CODEWITHSUNDEEP
java
user826323
user826323

Reputation: 2338

java - unique difference between abstract class and concrete class

I know few differences between abstract class and concrete class. I know that you can't create an instance with abstract class unlike concrete class, abstract class can have 'abstract' methods.

But i have an example like the following. A lot of times, we see the following examples at work. I will just skip some common methods that can be defined in the Parent class.

public abstract class Parent {
    public void init() {
       doInit();
    }
    public abstract void doInit();
}

public class Child extends Parent {
    public void doInit() {
       // implementation
    }
}

I think that we can do the same thing with a concrete class like the following.

public class Parent {
    public void init() {
      doInit();
    }
    public void doInit() {
       // Empty
    }
}

I am curious to see if there is any unique situation that we have to use abstract class. Is there any significant difference during runtime with the above example?

Thank you.

Upvotes: 3

Views: 13905

Answers (4)

Kshitish
Kshitish

Reputation: 21

A concrete class is one which has implementation (code inside) for all the methods. It does not matter whether it is derived from some other class.

public abstract class IAmAbstract{
    public void writeMe(){
        System.out.println("I am done with writing");
    }
}

public class IAmConcrete extends IAmAbstract{
    public void writeMe(){
        System.out.println("I am still writing");
    }
}

Upvotes: 2

Cajetan Rodrigues
Cajetan Rodrigues

Reputation: 172

Of course you can do it that way, but it all depends on the right business logic.There might arise a situation where you'd want to enforce a policy on people extending your code.

For example, I write an Employee class and you extend my class for writing a ProjectManager class. But suppose the business does not allow direct instantiation of Employee (like I said, just an example). So I declare my Employee class as abstract, thereby enforcing upon all extenders (read:you) of my class the rule that they can't instantiate Employee directly. (It will happen indirectly through the inheritance chain, of course, i.e. parent objects are created before child objects.)

Used properly, a person at place A controls how another person at place B will code.

Upvotes: 2

Aatch
Aatch

Reputation: 1856

Abstract classes have a variety of useful properties in use with software design.

Other than the obvious differences, such as being unable to be instantiated and being able to hold abstract methods. They are useful for defining common, yet overridable, functions, holding static methods that deal with it's children in a logical manner.

My favorite is the abstract factory pattern though.

By making a factory that is the parent of all the classes it may create, it can force functionality required for creation, this actually causes an odd artefact where technically tighter-coupled code is actually easier to maintain.

Upvotes: 0

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726479

The reason to use abstract class in this situation is to force everyone inheriting your base class to override the abstract doInit method. Without the class and the method being abstract, they may forget to do so, and the compiler would not catch them.

In addition to this pragmatic purpose, abstract classes provide a powerful way to communicate your design idea to the readers of your code. An abstract class tells the reader that the methods inside provide some common implementation for a group of related classes, rather than implementing a single concept that you are modeling. Very often communicating your intent to your readers is as important as it is to write correct code, because otherwise they might break something while maintaining your code.

It is customary in Java to call abstract classes Abstract...; in your example that would be AbstractParent.

Upvotes: 7

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