Polymorphism

Advanced Programming

SUT • Spring 2019

Outline

  • Polymorphism

  • Final Methods

Polymorphism

Animals can talk!

Talk request

  • You can send a talk request to an animal

  • What does it do?

  • It depends on type of the animal

Musical Instruments

  • The same note

  • Different sounds on different instruments

Polymorphic Behavior

  • Common interface

    • The same request

  • Different behaviors

  • Depending on the type of object

Polymorphism

  • The same interface

    • animal.talk()

    • instrument.play(int note)

  • But different implementation

    • in different classes

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;

UpCasting

  • Upcasting

    • Shape   s = new Rectangle();

    • Circle c = new Circle();

    • Shape   s =  c;

  • Upcasting is always valid

DownCasting

  • Down casting

    • Shape s = …

    • Circle c  = s;  

    • Circle c = (Circle) s;

    • Needs type cast

    • May cause errors

What About Method Calls?

  • Shape   s = new Rectangle();

    • s.draw();

    • double d = s.getArea();

  • Circle c = new Circle();

  • Shape s =  c;

    • s.draw();

    • double d = s.getArea();

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

  • Also known as Static Binding

  • When a method is called, compiler knows which method is called

  • 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

  • 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

Parent p = new Parent();
p.f();

Child c = new Child();
c.f();
p = c;
p.f();

Virtual Methods

  • 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

  • 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

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

  • Later!

Final

Abstract Behaviors

  • 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 public void f();

Final Variables

  • 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

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

  • You can not inherit from final classes

  • No class can extend final classes

Review of final Keyword

  • Final data

    • Const

    • Local variables

    • Method parameters

    • Member variables

    • Primitives = constant values

    • Objects = constant references

  • A compile-time constant that won’t ever change

  • A value initialized at run time that you don’t want changed

Review of final Keyword (2)

  • Final Methods

    • No override

  • Final Class

    • No sub-class

  • final keyword on data

    • Different from final classes & methods

Finalism and Performance

  • 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

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)");
    }
}
  • 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) {…}

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