What is Inheritance in Java Based on Class?

Inheritance :

Creating a class from an existing class is called inheritance.
The existing class is called the Super Class / Parent class / Base class.
The derived class is called the Subclass / Child class / Derived class.
We can achieve inheritance with the help of extends keywords.
Inheritance related to reusability of the code .

1: Single Inheritance

Creating a class with the help of single super class is called Single Inheritance .
Here only one sub class and only one super class .

//Single Inheritance

class Vehicle
{
}
class Car extends Vehicle
{
  void main(){}
}

2: Multilevel Inheritance

Creating a class with the help of Sub-class is known as Multilevel Inheritance .
Here more then One level present .

//Multilevel Inheritance 

class ElectronicDevice
{
}
class MobileDevice extends ElectronicDevice 
{

}
class SmartPhone extends MobileDevice
{
    void main(){}
}

3: Multiple Inheritance

creating a class with the help of more than one super class is called Multiple Inheritance .
java class does not support multiple inheritance (b/c ambiguity to call ).
But we achieve multiple inheritance with the help of interface .

//Multiple Inheritance

class A
{
}
class B
{
}

 class Car extends A ,B
{
  void main(){}
}

 error: '{' expected
 class Car extends A ,B
                    ^
1 error

4: Hierarchical Inheritance

creating more than one sub classes with the help of single super class is known as Hierarchical Inheritance .
here one super class but more than one sub class .

//Hierarchical inheritance 

class Vehicle
{
}
class Car extends Vehicle
{
}

 class Bike extends Vehicle 
{
  void main(){
    IO.println("hi java");
  }
}

5: Hybrid Inheritance

-It is a combination of more than one type of inheritance.

// Hybrid inheritance 
class A
{
}
class B extends A
{
}

 class D extends A 
{

}

class C extends B
{
    void main(){}
}

Why does Java not support multiple inheritance?

Why can’t a child class reference variable hold a parent class object?

// Why can’t a child class reference variable hold a parent class object?

class Vehicle{
int x=10;
int y=20;

}

 class Car extends Vehicle {
     int a=100;
     int b=200;
    public static void main(String[]   args) {
        Vehicle v1 = new Vehicle();
                Car c1 = new Car();
        Vehicle v2 = new Car();

    }
}

Notes

Car c2 = new Vehicle();

error: incompatible types: Vehicle cannot be converted to Car
                Car c2 = new Vehicle();

note

Here , Car class reference variable ( c2 ) requirement a , b , x and y non- static variables but vehicle object having only x and y . that's why it is Invalid . Hence , child class reference variable can not hold Parent / Super class Object .
But Super class reference variable ( v2 ) can hold sub - class Object .
here , Car class is sub class and car object having x, y ,a, and b non- static variable but Vehicle class reference variable requirement only x and y . (no problem because car object fulfill the requirement of Vehicle class reference variable . )