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1. ProgrammingWithJava
ICS201
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Chapter 8
Polymorphism

2. ProgrammingWithJava
ICS201 Object-Oriented Concept
 Object & Class
 Encapsulation
 Inheritance
 Polymorphism
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3. ProgrammingWithJava
ICS201
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Polymorphism
 polymorphism: many forms

4. ProgrammingWithJava
ICS201
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Polymorphism
 Polymorphism (from the Greek, meaning "many
forms", or something similar)
 The mechanism by which several methods can have the
same name and implement the same abstract
operation.
 Requires that there be multiple methods of the
same name
 The choice of which one to execute depends on the
object that is in a variable
 Reduces the need for programmers to code many if-
else or switch statements

5. ProgrammingWithJava
ICS201 Polymorphism
DrawChart
DrawChart(1)
DrawChart(1,2,1,2)
DrawChart(1,1,1)
DrawTriangle(1,1,1)
DrawRect(1,2,1,2)
DrawCircle(1)
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6. ProgrammingWithJava
ICS201
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Polymorphism Example

7. ProgrammingWithJava
ICS201 Polymorphism
 Add(integer, integer)
 Add(string, string)
 Add(string, integer)
 Add(1,1)  2
 Add(“Hello”, “World”) 
“HelloWorld”
 Add(“Hello”, 2)  “Hello2”

8. ProgrammingWithJava
ICS201 Polymorphism
:PaySlip
:HourlyPaidEmployee
:WeeklyPaidEmployee
:MonthlyPaidEmployee

getTotalPay()
calculatePay()
calculatePay()
calculatePay()
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9. ProgrammingWithJava
ICS201
Which bill() version ?
Which bill() version ?
Which bill() versions ?
Introductory example to Polymorphism

10. ProgrammingWithJava
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Polymorphism
 Methods can be overloaded. A method has the same
name as another member methods, but the type and/or
the count of parameters differs.
class Integer
{
float val;
void add( int amount )
{
val = val + amount;
}
void add( int num, int den)
{
val = float(num) / den;
}
}

11. ProgrammingWithJava
ICS201
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Polymorphism
 Polymorphism is the ability to associate many meanings
to one method name
 It does this through a special mechanism known as
late binding or dynamic binding
 Inheritance allows a base class to be defined, and other
classes derived from it
 Code for the base class can then be used for its own
objects, as well as objects of any derived classes
 Polymorphism allows changes to be made to method
definitions in the derived classes.

12. ProgrammingWithJava
ICS201
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Late Binding
 The process of associating a method definition with a
method invocation is called binding.
 If the method definition is associated with its call when
the code is compiled, that is called early binding.
 If the method definition is associated with its call when
the method is invoked (at run time), that is called late
binding or dynamic binding.

13. ProgrammingWithJava
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The Sale and DiscountSale Classes
 The Sale class contains two instance variables
 name: the name of an item (String)
 price: the price of an item (double)
 It contains two constructors
 A no-argument constructor that sets name to "No
name yet", and price to 0.0
 A two-parameter constructor that takes in a String
(for name) and a double (for price)

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The Sale and DiscountSale Classes
 The Sale class also has
 a set of accessors (getName, getPrice),
 mutators (setName, setPrice),
 overridden methods equals and toString
 a static announcement method.
 a method bill, that determines the bill for a sale, which
simply returns the price of the item.
 It has also two methods, equalDeals and lessThan,
each of which compares two sale objects by comparing
their bills and returns a boolean value.

15. ProgrammingWithJava
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The Sale and DiscountSale Classes
 The DiscountSale class inherits the instance variables and
methods from the Sale class.
 In addition, it has its own instance variable, discount (a
percent of the price),
 it has also its own suitable constructor methods,
 accessor method (getDiscount),
 mutator method (setDiscount),
 overriden toString method, and static announcement
method.
 It has also its own bill method which computes the bill as a
function of the discount and the price.

16. ProgrammingWithJava
ICS201The Sale and DiscountSale Classes
 The Sale class lessThan method
 Note the bill() method invocations:
public boolean lessThan (Sale otherSale)
{
if (otherSale == null)
{
System.out.println("Error:null object");
System.exit(0);
}
return (bill( ) < otherSale.bill( ));
}

17. ProgrammingWithJava
ICS201The Sale and DiscountSale Classes
 The Sale class bill() method:
public double bill( )
{
return price;
}
 The DiscountSale class bill() method:
public double bill( )
{
double fraction = discount/100;
return (1 - fraction) * getPrice( );
}

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The Sale and DiscountSale Classes
 Given the following in a program:
. . .
Sale simple = new sale(“xx", 10.00);
DiscountSale discount = new DiscountSale(“xx", 11.00, 10);
. . .
if (discount.lessThan(simple))
System.out.println("$" + discount.bill() +
" < " + "$" + simple.bill() +
" because late-binding works!");
. . .
 Output would be:
$9.90 < $10 because late-binding works!

19. ProgrammingWithJava
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The Sale and DiscountSale Classes
 In the previous example, the boolean expression in the if
statement returns true.
 As the output indicates, when the lessThan method in
the Sale class is executed, it knows which bill() method
to invoke
 The DiscountSale class bill() method for discount,
and the Sale class bill() method for simple.
 Note that when the Sale class was created and compiled,
the DiscountSale class and its bill() method did not yet
exist
 These results are made possible by late-binding

20. ProgrammingWithJava
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The final Modifier
 A method marked final indicates that it cannot be
overridden with a new definition in a derived class
 If final, the compiler can use early binding with the
method
public final void someMethod() { . . . }
 A class marked final indicates that it cannot be used as a
base class from which to derive any other classes

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Upcasting and Downcasting
 Upcasting is when an object of a derived class is assigned
to a variable of a base class (or any ancestor class):
Example:
class Shape {
int xpos, ypos ;
public Shape(int x , int y){
xpos = x ;
ypos = y ;
}
public void Draw() {
System.out.println("Draw method called of class Shape") ;
}
}

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Upcasting and Downcasting
class Circle extends Shape {
int r ;
public Circle(int x1 , int y1 , int r1){
super(x1 , y1) ;
r = r1 ;
}
public void Draw() {
System.out.println("Draw method called of class Circle") ;
}
}
class UpcastingDemo {
public static void main (String [] args) {
Shape s = new Circle(10 , 20 , 4) ;
s.Draw() ;
}
}
Output:
Draw method called of class Circle

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Upcasting and Downcasting
 When we wrote Shape S = new Circle(10 , 20 , 4), we
have cast Circle to the type Shape.
 This is possible because Circle has been derived from Shape
 From Circle, we are moving up to the object hierarchy to
the type Shape, so we are casting our object “upwards” to
its parent type.

24. ProgrammingWithJava
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Upcasting and Downcasting
 Downcasting is when a type cast is performed from a
base class to a derived class (or from any ancestor class
to any descendent class).
Example:
class Shape {
int xpos, ypos ;
public Shape(int x , int y){
xpos = x ;
ypos = y ;
}
public void Draw() {
System.out.println("Draw method called of class Shape") ;
}
}

25. ProgrammingWithJava
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Upcasting and Downcasting
class Circle extends Shape {
int r ;
public Circle(int x1 , int y1 , int r1){
super(x1 , y1) ;
r = r1 ;
}
public void Draw() {
System.out.println("Draw method called of class Circle") ;
}
public void Surface() {
System.out.println("The surface of the circle is " +
((Math.PI)*r*r));
}
}

26. ProgrammingWithJava
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Upcasting and Downcasting
class DowncastingDemo {
public static void main (String [] args) {
Shape s = new Circle(10 , 20 , 4) ;
((Circle) s).Surface() ;
}
}
Output:
The surface of the circle is 50.26

27. ProgrammingWithJava
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Upcasting and Downcasting
 When we wrote Shape s = new Circle(10 , 20 , 4) we have
cast Circle to the type shape.
 In that case, we are only able to use methods found in
Shape, that is, Circle has inherited all the properties and
methods of Shape.
 If we want to call Surface() method, we need to down-
cast our type to Circle. In this case, we will move down
the object hierarchy from Shape to Circle :
 ((Circle) s).Surface() ;

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Downcasting
 It is the responsibility of the programmer to use
downcasting only in situations where it makes sense
 The compiler does not check to see if downcasting is a
reasonable thing to do
 Using downcasting in a situation that does not make sense
usually results in a run-time error.