What is the difference between interface and abstract classes?

Answer 1

Interface:
Interface are just like a Contract what a class gets into & the class should follow the contract.
Incomplete methods inherited from an Interface should be Completed in a Class.
If you don't Complete the method in the class then you will get an Error.

Difference in Example Wise :
Public interface A
{
public void test ();
//An Incomplete Method

}

Public class B implements A
{
public static void main(String[] args)
{
B b= new B();
b.test();
}
}

Key Difference for you :
- Interface can story only incomplete methods
- By default every method is abstract in an Interface.
- Every variable by default in an Interface is Public Static & Final.
- Multiple Inheritance is Supported in Interface's.
- We can Never create the Object of Interface.
- We can Not keep a Main Method in an Interface.

Abstract Class:
Abstract classes are Partial incomplete in there Nature.
Every method should have abstract keyword to specify that it is Incomplete.

Difference in Example Wise :
public abstract class A
{
public abstract void test1();
//An Incomplete Method

public void test2();
{
System.out.println("Inside A");
}
}

Public class B implements A
{
public void test1()
{
System.out.println("Inside A");
}

public static void main(String[] args)
{
B b= new B();
b.test1();
b.test2();
}
}

Key Difference for you :
- An abstract class can consist of both Complete & Incomplete methods in it.
- Every method by default is Non-Abstract.
- We should use abstract keyword to specificy in complete method.
- Variable by default is Not Final.
- Multiple Inheritance in Abstract class is not possible.
- Inheritance between an Abstract class & Interface is possible.

Answer 2

how come there are so many questions about interfaces and abstract classes?

https://www.sololearn.com/Discuss/363907/?ref=app

Answer 3

As far as I know from C# and Java:

Abstract Class:
- needs to be inherited
- can contain state members and implementations of methods
- derived classes don't require to implement the abstract methods

Interfaces:
- needs to be implemented
- doesn't have any form of implementation
- classes that implement interfaces require to implement every method of each interface used

You can only use one abstract class because of the single-inheritance limitations of C# and Java, but you can implement multiple interfaces.

Feel free to add/correct me if I missed something.