Understanding Polymorphism in Java Programming

Object Oriented Programming Concepts in JAVA Polymorphism

Agenda 2 Polymorphism Abstract method and Class Final Methods

Animals can talk! 3

Talk request 4 You can send a talk request to an animal What does it do? It depends on type of the animal

Musical Instruments 5 The same note Different sounds on different instruments

Polymorphic Behavior 6 Common interface The same request Different behaviors Depending on the type of object

Polymorphism 7 The same interface animal.talk() instrument.play(int note) But different implementation in different classes

Polymorphism 8 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;

UpCasting 9 Upcasting Shape s = new Rectangle(); Circle c = new Circle(); Shape s = c; Upcasting is always valid

DownCasting 10 Downcasting Shape s = Circle c = s; Circle c = (Circle) s; Needs type cast May cause errors

What About Method Calls? 11 Shape s = new Rectangle(); s.draw(); double d = s.getArea(); Circle c = new Circle(); Shape s = c; s.draw(); double d = s.getArea();

12

Compile-time Method Binding 13 Also known as Static Binding When a method is called, compiler knows which method is called The translation is done in compile-time

Run-time Method Binding 14 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

Virtual Methods 15 In some languages (like C++) you can specify the binding mechanism for methods If a method is declared as virtual,dynamic binding is used for that method

Applications of Polymorphism 16 Polymorphic behavior Suppose you have so many objects in a GUI application All of them have draw() operation You simply call draw() on every object It knows how to draw itself Classes : Drawable(superclass), Player, Referee, Ball,

No Polymorphism 17

With Polymorphism 18

Hint on Array Initialization 19

Animal Example 20

21 Cat & Dog

Polymorphic Animals! 22

Final in JAVA

Final Methods 24 You can not override final methods final keyword Static method binding for final methods Private methods are implicitly final Static methods are implicitly final Static methods are statically bound Invoked reference is not important No polymorphism for static variables

Final Variables 25 You can define variables as final The value of final variable will remain constant You can not change the value of final variables You should immediately assign a value to final variables Final parameter Final local variable Final property Final static variable

Final Variables 26

Final Classes 27 You can not inherit from final classes No class can extend final classes

Review of final Keyword 28 Final data Const Local variables Method parameters Member variables Primitives constant values Objects constant references A compile-time constantthat won t ever change A value initialized at run time that you don t want changed

Review of final Keyword (2) 29 Final Methods No override Final Class No sub-class final keyword on data Different from final classes & methods

Finalism and Performance 30 Final methods can be invoked inline Compiler can bind final methods statically Static binding So it may bring a better performance It is now discouraged to use final to try to help the optimizer Especially with Java 6+ Don t worry about performance Java optimizer

Final Classes final class Executive extends Manager { . . . }

Final Methods You can also make a specific method in a class final. If you do this, then no subclass can override that method. (All methods in a final class are automatically final.) class Employee { . . . public final String getName() { return name; } . . . }

More on Polymorphism

class Parent{ public void f(){ } } class Child extends Parent{ public void f(){ System.out.println("f() in Child"); } } System.out.println("f() in Parent"); 34 public class SomeClass { public void method(Parent p){ System.out.println("method(Parent)"); } public void method(Child p){ System.out.println("method(Child)"); } }

What is the output of: 35 Child child = new Child(); Parent parent = new Parent(); Parent parentRefToChild = new Child(); parent.f(); child.f(); parentRefToChild.f(); Output: f() in Parent f() in Child f() in Child

What is the output of: 36 SomeClass square = new SomeClass(); square.method(parent); square.method(child); square.method(parentRefToChild); Output: method(Parent) method(Child) method(Parent) Important Note: Polymorphic behavior for reference the reference before dot Not for the parameters

Note: Overloading 37 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

To Overload or to Override 38 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

What is the output of: 39 SomeSubClass ref = new SomeSubClass(); ref.method(parent); ref.method(child); ref.method(parentRefToChild); Output: method(Parent) method(Child) method(Parent)

A Question 40 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?!

Abstract Classes

Abstract Classes Java uses Abstract classes & Interfaces to further strengthen the idea of inheritance. To see the role of abstract of classes, suppose that the pay method is not implemented in the HourlyEmployee subclass. Obviously, the pay method in the Employee class will be assumed, which will lead to wrong result. One solution is to remove the pay method out and put it in another extension of the Employee class, MonthlyEmployee. The problem with this solution is that it does not force subclasses of Employee class to implement the pay method.

Review of Abstract Classes (Cont'd) The solution is to declare the pay method of the Employee class as abstract, thus, making the class abstract. abstract class Employee { protected String name; protected double payRate; public Employee(String empName, double empRate) { name = empName; payRate = empRate; } public String getName() {return name;} public void setPayRate(double newRate) {payRate = newRate;} abstract public double pay(); public void print() { System.out.println("Name: " + name); System.out.println("Pay Rate: "+payRate); } }

Review of Abstract Classes (Cont'd) The following extends the Employee abstract class to get MonthlyEmployee class. class MonthlyEmployee extends Employee { public MonthlyEmployee(String empName, double empRate) { super(empName, empRate); } public double pay() { return payRate; } } The next example extends the MonthlyEmployee class to get the Executive class.

Review of Abstract Classes (Cont'd) class Executive extends MonthlyEmployee { private double bonus; public Executive(String exName, double exRate) { super(exName, exRate); bonus = 0; } public void awardBonus(double amount) { bonus = amount; } public double pay() { double paycheck = super.pay() + bonus; bonus = 0; return paycheck; } public void print() { super.print(); System.out.println("Current bonus: " + bonus); } } HourlyEmployee Employee Executive MonthlyEmployee

Review of Abstract Classes (Cont'd) The following further illustrates the advantages of organizing classes using inheritance - same type, polymorphism, etc. public class TestAbstractClass { public static void main(String[] args) { Employee[] list = new Employee[3]; list[0] = new Executive("Jarallah Al-Ghamdi", 50000); list[1] = new HourlyEmployee("Azmat Ansari", 120); list[2] = new MonthlyEmployee("Sahalu Junaidu", 9000); ((Executive)list[0]).awardBonus(11000); for(int i = 0; i < list.length; i++) if(list[i] instanceof HourlyEmployee) ((HourlyEmployee)list[i]).addHours(60); for(int i = 0; i < list.length; i++) { list[i].print(); System.out.println("Paid: " + list[i].pay()); System.out.println("*************************"); } } } The Program Output

Remember Animals Class Hierarchy 47

Abstract Behaviors 48 Does any animal swim? No. So to swim is not a behavior of animals. Any animal has a voice to talk is a behavior of animals But what is the voice of an animal? How does an animal talk ? It depends to the specific type of animal Dog: Hop! Hop! Cat: Mewww!

Abstract Behaviors (2) 49 talk is an abstract behavior of Animal All animals can talk But we can not specify how an animal talks It depends to the specific class of animal talk is a concrete behavior of Dog Hop! Hop! swim is not a behavior of Animal All animals can not swim swim is a concrete behavior of Fish

Remember Shape Classes 50