OOP with Java - Part 3

Java/J2EE Programming Training OOP with Java Contd.

Classification: Restricted Agenda • Review of last class concepts • Types of Inheritance and a look at Aggregation • Polymorphism • Method overloading • Method overriding

Inheritance in Java • a mechanism in which one object acquires all the properties and behaviours of parent object. • Inheritance represents the IS-A relationship, also known as parent- child relationship. • Why use inheritance in java oFor Method Overriding (so runtime polymorphism can be achieved). oFor Code Reusability. Classification: Restricted Page 3

Inheritance in Java Classification: Restricted Page 4

Types of Inheritance Classification: Restricted Page 5

Inheritance Not Supported in Java • When a class extends multiple classes i.e. known as multiple inheritance. Classification: Restricted Page 6

Why multiple inheritance not supported in Java? Classification: Restricted Page 7 class A{ void msg(){System.out.println("Hello");} } class B{ void msg(){System.out.println("Welcome");} } class C extends A,B{//suppose if it were public static void main(String args[]){ C obj=new C(); obj.msg();//Now which msg() method would be invoked? Compile-time error } }

Aggregation in Java • If a class have an entity reference, it is known as Aggregation. • Aggregation represents HAS-A relationship. • Why use Aggregation? oFor Code Reusability. Classification: Restricted Page 8

Aggregation in Java Classification: Restricted Page 9 class Operation{ int square(int n){ return n*n; } } class Circle{ Operation op;//aggregation double pi=3.14; double area(int radius){ op=new Operation(); int rsquare=op.square(radius);//code reusability (i.e. delegates the method call). return pi*rsquare; } public static void main(String args[]){ Circle c=new Circle(); double result=c.area(5); System.out.println(result); } }

Example: Employee HAS-A Address Classification: Restricted Page 10 • Demo

Method Overriding in Java Classification: Restricted Page 12 • If subclass (child class) has the same method as declared in the parent class, it is known as method overriding in java. • If subclass provides the specific implementation of the method that has been provided by one of its parent class, it is known as method overriding. • Method overriding is used to provide specific implementation of a method that is already provided by its super class. • Method overriding is used for runtime polymorphism • Rules: • method must have same name as in the parent class • method must have same parameter as in the parent class. • must be IS-A relationship (inheritance).

Example of Method Overriding Classification: Restricted Page 13 class Vehicle{ void run(){System.out.println("Vehicle is running");} } class Bike2 extends Vehicle{ void run(){System.out.println("Bike is running safely");} public static void main(String args[]){ Bike2 obj = new Bike2(); obj.run(); }

Classification: Restricted Page 14 Point to ponder.. Can we override static methods in Java? Why? Think… (This is one of the exercises for today)

Method Overloading vs Method Overriding Classification: Restricted Page 15 No. Method Overloading Method Overriding 1) Method overloading is used to increase the readability of the program. Method overriding is used to provide the specific implementation of the method that is already provided by its super class. 2) Method overloading can be performed within class. Method overriding occurs in two classes that have IS-A (inheritance) relationship. 3) In case of method overloading, parameter must be different. In case of method overriding, parameter must be same. 4) Method overloading is the example of compile time polymorphism. Method overriding is the example of run time polymorphism. 5) In Java, method overloading can't be performed by changing return type of the method only. Return type can be same or different in method overloading. But you must have to change the parameter. Return type must be same or covariant in method overriding.

super keyword in Java Classification: Restricted Page 16 • The super keyword in Java is a reference variable that is used to refer immediate parent class object. • Usage of super Keyword o super is used to refer immediate parent class instancevariable. o super() is used to invoke immediate parent classconstructor. o super is used to invoke immediate parent classmethod.

super used to refer immediate parent class instance variable class Vehicle{ int speed=50; } class Bike3 extends Vehicle{ int speed=100; void display(){ System.out.println(speed);//will print speed of Bike System.out.println(super.speed);//will print speed of Vehicle } public static void main(String args[]){ Bike3 b=new Bike3(); b.display(); } } Classification: Restricted Page 17

super is used to invoke parent class constructor Classification: Restricted Page 18 class Vehicle{ Vehicle(){System.out.println("Vehicle is created");} } class Bike5 extends Vehicle{ Bike5(){ super();//will invoke parent class constructor System.out.println("Bike is created"); } public static void main(String args[]){ Bike5 b=new Bike5(); } }

super can be used to invoke parent class method Classification: Restricted Page 19 class Person{ void message(){System.out.println("welcome");} } class Student16 extends Person{ void message(){System.out.println("welcome to java");} void display(){ message();//will invoke current class message() method super.message();//will invoke parent class message() method } public static void main(String args[]){ Student16 s=new Student16(); s.display(); } }

super not required here Classification: Restricted Page 20 class Person{ void message(){System.out.println("welcome");} } class Student17 extends Person{ void display(){ message();//will invoke parent class message() method } public static void main(String args[]){ Student17 s=new Student17(); s.display(); } }

final keyword in Java Classification: Restricted Page 22 • final keyword applied to • Variable: stops value change • Methods: stops overriding • Class: stops inheritance

final variable Classification: Restricted Page 23 final variable once assigned a value can never be changed. class Bike9{ final int speedlimit=90;//final variable void run(){ speedlimit=400; //compile-time error } public static void main(String args[]){ Bike9 obj=new Bike9(); obj.run(); } }//end of class

final method – cannot override Classification: Restricted Page 24 class Bike{ final void run(){System.out.println("running");} } class Honda extends Bike{ void run(){System.out.println("running safely with 100kmph");} //compile-time error public static void main(String args[]){ Honda honda = new Honda(); honda.run(); } }

final class – cannot extend Classification: Restricted Page 25 final class Bike{} class Honda1 extends Bike{ //compile-time error void run(){System.out.println("running safely with 100kmph");} public static void main(String args[]){ Honda1 honda= new Honda(); honda.run(); } }

Blank final variables must be initialized only in constructor Classification: Restricted Page 26 class Bike10{ final int speedlimit; // blank final variable Bike10(){ speedlimit=70; // initialized in constructor System.out.println(speedlimit); } public static void main(String args[]){ new Bike10(); } }

final parameter example Classification: Restricted Page 27 class Bike11{ int cube(final int n){ n=n+2; //can't be changed as n is final n*n*n; } public static void main(String args[]){ Bike11 b=new Bike11(); b.cube(5); } }

Point to ponder…Can constructors be declared final? Classification: Restricted Page 28 Think… (Exercise for today)

Java Runtime Polymorphism – Example of Upcasting Classification: Restricted Page 29 class Animal{ void eat(){System.out.println("eating");} } class Dog extends Animal{ void eat(){System.out.println("eating fruits");} } class BabyDog extends Dog{ void eat(){System.out.println("drinking milk");} public static void main(String args[]){ Animal a1,a2,a3; a1=new Animal(); a2=new Dog(); a3=new BabyDog(); a1.eat(); a2.eat(); a3.eat(); } }

Static and dynamic binding Classification: Restricted Page 30 Static Binding: Type of object determined at Compile time. class Dog{ private void eat(){System.out.println("dog is eating...");} public static void main(String args[]){ Dog d1=new Dog(); d1.eat(); } } Dynamic Binding: Type of object determined at run time class Animal{ void eat(){System.out.println("animal is eating...");} } class Dog extends Animal{ void eat(){System.out.println("dog is eating...");} public static void main(String args[]){ Animal a=new Dog(); a.eat(); } }

instanceof operator Classification: Restricted Page 31 • used to test whether the object is an instance of the specified type (class or subclass or interface). class Simple1{ public static void main(String args[]){ Simple1 s=new Simple1(); System.out.println(s instanceof Simple1);//true } } class Animal{} class Dog1 extends Animal{//Dog inherits Animal public static void main(String args[]){ Dog1 d=new Dog1(); System.out.println(d instanceof Animal);//true } }

class Dog2{ public static void main(String args[]){ Dog2 d=null; System.out.println(d instanceof Dog2);//false } } Classification: Restricted Page 32 instanceof operator

Downcasting Classification: Restricted Page 33 Dog d=new Animal();//Compilation error Dog d=(Dog)new Animal(); //Compiles successfully but ClassCastException could be thrown at runtime

OOP with Java - Part 3

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OOP with Java - Part 3