1. Polymorphism

2. 3 main programming mechanisms that constitute OOP:
Encapsulation
Inheritance
Polymorphism

3. Polymorphism
The ability to associate many meanings to one method name by means of a special mechanism known as late binding or dynamic binding.
Allows one to make changes in the method definition for the derived classes and have those changes apply to the software written in the base class.

4. Binding
Binding – the process of associating a method definition with a method invocation

5. Binding types early/static binding late/dynamic binding
the method definition is associated with the method invocation when the code is compiled
late/dynamic binding
the method invocation is associated with the method invocation when the method is invoked (at run time)
Java uses late binding except for a few cases.

6. Late binding example class Figure draw() method that draws a point
center() method that moves the object to the center of the screen and calls draw()
superclass for drawing with the following subclasses:
Rectangle
draw() method that draws a rectangle
Circle
draw() method that draws a circle
Oval
draw() method that draws an oval

7. Late binding example We add a new subclass of Figure called Triangle.
Do we need to recompile Figure (because Figure’s center() method will call Triangle’s draw() method)?

8. Late binding example We add a new subclass of Figure called Triangle.
Do we need to recompile Figure (because Figure’s center() method will call Triangle’s draw() method)? NO!
What mechanism makes this work?

9. Late binding example We add a new subclass of Figure called Triangle.
Do we need to recompile Figure (because Figure’s center() method will call Triangle’s draw() method)? NO!
What mechanism make this work?
Late binding!

10. Late binding example
What would happen (when Figure’s center() calls draw() for a Triangle) if we didn’t have late binding but had early binding instead?

11. Late binding example
What would happen (when Figure’s center() calls draw() for a Triangle) if we didn’t have late binding but had early binding instead?
Figure’s draw() would be called instead of Triangle’s draw().

12. Late binding Late binding is not “free.”
Some additional overhead at runtime is required.

13. final Recall the final keyword. What happens for an instance variable?
What happens for a method?
What happens for a class?

14. Late binding exceptions
Java does not use late binding with:
Private methods
Methods marked final
Static methods
Static binding is used instead.

15. Example class Sale { public static void announcement ( ) {
System.out.println( “This is the Sale class.” ); }
public void showAd ( ) {
System.out.println( “buy sale” );
class DiscountSale extends Sale {
System.out.println( “This is the DiscountSale class.” );
System.out.println( “buy discount sale” );

16. public class SaleTest {
public static void main ( String args[] ) {
Sale s = new Sale();
DiscountSale d = new DiscountSale();
s.announcement();
d.announcement();
s.showAd();
d.showAd();
s = d;
System.out.println( s.toString() ); }
class Sale {
public static void announcement ( ) {
System.out.println( “This is the Sale class.” ); }
public void showAd ( ) {
System.out.println( “buy sale” );
class DiscountSale extends Sale {
System.out.println( “This is the DiscountSale class.” );
System.out.println( “buy discount sale” );
This is the Sale class.
This is the DiscountSale class.
buy sale
buy discount sale

17. public class SaleTest {
public static void main ( String args[] ) {
Sale s = new Sale();
DiscountSale d = new DiscountSale();
s.announcement();
d.announcement();
s.showAd();
d.showAd();
s = d;
System.out.println( s.toString() ); }
class Sale {
public static void announcement ( ) {
System.out.println( “This is the Sale class.” ); }
public void showAd ( ) {
System.out.println( “buy sale” );
class DiscountSale extends Sale {
System.out.println( “This is the DiscountSale class.” );
System.out.println( “buy discount sale” );
This is the Sale class.
This is the DiscountSale class.
buy sale
buy discount sale

18. public class SaleTest {
public static void main ( String args[] ) {
Sale s = new Sale();
DiscountSale d = new DiscountSale();
s.announcement();
d.announcement();
s.showAd();
d.showAd();
s = d;
System.out.println( s.toString() ); }
class Sale {
public static void announcement ( ) {
System.out.println( “This is the Sale class.” ); }
public void showAd ( ) {
System.out.println( “buy sale” );
class DiscountSale extends Sale {
System.out.println( “This is the DiscountSale class.” );
System.out.println( “buy discount sale” );
This is the Sale class.
This is the DiscountSale class.
buy sale
buy discount sale

19. public class SaleTest {
public static void main ( String args[] ) {
Sale s = new Sale();
DiscountSale d = new DiscountSale();
s.announcement();
d.announcement();
s.showAd();
d.showAd();
s = d;
System.out.println( s.toString() ); }
class Sale {
public static void announcement ( ) {
System.out.println( “This is the Sale class.” ); }
public void showAd ( ) {
System.out.println( “buy sale” );
class DiscountSale extends Sale {
System.out.println( “This is the DiscountSale class.” );
System.out.println( “buy discount sale” );
This is the Sale class.
This is the DiscountSale class.
buy sale
buy discount sale

20. Static vs. dynamic binding example
class TestStaticBinding { public static void main ( String args[] ) { Test t1 = new Fred(); t1.stat(); t1.notStat(); Fred f = (Fred) t1; f.stat(); } class Test { static void stat ( ) { System.out.println( "this is TestStaticBinding1" ); } void notStat ( ) { System.out.println( "this is TestStaticBinding2" ); } class Fred extends Test { static void stat ( ) { System.out.println( "this is Fred1" ); } void notStat ( ) { System.out.println( "this is Fred2" ); }

21. Downcasting & upcasting

22. Casting
What are casts?
Where have we seen/used casts before?

23. Casting What are casts? Where have we seen/used casts before?
Converting from one type to another
Where have we seen/used casts before?
double d = 0.9;
int i1 = (int) d;
int i2 = (int) (d + 0.5);

24. Downcasting and upcasting
Upcast = assigning an object of a derived class to a variable of a base class (or any ancestor class)
straightforward
Downcast = a type cast from a base class to a derived class (or from any ancestor class to any descendent class)
troublesome

25. Downcasting
When impossible, it will generate an error at either compile time or a run time.
Required by equals() method (when downcasting from Object)
instanceof may be used to check if downcast will work

26. clone() method

27. clone() method defined in Object as:
protected Object clone()
every object inherits a clone() method
(supposed to) return a deep copy of the calling object
you are expected to override it
like a copy ctor but there are cases where clone() works but the copy ctor does not.

28. Unofficial version of clone()
public Class_Name clone ( ) {
return new Class_Name( this ); }
Later, we will define the “official” version.

29. Cloning array elements
public static Sale[] goodCopy ( Sale a[] ) {
Sale b[] = new Sale[ a.length ];
for (int i=0; i<a.length; i++)
b[i] = a[i].clone();
return b; }
Does this work? Yes.
Does this provide a “deep” copy? Yes, as long as clone() does.

30. Cloning array elements
public static Sale[] goodCopy ( Sale a[] ) {
Sale b[] = new Sale[ a.length ];
for (int i=0; i<a.length; i++)
b[i] = a[i].clone();
return b; }
Does it work if elements of a[] are not Sale objects but are derived from Sale?

31. Cloning array elements
public static Sale[] goodCopy ( Sale a[] ) {
Sale b[] = new Sale[ a.length ];
for (int i=0; i<a.length; i++)
b[i] = a[i].clone();
return b; }
Does it work if elements of a[] are not Sale objects but are derived from Sale? Yes.
Why?

32. Cloning array elements
public static Sale[] goodCopy ( Sale a[] ) {
Sale b[] = new Sale[ a.length ];
for (int i=0; i<a.length; i++)
b[i] = a[i].clone(); //polymorphic
return b; }
Does it work if elements of a[] are not Sale objects but are derived from Sale? Yes.
Why? Because clone() is overridden.

33. Cloning array elements
Does it work using a copy ctor?
public static Sale[] goodCopy ( Sale a[] ) {
Sale b[] = new Sale[ a.length ];
for (int i=0; i<a.length; i++)
b[i] = new Sale( a[i] ); //not polymorphic
return b; }
This doesn’t work for subclasses of Sale!