Inheritance in Java

• Inheritance in java is a mechanism in which one object acquires all the properties and behaviors of parent object. • The idea behind inheritance in java is that you can create new classes that are built upon existing classes. When you inherit from an existing class, you can reuse methods and fields of parent class, and you can add new methods and fields also. • Inheritance represents the IS-A relationship, also known as parent-child relationship. Use of inheritance in java

• For Method Overriding (so runtime polymorphism can be achieved).

• For Code Reusability.

Syntax of Java Inheritance class Subclass-name extends Superclass-name { //methods and fields }

• The extends keyword indicates that you are making a new class that derives from an existing class. The meaning of "extends" is to increase the functionality.

• In the terminology of Java, a class which is inherited is called parent or super class and the new class is called child or subclass.

Java Inheritance Example

• As displayed in the above figure, Programmer is the subclass and Employee is the superclass. • Relationship between two classes is Programmer IS-A Employee. • It means that Programmer is a type of Employee. Java Inheritance Example class Employee{ float salary=40000; } class Programmer extends Employee{ int bonus=10000; public static void main(String args[]){ Programmer p=new Programmer(); System.out.println("Programmer salary is:"+p.salary); System.out.println("Bonus of Programmer is:"+p.bonus); } } Output : Programmer salary is:40000.0 Bonus of programmer is:10000 Types of inheritance in java

• On the basis of class, there can be three types of inheritance in java: single, multilevel and hierarchical.

• In java programming, multiple and hybrid inheritance is supported through interface only. We will learn about interfaces later. Single Inheritance Example class Animal{ void eat(){System.out.println("eating...");} } class Dog extends Animal{ void bark(){System.out.println("barking..."); } class TestInheritance{ } public static void main(String args[]){ Dog d=new Dog(); d.bark(); d.eat(); } } Output: barking... eating...

Multilevel Inheritance Example class Animal{ void eat(){System.out.println("eating...");} } class Dog extends Animal{ void bark(){System.out.println("barking...");} } class BabyDog extends Dog{ void weep(){System.out.println("weeping...");} }

Multilevel Inheritance Example class TestInheritance2{ public static void main(String args []){ BabyDog d=new BabyDog(); d.weep(); d.bark(); d.eat(); }}

Output: weeping... barking... eating... Hierarchical Inheritance Example class Animal{ void eat(){System.out.println("eating...");} } class Dog extends Animal{ void bark(){System.out.println("barking...");} } class Cat extends Animal{ void meow(){System.out.println("meowing...");} } Hierarchical Inheritance Example class TestInheritance3{ public static void main(String args []){ Cat c=new Cat(); c.meow(); c.eat(); //c.bark();//C.T.Error }}

Output: meowing... eating... Method Overriding in Java • If subclass (child class) has the same method as declared in the parent class, it is known as method overriding in java.

• In other words, 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. Usage of Java 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 for Java Method Overriding

• 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 • In this example, we have defined the run method in the subclass as defined in the parent class but it has some specific implementation. The name and parameter of the method is same and there is IS-A relationship between the classes, so there is method overriding. class Vehicle{ void run(){System.out.println("Vehicle is running");} } Example of method overriding 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(); } super keyword in java

• The super keyword in java is a reference variable which is used to refer immediate parent class object. • Whenever you create the instance of subclass, an instance of parent class is created implicitly which is referred by super reference variable.

super keyword in java

1. super can be used to refer immediate parent class instance variable.

2. super can be used to invoke immediate parent class method.

3. super() can be used to invoke immediate parent class constructor. super : used to invoke immediate parent class instance variable. class Animal{ class TestSuper1{ String color="white"; public static void main(String args[]){ Dog d=new Dog(); } d.printColor(); }} class Dog extends Animal{ Output: black String color="black"; white void printColor(){ System.out.println(color);//prints color of Dog class System.out.println(super.color);//prints color of Animal class

} } super : used to refer immediate parent class method. class Animal{ void eat(){System.out.println("eating...");} } class Dog extends Animal{ void eat(){System.out.println("eating bread...");} void bark(){System.out.println("barking...");} void work() { class TestSuper2{ public static void main(String args[]){ super.eat(); Dog d=new Dog(); bark(); d.work(); }} } } Output: eating... barking... super : used to invoke immediate parent class conctructor. class Animal{ Animal(){System.out.println("animal is created");} } class TestSuper3{ public static void main(String args[]){

class Dog extends Animal{ Dog d=new Dog(); Dog(){ } } super(); Output animal is created System.out.println("dog is created"); dog is created } } Another super example class Person{ int id; String name; Person(int id,String name){ this.id=id; this.name=name; } } Another super example…continued class Emp extends Person{ float salary; Emp(int id,String name,float salary) { super(id,name);//reusing parent constructor this.salary=salary; } void display(){System.out.println(id+" "+name+" "+salary);} } Another super example…continued class TestSuper5{ public static void main(String[] args) { Emp e1=new Emp(1,"ankit",45000f); e1.display(); } } Output: 1 ankit 45000

Final Keyword In Java

The final keyword in java is used to restrict the user. The java final keyword can be used in many context. Final can be:

• variable

• method

• class

Final Keyword In Java

The final keyword in java is used to restrict the user. The java final keyword can be used in many context. Final can be: • variable • method • Class The final keyword can be applied with the variables, a final variable that have no value it is called blank final variable or uninitialized final variable. It can be initialized in the constructor only. The blank final variable can be static also which will be initialized in the static block only. Java final variable • If you make any variable as final, you cannot change the value of final variable(It will be constant). class Bike9{ final int speedlimit=90;//final variable void run(){ speedlimit=400; } public static void main(String args[]){ Bike9 obj=new Bike9(); obj.run(); } }//end of class Output:Compile Time Error Java final method • If you make any method as final, you cannot override it. Example of final method class Bike{ final void run(){System.out.println("running");} } class Honda extends Bike{ void run(){System.out.println("running safely with 100kmph");} //Error

public static void main(String args[]){ Honda honda= new Honda(); honda.run(); } } Output: Compile Time Error

Java final class

• If you make any class as final, you cannot extend it. • Example final class Bike{} class Honda1 extends Bike{ //Error void run(){System.out.println("running safely with 100kmph");}

public static void main(String args[]){ Honda1 honda= new Honda(); honda.run(); } } Output:Compile Time Error

Blank or uninitialized final variable • A final variable that is not initialized at the time of declaration is known as blank final variable.

• If you want to create a variable that is initialized at the time of creating object and once initialized may not be changed, it is useful. For example PAN CARD number of an employee.

• It can be initialized only in constructor. Example of blank final variable class Student{ int id; String name; final String PAN_CARD_NUMBER; ... } Dynamic Method Dispatch • Runtime polymorphism or Dynamic Method Dispatch is a process in which a call to an overridden method is resolved at runtime rather than compile-time.

• In this process, an overridden method is called through the reference variable of a superclass. The determination of the method to be called is based on the object being referred to by the reference variable. Upcasting • When reference variable of Parent class refers to the object of Child class, it is known as upcasting. For example:

class A{} class B extends A{} A a=new B();//upcasting

Dynamic Method Dispatch Example class Bank{ float getRateOfInterest(){return 0;} } class SBI extends Bank{ float getRateOfInterest(){return 8.4f;} } class ICICI extends Bank{ float getRateOfInterest(){return 7.3f;} } class AXIS extends Bank{ float getRateOfInterest(){return 9.7f;} } Dynamic Method Dispatch Example class TestPolymorphism{ public static void main(String args[]){ Bank b; b=new SBI(); System.out.println("SBI Rate of Interest: "+b.getRateOfInterest()); b=new ICICI(); System.out.println("ICICI Rate of Interest: "+b.getRateOfInterest()); b=new AXIS(); System.out.println("AXIS Rate of Interest: "+b.getRateOfInterest());

} }

Abstractionin Java

• Abstraction is a process of hiding the implementation details and showing only functionality to the user. Ways to achieve Abstaction There are two ways to achieve abstraction in java • Abstract class (0 to 100%) • Interface (100%)

Abstract class in Java

• A class that is declared with abstract keyword, is known as abstract class in java.

• It can have abstract and non-abstract methods (method with body).

• It needs to be extended and its method implemented.

• It cannot be instantiated. Abstract class in Java abstract class Shape{ abstract void draw(); } //In real scenario, implementation is provided by others i.e. unk //nown by end user class Rectangle extends Shape{ void draw(){System.out.println("drawing rectangle");} } class Circle1 extends Shape{ void draw(){System.out.println("drawing circle");} }

Abstract class in Java

//In real scenario, method is called by programmer or u //ser class TestAbstraction1{ public static void main(String args[]){ Shape s=new Circle1();//In real scenario, object is provi //ded through method e.g. getShape() method s.draw(); } } Interface in Java

• An interface in java is a blueprint of a class. It has static constants and abstract methods. • The interface in java is a mechanism to achieve abstraction. • There can be only abstract methods in the java interface not method body. • It is used to achieve abstraction and multiple inheritance in Java. • Java Interface also represents IS-A relationship. • It cannot be instantiated just like abstract class. Why use Java interface?

There are mainly three reasons to use interface. They are given below.

• It is used to achieve abstraction.

• By interface, we can support the functionality of multiple inheritance.

• It can be used to achieve loose coupling.

Relationship between classes and interfaces • As shown in the figure given below, a class extends another class, an interface extends another interface but a class implements an interface. Java Interface Example

• In this example, Printable interface has only one method, its implementation is provided in the A class. interface printable{ void print(); } class A implements printable{ public void print(){System.out.println("Hello");} public static void main(String args[]){ A obj = new A(); obj.print(); } }

Java Interface Example2 //Interface declaration: by first user interface Drawable{ void draw(); } //Implementation: by second user class Rectangle implements Drawable{ public void draw(){System.out.println("drawing rectangle");} } class Circle implements Drawable{ public void draw(){System.out.println("drawing circle");} } Java Interface Example2…continued //Using interface: by third user class TestInterface1{ public static void main(String args[]){ Drawable d=new Circle(); //In real scenario, object is provided //by method e.g. getDrawable() d.draw(); } } Why multiple inheritance is not supported in java? • To reduce the complexity and simplify the language, multiple inheritance is not supported in java. • Consider a scenario where A, B and C are three classes. The C class inherits A and B classes. If A and B classes have same method and you call it from child class object, there will be ambiguity to call method of A or B class. • Since compile time errors are better than runtime errors, java renders compile time error if you inherit 2 classes. So whether you have same method or different, there will be compile time error now.

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? } }

Output :

Compile Time Error Multiple Inheritance in Java • If a class implements multiple interfaces, or an interface extends multiple interfaces i.e. known as multiple inheritance. Multiple Inheritance in Java interface Printable{ void print(); } interface Showable{ void show(); } class A implements Printable , Showable{ public void print(){System.out.println("Hello");} public void show(){System.out.println("Welcome");}

public static void main(String args[]){ A obj = new A (); obj.print(); obj.show(); } }

Interface inheritance • A class implements interface but one interface extends another interface interface Printable{ void print(); } interface Showable extends Printable{ void show(); } class TestInterface4 implements Showable{ public void print(){System.out.println("Hello");} public void show(){System.out.println("Welcome");} public static void main(String args[]){ TestInterface4 obj = new TestInterface4(); obj.print(); obj.show(); } }

Difference between abstract class and interface Abstract class Interface Interface can have only abstract methods. 1) Abstract class can have abstract and Since Java 8, it can have default and static non-abstract methods. methods also.

2) Abstract class doesn't support multiple Interface supports multiple inheritance. inheritance.

3) Abstract class can have final, non-final, Interface has only static and final variables. static and non-static variables.

4) Abstract class can provide the Interface can't provide the implementation implementation of interface. of abstract class.

5) The abstract keyword is used to declare The interface keyword is used to declare abstract class. interface.

6) Example: Example: public abstract class Shape{ public interface Drawable{ public abstract void draw(); void draw(); } }