Polymorphism

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. Late binding • AKA dynamic binding • Binding – the process of associating a method definition with a method invocation • Early binding – the method definition is associated with the method invocation when the code is compiled; AKA static binding • Late 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.

5. Late binding example • 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

6. 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)?

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)? NO! • What mechanism makes this work?

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 make this work? • Late binding!

9. 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?

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? • Figure’s draw() would be called instead of Triangle’s draw().

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

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

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

14. Static binding & static methods public class Sale { //… public static void announcement ( ) { System.out.println( “This is the Sale class.” ); } } public class DiscountSale extends Sale { //… public static void announcement ( ) { System.out.println( “This is the DiscountSale class.” ); } }

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

16. Downcasting & upcasting

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

18. Casting • What are casts? • 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);

19. 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

20. 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

21. clone() method

22. 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.

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

24. 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.

25. 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?

26. 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?

27. 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.

28. 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.