Polymorphism
Advanced Programming
SUT • Spring 2019
Outline
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Polymorphism
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Final Methods
Polymorphism
Animals can talk!
Talk request
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You can send a talk request to an animal
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What does it do?
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It depends on type of the animal
Musical Instruments
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The same note
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Different sounds on different instruments
Polymorphic Behavior
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Common interface
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The same request
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Different behaviors
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Depending on the type of object
Polymorphism
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The same interface
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animal.talk()
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instrument.play(int note)
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But different implementation
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in different classes
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Polymorphism
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Suppose Child is a subclass of Parent class.
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Remember: A Child’s object is also a Parent’s object
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is-a relationship
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So these lines are valid:
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Child c = new Child();
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Parent p = new Parent();
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p = c;
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But this line is invalid:
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c = p;
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UpCasting
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Upcasting
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Shape s = new Rectangle();
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Circle c = new Circle();
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Shape s = c;
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Upcasting is always valid
DownCasting
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Down casting
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Shape s = …
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Circle c = s;
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Circle c = (Circle) s;
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Needs type cast
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May cause errors
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What About Method Calls?
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Shape s = new Rectangle();
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s.draw();
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double d = s.getArea();
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Circle c = new Circle();
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Shape s = c;
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s.draw();
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double d = s.getArea();
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Faculty & Employee
class Parent {
public void f() {
System.out.println("f() in Parent");
}
}
public class Child extends Parent {
public void f() {
System.out.println("f() in Child");
}
public static void main(String[] args) {
Parent p = new Parent();
p.f();
Child c = new Child();
c.f();
p = c;
p.f();
}
}
Compile-time Method Binding
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Also known as Static Binding
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When a method is called, compiler knows which method is called
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The translation is done in compile-time
class SomeClass{
public void f() {
System.out.println("f");
}
public void g() {
f();
System.out.println("g");
}
}
Run-time Method Binding
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Also known as Dynamic Binding
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When you call a method on a superclass reference
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Actual method is bound in runtime
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(If it is overridden)
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Performance overload
Parent p = new Parent();
p.f();
Child c = new Child();
c.f();
p = c;
p.f();
Virtual Methods
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In some languages (like C++) you can specify the binding mechanism for methods
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If a method is declared as virtual, dynamic binding is used for that method
Applications of Polymorphism
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Polymorphic behavior
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Suppose you have so many objects in a GUI application
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All of them have draw() operation
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You simply call draw() on every object
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It knows how to draw itself
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Classes : Drawable(superclass), Player, Referee, Ball, …
No Polymorphism
Player[] players;
Refree[] refs;
Ball ball;
for (Player player: players){
player.draw();
}
for (Referee ref: refs){
ref.draw();
}
ball.draw();
With Polymorphism
Player[] players;
Refree[] refs;
Ball ball;
for (Player player: players){
player.draw();
}
for (Referee ref: refs){
ref.draw();
}
ball.draw();
Drawable[] drawables;
for (Drawable drawable: drawables) {
drawable.draw();
}
Hint on Array Initialization
Cat maloos = new Cat("Maloos");
Cat loos = new Cat("Loos");
Dog fido = new Dog("Fido");
Animal[] animals = new Animals[]{maloos, loos, fido};
Animal[] animals = new Animals[3];
animals[0] = maloos;
animals[1] = loos;
animals[2] = fido;
Animal[] animals = new Animals[]{new Cat("Maloos"), new Cat("Loos"),
new Dog("Fido")};
Animal[] animals = new Animals[3];
animals[0] = new Cat("Maloos");
animals[1] = new Cat("Loos");
animals[2] = new Dog("Fido");
Sample
class Cat extends Animal {
public Cat(String name){
super(name);
}
public String talk(){
return "Meoww!!!";
}
}
class Dog extends Animal {
public Dog(String name) {
super(name);
}
public String talk(){
return "Hop! Hop!";
}
}
Animal Example
public abstract class Animal {
private final String name;
protected Animal(String name){
this.name = name;
}
public String getName(){
return name;
}
public abstract String talk();
}
Polymorphic Animals!
Animal[] animals = new Animals[]{
new Cat("Maloos"),
new Cat("Loos"),
new Dog("Fido")
};
for(Animal animal: animals){
System.out.println(animal.getName() + ": " + animal.talk());
}
More on Polymorphism
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Later!
Final
Abstract Behaviors
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You can not override final methods
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final keyword
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Static method binding for final methods
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Private methods are implicitly final
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Static methods are implicitly final
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Static methods are statically bound
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Invoked reference is not important
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No polymorphism for static variables
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final public void f();
Final Variables
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You can define variables as final
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The value of final variable will remain constant
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You can not change the value of final variables
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You should immediately assign a value to final variables
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Final parameter
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Final local variable
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Final property
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Final static variable
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private final String name;
Final Variables
class SomeClass{
private final String name;
public final int val = 12;
void f(final int a) {
final int b = a + 1;
}
void g() {
final String s = "123";
}
protected SomeClass(String name) {
this.name = name;
}
}
Final Classes
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You can not inherit from final classes
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No class can extend final classes
Review of final Keyword
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Final data
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Const
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Local variables
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Method parameters
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Member variables
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Primitives = constant values
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Objects = constant references
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A compile-time constant that won’t ever change
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A value initialized at run time that you don’t want changed
Review of final Keyword (2)
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Final Methods
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No override
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Final Class
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No sub-class
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final keyword on data
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Different from final classes & methods
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Finalism and Performance
Final methods can be invoked inline
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Compiler can bind final methods statically
Static binding
So it may bring a better performance…
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It is now discouraged to use final to try to help the optimizer
Especially with Java 6+
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Don’t worry about performance
Java optimizer
More on Polymorphism
Contents
class Parent{
public void f(){
System.out.println("f() in Parent");
}
}
class Child extends Parent{
public void f(){
System.out.println("f() in Child");
}
}
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:
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:
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
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)");
}
}
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method() is overloaded in SomeSubClass
- It is not overridden
- Two independent methods
What is the output of:
SomeSubClass ref = new SomeSubClass();
ref.method(parent);
ref.method(child);
ref.method(parentRefToChild);
- Output:
- method(Parent)
- method(Child)
- method(Parent)
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:
- But not like this:
- Why?!
public boolean equals(Object obj) {…}
public boolean equals(Person obj) {…}
Title Text
Polymorphism
By Behnam Hatami
Polymorphism
Polymorphism / Advanced Programming Course @ SUT, Spring 2019
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