Interfaces and Inheritance in Java
A class can extend another class and can implement one and more than one Java interface. Also, this topic has a major influence on the concept of Java and Multiple Inheritance.
Example:
interface intfA {
void m1();
}
interface intfB {
void m2();
}
// class implements both interfaces
// and provides implementation to the method.
class sample implements intfA, intfB
{
@Override
public void m1() {
System.out.println("Welcome: inside the method m1");
}
@Override
public void m2() {
System.out.println("Welcome: inside the method m2");
}
}
class GFG
Output
Welcome: inside the method m1 Welcome: inside the method m2
Hierarchy
Note: This Hierarchy will be followed in the same way, we cannot reverse the hierarchy while inheritance in Java. This means that we cannot implement a class from the interface because Interface-to-Class inheritance is not allowed, and it goes against the fundamental principles of class-based inheritance. This will also breach the relationship known as the Is-A relationship, where a subclass is a more specialized version of its superclass.
Interface Inheritance
An Interface can extend another interface. 
Inheritance is inheriting the properties of the parent class into the child class.
- Inheritance in Java is a mechanism in which one object acquires all the properties and behaviors of a 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 the parent class.
- You can also add new methods and fields in your current class.
- Inheritance represents the IS-A relationship which is also known as the parent-child relationship.
Example:
Dog IS_A Animal
Car IS_A Vehicle
Employee IS_A Person
Surgeon IS_A Doctor etc.
// Animal is a Parent class
class Animal
{
public void eat()
{
System.out.println("Animal is eating");
}
}
// Dog is derived from Animal class
class Dog extends Animal
{
public static void main(String args[])
{
// Creating object of Dog class
Dog d = new Dog();
// Dog can access eat() method
// of Animal class
d.eat();
}
}
Output
Animal is eating
Syntax of Java Inheritance
class <Subclass-name> extends <Superclass-name> {
//methods and fields
}
Note: 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.
Example of Java Inheritance
// Parent Class
class Person1
{
// Variables
int id;
String name;
// Java Methods
void set_Person(int id, String name) {
try {
this.id = id;
this.name = name;
}
catch (Exception ex) {
ex.printStackTrace();
}
}
void disp_Person() {
System.out.println(id + "\n" + name + "\n");
}
}
// Child Class
class Employee1 extends Person1
{
int sal;
String desgn;
void set_Emp(int id, String name, String desgn, int sal)
{
try {
set_Person(id, name);
this.desgn = desgn;
this.sal = sal;
}
catch (Exception ex) {
ex.printStackTrace();
}
}
void disp_Emp()
{
disp_Person();
System.out.println(desgn + "\n" + sal);
}
// Main function
public static void main(String args[])
{
Employee1 e1 = new Employee1();
e1.set_Emp(1001, "Manjeet", "AP", 20000);
e1.disp_Emp();
}
}
Output
1001 Manjeet AP 20000
Types of Inheritance in Java
- Java supports three types of inheritance in Java: single-level, multilevel, and hierarchical inheritance in the case of classes to avoid ambiguity.
- In Java programming, multiple and hybrid inheritance is supported through the interface only.
1. Single Inheritance
When a class inherits another class, it is known as a single inheritance.
// Single-Level inheritance
// Class B ---> Class A, which means
// that class B is derived from Class A
class A {
int a;
void set_A(int x) {
a = x;
}
}
// Class B have access to all public and
// protected methods and data members of Class A
class B extends A {
int b, product;
void set_B(int x) {
b = x;
}
void cal_Product(){
product = a * b;
System.out.println("Product = " + product);
}
public static void main(String[] args)
{
B b = new B();
b.set_A(5);
b.set_B(5);
b.cal_Product();
}
}
Output
Product = 25
2. Multilevel Inheritance
When there is a chain of inheritance, it is known as multilevel inheritance.
// Multilevel inheritance
// Class C ---> Class B ---> Class A
// Class C is derived from Class B which
// in correspondence derived from Class A
class A {
int a;
void set_A(int x) {
a = x;
}
}
// Child of Class A
class B extends A {
int b;
void set_B(int x) {
b = x;
}
}
// Child of Class B but have access to
// methods of both classes, i.e., Class A and B
class C extends B {
int c, product;
void cal_Product()
{
product = a * b;
System.out.println("Product = " + product);
}
public static void main(String[] args)
{
C c = new C();
// Class C accesses methods of both class A and B
c.set_A(5);
c.set_B(5);
c.cal_Product();
}
}
Output
Product = 25
3. Hierarchical Inheritance
When two or more classes inherit a single class, it is known as hierarchical inheritance.
// Hierarchical inheritance
// Class C ---> Class A <--- Class B
// Both Class B and C inherits Class A
class A {
int a;
void set_A(int x) {
a = x;
System.out.println("Setting A's value to = " + x);
}
}
// Class B derived from Class A
class B extends A
{
int b;
void set_B(int x) {
b = x;
System.out.println("Setting B's value to = " + b);
}
}
// Class C also derived from Class A
class C extends A
{
int c;
void set_C(int x) {
c = x;
System.out.println("Setting C's value to = " + c);
}
}
public class Geeks
{
public static void main(String[] args)
{
C c = new C();
c.set_C(5);
c.set_A(50);
B b = new B();
b.set_B(10);
b.set_A(15);
}
}
Output
Setting C's value to = 5 Setting A's value to = 50 Setting B's value to = 10 Setting A's value to = 15
4. Inheritance in Interfaces
// Java program to demonstrate inheritance in
// interfaces.
import java.io.*;
interface intfA
{
void geekName();
}
interface intfB extends intfA
{
void geekInstitute();
}
// class implements both interfaces and provides
// implementation to the method.
class sample implements intfB
{
@Override
public void geekName() {
System.out.println("Rohit");
}
@Override
public void geekInstitute() {
System.out.println("JIIT");
}
public static void main(String[] args)
{
sample ob1 = new sample();
// calling the method implemented
// within the class.
ob1.geekName();
ob1.geekInstitute();
}
}
Output:
Rohit
JIIT
An interface can also extend multiple interfaces.
// multiple inheritance in interfaces
import java.io.*;
interface intfA
{
void geekName();
}
interface intfB
{
void geekInstitute();
}
// always remember that interfaces always
// extends interface but a class always
// implements a interface
interface intfC extends intfA, intfB {
void geekBranch();
}
// class implements both interfaces and provides
// implementation to the method.
class sample implements intfC
{
public void geekName() {
System.out.println("Rohit");
}
public void geekInstitute() {
System.out.println("JIIT");
}
public void geekBranch() {
System.out.println("CSE");
}
public static void main(String[] args)
{
sample ob1 = new sample();
// calling the method implemented
// within the class.
ob1.geekName();
ob1.geekInstitute();
ob1.geekBranch();
}
}
Output
Rohit JIIT CSE
Why Multiple Inheritance is not supported through a class in Java, but it can be possible through the interface?
Multiple Inheritance is not supported by class because of ambiguity. In the case of interface, there is no ambiguity because the implementation of the method(s) is provided by the implementing class up to Java 7. From Java 8, interfaces also have implementations of methods. So if a class implements two or more interfaces having the same method signature with implementation, it is mandated to implement the method in class also.
Must Read: Java and Multiple Inheritance for more details.
