Published 31 Aug, 2022

Java - How is abstract class different from concrete class?

Category Java
Modified : Sep 29, 2022
56

I understand WHY we need Abstract Class in Java - to create sub-classes. But the same can be achieved by concrete class. e.g. Class Child extends Parent. Here Parent can very well be abstract & concrete. So why do we have ABSTRACT??

Answers

There are 4 suggested solutions here and each one has been listed below with a detailed description. The following topics have been covered briefly such as Java, Class, Abstract. These have been categorized in sections for a clear and precise explanation.

92

Abstract classes cannot be instantiated directly. Declaring a class as abstract means that you do not want it to be instantiated and that the class can only be inherited. You are imposing a rule in your code.

If you extend your Parent/Child relationship example further to include a Person class then it would make good sense for Person to be abstract. Parent is a concrete idea and so is child. Person is an abstract concept in reality as well as in code.

One benefit is that you explicitly define and protect the idea of the abstract class. When you declare a class as an abstract there's no way that you or anyone else using your code uses it incorrectly by instantiating it. This reasoning is similar to why we specify functions and fields as public, private or protected. If you declare a function or member as private you are in effect protecting it from improper access from client code. Privates are meant to be used within the class and that's it. Abstract classes are meant to be inherited and that's that.

Now, do you have to use abstract classes and define functions and fields as private instead of public? No, you don't. But these concepts are provided to help keep code clean and well-organized. The abstract class is implemented in all object-oriented languages to my knowledge. If you look around you will see that C++, C#, VB.NET etc. all use this concept.

A better, specific example:

Shape hierarchy UML Diagram

In the example above the Shape class should be abstract because it is not useful on its own.


51

An abstract class is meant to be used as the base class from which other classes are derived. The derived class is expected to provide implementations for the methods that are not implemented in the base class. A derived class that implements all the missing functionality is called a concrete class


38

The point of abstraction is not to create sub-classes. It's more about creating Seams in your code. You want code to be test-able and decoupled which lead to the ultimate goal of maintainability. For similar reasons, abstraction also buys us the ability to replace a bit of code without rippling side effects.


27

For those who seek only differences in pure technical approach, the clearest difference between concrete parent classes and abstract parent classes is the obligation for children to include/implement specific methods.

A concrete parent class cannot force/oblige its children to include/implement a method. An abstract parent class oblige its children to do that by declaring abstract methods.

Apart from the above, it comes to design and functional requirements to dictate the use of abstract class. Such examples can be found on javax.servlet.http.HttpServlet class