What is exact difference between Inheritance and Abstract class?

Question

I know the fundamentals of OOP concepts[Inheritance, Abstraction, Encapsulation, Polymorphism]

We use Inheritance in case of Parent-Child relationship[Child can have all functionalities which Parent have and can add more functionality to itself too]

And we use Abstract class(In java) for a partial set of default implementations of methods in a class, which also can be implemented by simple Inheritance.

Look below example which makes my point clear.

Inheritance:

Parent class

public class Parent {

    // This method will remain same for all child classes.No need to override
    public void abc() {
        System.out.println("Parent here");
    }

    // This methods need to be overridden from child class
    public int getROI() {
        return 0;
    }
}

Child class

public class Child extends Parent{

    @Override
    public int getROI(){
        return 5;
    }

    public static void main(String[] args) {
        Child child =new Child();
        child.abc();
        System.out.println(child.getROI());
    }
}

Abstract Class:

Parent class

abstract class Parent {

    // This method will remain same for all child classes.No need to override
    public void abc() {
        System.out.println("Parent here");
    }

    // This methods need to be implemented from child class
    public abstract int getROI();
}

Child class

public class Child extends Parent{

    public int getROI(){
        return 5;
    }

    public static void main(String[] args) {
        Child child =new Child();
        child.abc();
        System.out.println(child.getROI());
    }
}

For above programs o/p will be same.

O/P:    
Parent here
5

So I think,

Inheritance: We need to override the method in child class

Abstract class: Put abstract keyword in method name and need to implement the method in child class

So Inheritance and abstract class is same regardless of abstract keyword

So we can implement abstract class using inheritance, here just method signature change classes(That's my belief).

Is there any significant difference?

Answer 1

Inheritance is for inheriting properties and having some of its own as well.

Abstract is to restrict from being instantiated.

Example:
Lets take Vehicle and VehiclePart. But Vehicle as such is very abstract and not complete. So we want Vehicle class abstract because we don't want to instantiate it directly. Car is more meaningful entity than Vehicle and car is a Vehicle. So car extends vehicle and it is not abstract.

abstract class Vehicle{
    String name;
}

abstract class VehiclePart{
    String name;
    Date expiry;
}

class Car extends Vehicle{
     List<VehicleParts> parts;
}

class RacingCar extends Vehicle{

}

class Gear extends VehiclePart{
   int numOfGears;
}

Inheritance: We need to override the method in child class

Nope. in the above example you can see Car is inheriting properties like name from Vehicle. Overriding is optional. Like RacingCar can override methods of Car and make it a little bit custom. But basically it is getting(inheriting) some properties from base class. Like all the basic properties of a car will in Car and not in RacingCar. RacingCar will have properties specific to it.

Abstract class: Put abstract keyword in method name and need to implement the method in child class

Nope. It is just to restrict its instantiation. Eg. We don't want to instantiate Vehicle object because there is no meaning to it. A vehicle has to be something like car, bus etc etc. It can't just be a vehicle. So we put abstract and restrict instantiation.

Answer 2

After java 8 you can have static and default methods in Interface. So it makes the interface much similar to abstract class.

But Still abstract class is class so we can have constructor, instance variable, getter and setter to change the state of objects. These all functionalities not provided by interface .That is main difference between interface and abstract class after java 8.