1. Advanced Programming in Java
Polymorphism
Mehdi Einali

2. agenda Polymorphism Polymorphism Application
Polymorphism and Method Call
Class Diagram

3. Polymorphism

4. Animals can talk!

5. Talk request You can send a talk request to an animal What does it do?
It depends on type of the animal

6. Musical Instruments The same note
Different sounds on different instruments

7. Polymorphic Behavior Common interface Different behaviors
The same request
Different behaviors
Depending on the type of object

8. Polymorphism The same interface But different implementation
animal.talk()
instrument.play(int note)
But different implementation
in different classes

9. Polymorphism Suppose Child is a subclass of Parent class.
Remember : A Child’s object is also a Parent’s object
is-a relationship
So these lines are valid:
Child c = new Child();
Parent p = new Parent();
p = c;
But this line is invalid:
c = p;

10. UpCasting Upcasting Upcasting is always valid
Shape s = new Rectangle();
Circle c = new Circle();
Shape s = c;
Upcasting is always valid

11. DownCasting Downcasting Shape s = … Circle c = s;
Circle c = (Circle) s;
Needs type cast
On your own risk
May cause errors
ClassCastException

12. What About Method Calls?
Shape s = new Rectangle();
s.draw();
double d = s.getArea();
Circle c = new Circle();
Shape s = c;

14. Compile-time Method Binding
Also known as Static Binding
When a method is called, compiler knows which method is called
The translation is done in compile-time
Private methods
Final methods
Static methods

15. Run-time Method Binding
Also known as Dynamic Binding
When you call a method on a superclass reference
Actual method is bound in runtime
(If it is overridden)
Performance overload

16. Applications of Polymorphism

17. Applications of Polymorphism
Polymorphic behavior
Classes : Player, Referee, Ball, …
Drawable(superclass)

18. No Polymorphism

19. With Polymorphism

20. Animal Example

21. Cat and dog

22. Polymorphic Animals!

23. Hint on Array Initialization

24. Polymorphism and method call

25. class Animal{
public void f(){
System.out.println("f() in Parent"); }
class Cat extends Animal{
System.out.println("f() in Child");
public class SomeClass {
public void method(Animal p){
System.out.println("method(Parent)");
public void method(Cat p){
System.out.println("method(Child)");

26. What is the output of: Cat cat= new Cat();
Animal animal= new Animal();
Animal animalCat= new Cat();
animal.f();
cat.f();
animalCat.f();
Output:
f() in Parent
f() in Child

27. What is the output of: SomeClass caller = new SomeClass();
caller.method(animal);
caller.method(cat);
caller.method(animalCat);
Important Note:
Polymorphic behavior for method call
Method call evaluated based on content not reference
Not for the parameters
Parameter evaluated based on reference not content
More specific to broader (based on reference!!)
Output:
method(Parent)
method(Child)

28. Note: Overloading method() is overloaded in SomeClass
public class SomeClass {
public void method(Parent p){
System.out.println("method(Parent)"); }
public void method(Child p){
System.out.println("method(Child)");
method() is overloaded in SomeClass
Two independent methods

29. To Overload or to Override
class SomeClass {
public void method(Parent p){
System.out.println("method(Parent)"); }
class SomeSubClass extends SomeClass{
public void method(Child p){
System.out.println("method(Child)");
method() is overloaded in SomeSubClass
It is not overridden
Two independent methods

30. What is the output of: SomeSubClass ref = new SomeSubClass();
ref.method(parent);
ref.method(child);
ref.method(parentRefToChild);
Output:
method(Parent)
method(Child)

31. Output:
method(Parent)
method(Child)
method(Parent)
method(Child)

32. Output:
method(Parent)
method(Parent)

33. Output:
Exception : there is no method for [method(animal), method(animalCat)]
Exception : there is no method for [method(animal), method(animalCat)]

34. It is about external behavior not internal data or static attributes
Is-a means every cat has animal behavior too, it is not about use every cat instead of animal (such as method param).
It is about external behavior not internal data or static attributes
What about is-a relationship between cat and animal?
What about Polymorphism between cat and animal?
Polymorphism means every inherited behavior in animal must evaluated based on its overriden childish behavior!
It is not applicable here!
What about Liskov substitution principle between cat and animal?
Liskov is about changing parent with child and expect same external behavior. It is rephrase of is-a relationship.
Output:
method(Parent)
Exception for barfoo.method

35. It is no applicable here!
foo is a right bar .
nothing wrong!
It is no applicable here!
What about is-a relationship between foo and bar?
What about Polymorphism between foo and bar?
Polymorphism means every inherited behavior in bar must evaluated based on its overridden childish behavior!
What about Liskov substitution principle between foo and bar?
Same as is-a relationship. it is not applicable here!
There is not overridden method in bar. Two overloaded method in bar and foo.
method(Parent)
method(Child)

36. A Question When we override equals() method
why do we pass Object as the parameter?
For example class Person has an equals method like this:
public boolean equals(Object obj) {…}
But not like this:
public boolean equals(Person obj) {…}
Why?!

37. Class diagram

38. UML – Unified Modeling Language
The Unified Modeling Language (UML) is a standard language for specifying, visualizing, constructing and documenting the artifacts of software systems, as well as for business modeling and other non-software systems.

39. Composite Structure Diagram
UML Diagrams
Structural Diagrams
Class Diagram
Object Diagram
Component Diagram
Composite Structure Diagram
Deployment Diagram
Package Diagram
Behavioral Diagrams
State Machine Diagram
Use Case Diagram
Activity Diagram
Interaction Diagrams
Timing
Diagram
Interaction Overview Diagram
Communication Diagram
Sequence Diagram

40. class
A class diagram describes the types of objects in the system and the various kinds of static relationships that exist among them

41. کلاس خوب
نام واضح
یک تجرید مشخص از یک مولفه از فضای مسئله
نگاشت واضح به یک مفهوم
مسئولیت های کم و خوش تعریف – کلاس قدر قدرت – ضدالگو
سه تا پنج مسئولیت
انسجام بالا
وابستگی کم

42. Notes on class diagram
Classes should be simple, understandable and clear
High Cohesion/Low Coupling
Don’t mention every thing
comprehensiveness is the enemy of comprehensibility!
Avoid omnipotent classes

43. End

44. Martin fowler http://martinfowler.com/ ThoughtWorks New methodology
Technology radars
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