r;s.changeWidth(20); // syntax errorRectangle r = new Rectangle(5, 10);Shape s = r;((Rectangle)s).changeWidth(20); //Okay
Inheritance, Polymorphism, and Interfaces
Problems with Casting
The following code compiles but a Class CastException is thrown at runtime Casting must be done carefully and correctly If unsure of what type object will be the use the instanceof
operator or the
getClass()
Rectangle r = new Rectangle(5, 10);Circle c = new Circle(5);Shape s = c;((Rectangle)s).changeWidth(4); method^ expression instanceof ClassName
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Multiple Inheritance
Inheritance models the "is-a" relationshipbetween real world things one of the benefits is code reuse,completing programs faster, with less effort in the real world a thing can have "is-a"relationships with several other things– a Graduate Teaching Assistant is-a GraduateStudent. Graduate Teaching Assistant is-aFaculty Member– a Student is-a Person. a Student is aSortableObject
Inheritance, Polymorphism, and Interfaces
Interfaces
A Java
interface
is a "pure abstract
class".– Design only, no implementation.^ Interfaces are declared in a way similar toclasses but– consist only of public abstract methods– public final static fields^ A Java class extends exactly one otherclass, but can implement as many interfacesas desired
