Interfaces and Inheritance in Java

Prerequisites: Interfaces in Java, Java, and Multiple Inheritance A class can extend another class and/ can implement one and more than one interface.

Example:

Java

// Java program to demonstrate that a class can
// implement multiple interfaces
import java.io.*;
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
{
    public static void main (String[] args)
    {
        sample ob1 = new sample();

        // calling the method implemented
        // within the class.
        ob1.m1();
        ob1.m2();
    }
}

Output;

Welcome: inside the method m1
Welcome: inside the method m2

Interface inheritance : An Interface can extend other interface.

Inheritance is inheriting the properties of parent class into 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 parent- child relationship.

Eg:
Dog IS_A Animal
Car IS_A Vehicle
Employee IS_A Person
Surgeon IS_A Doctor etc.

Java

class Animal
{
    public void eat()
    {
    }
}

class Dog extends Animal
{
    public static void main(String args[])
    {
        Dog d=new Dog();

        d.eat();
    }
}

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.

Eg_1:

Java

import java.io.*;
class Person {
    int id;
    String name;
    void set_Person()
    {

        try {
            BufferedReader br = new BufferedReader(
                new InputStreamReader(System.in));
            System.out.println("Enter the Id:");
            id = Integer.parseInt(br.readLine());
            System.out.println("Enter the Name");
            name = br.readLine();
        }
        catch (Exception ex) {
            ex.printStackTrace();
        }
    }
    void disp_Person()
    {
        System.out.print(id + "\t" + name + "\t");
    }
}
class Employee extends Person {
    int sal;
    String desgn;
    void set_Emp()
    {
        try {
            set_Person();
            BufferedReader br = new BufferedReader(
                new InputStreamReader(System.in));
            System.out.println("Enter the Designation:");
            desgn = br.readLine();
            System.out.println("Enter the Salary:");
            sal = Integer.parseInt(br.readLine());
        }
        catch (Exception ex) {
            ex.printStackTrace();
        }
    }
    void disp_Emp()
    {
        disp_Person();
        System.out.println(desgn + "\t" + sal);
    }
    public static void main(String args[])
    {
        Employee e1 = new Employee();
        e1.set_Emp();
        e1.disp_Emp();
    }
}

Eg_2:

Java

class Person1 {
    int id;
    String name;
    void set_Person(int id, String name)
    {
        try {
            this.id = id;
            this.name = name;
        }
        catch (Exception ex) {
            ex.printStackTrace();
        }
    }
    void disp_Person()
    {
        System.out.print(id + "\t" + name + "\t");
    }
}
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.print(desgn + "\t" + sal);
    }
    public static void main(String args[])
    {

        Employee1 e1 = new Employee1();
        e1.set_Emp(1001, "Manjeet", "AP", 20000);
        e1.disp_Emp();
    }
}

Types of inheritance in java

 Java Support three types of inheritance in java: single level, multilevel and
hierarchical inheritance in case of classes to avoid ambiguity.
 In java programming, multiple and hybrid inheritance is supported through
interface only.

Single Inheritance Example

When a class inherits another class, it is known as a single inheritance. 

Java

class A {
    int a;
    void set_A(int x) {
      a = x;
    }
}
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();
    }
}

Multilevel Inheritance Example

When there is a chain of inheritance, it is known as multilevel inheritance.

Java

class A {
    int a;
    void set_A(int x) {
      a = x;
    }
}
class B extends A {
    int b;
    void set_B(int x) {
      b = x;
    }
}
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();
        c.set_A(5);
        c.set_B(5);
        c.cal_Product();
    }
}

Hierarchical Inheritance Example
When two or more classes inherit a single class, it is known as hierarchical
inheritance.
Eg:

Java

class A {
    int a;
    void set_A(int x) {
      a = x;
    }
}
class B extends A {
    int b;
    void set_B(int x) {
      b = x;
    }
}
class C extends A {
    int c;
    void set_C(int x) {
      c = x;
    }

Java

// 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.

Java

// Java program to demonstrate multiple inheritance
// in interfaces

import java.io.*;

interface intfA {
    void geekName();
}

interface intfB {
    void geekInstitute();
}

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. Refer to Java and Multiple Inheritance for details. 

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