What is the main advantage of making a class abstract

Question

Why do we declare a class as abstract? I know it cannot be instantiated, but why give a special keyword for it. Even a \'normal\' class will work just as well and can be eas

Answer 1

I think you misunderstand the point of abstract classes: they provide a partial implementation of some functionality, but not a complete implementation.

You suggested that abstract classes were redundant because you can define incomplete methods using public void methodname(){} -- which is certainly ok. However, let's say your clients inherit from a class defined in such a way, how do they know which methods to override? What happens if they forget to override a method? Now their derived class has an incomplete definition -- you don't want that.

The abstract keyword forces clients to provide implementations for certain methods, otherwise the code won't even compile. In other words, it provides a compile-time guarantee that classes you use or create are fully implemented.

Answer 2

Declaring the class abstract prevents any code from instantiating the class.

This enforces the design guideline to make non-leaf classes abstract.

It allows you to add abstract methods to your superclass (and implementations to the subclasses) later, without affecting any existing clients.

The abstract keyword works even if the non-leaf class does not currently have any abstract methods.

Answer 3

It's useful if you want to have a group of classes that inherit the same logical functions. But in the same time, the class only implements the basic logic, and doesn't contain any real functionality.

You should see it as a skeleton class.

For example, I once made a class with these specifications :

1. Controls the process of executing a command on another thread, and relays events of that process.

2. The class itself didn't have any functionality by itself (didn't implement the actual work() function)

So the result is an abstract class, that can be inherited, that already has a built in thread control, which all you need to do is just implement the work() method.