Why use an abstract class without abstract methods?

Question

I am now studying a java and I'm at the part of Abstract. I read sorta strange part to me that there is an abstract class which does not include any abstarct method.

Why do they use this kind of class?

In JDK 1.00 an abstract method was required in abstract classes. 1.1 relaxed that restriction, and added classes such as `java.awt.event.KeyAdapter`. http://download.java.net/jdk7/docs/api/java/awt/event/KeyAdapter.html — Tom Hawtin - tackline, Jul 28 '11 at 09:18

score 42 · Accepted Answer · answered Jul 28 '11 at 08:37

42

To prevent instantiation of that class and use it only as a base class. Child classes can use the general methods defined in the abstract class.

For example it doesn't make sense to create an instance of AbstractVehicle. But All vehicles can reuse a common registerMileage(int) method.

answered Jul 28 '11 at 08:37

Bozho

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+1 I think a "canonical" example would be `javax.servlet.HttpServlet`, which is `abstract` but has no abstract methods. The designers of that class simply don't want to allow you to register an `HttpServlet` as such in web.xml, as it's simply useless. – Costi Ciudatu Jul 28 '11 at 08:46
How about if I create an anonymous inner class out of the abstract class? In this way I can instantiate the abstract class without changing anything in it. – Abhishek Jain Jul 16 '17 at 18:30

Bohemian · Answer 2 · 2013-10-07T16:12:54.027

A common reason to do this is to have the abstract class provide exploding implementations of the abstract methods as a convenience to subclasses who don't have to implement all the abstract methods, just those they want to - the remaining ones will still explode but it won't matter if those execution paths aren't exercised.

HttpServlet is an example of this pattern in action. It has default implementations for all methods that handle the different request types, but they all throw an exception. The subclass must override these if they want to do something meaningful. It's OK to leave some handler methods not overridden as long as they are never called.

Maulik Kakadiya · Answer 3 · 2016-08-11T11:02:31.360

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Yes, we can have abstract class without any abstract method. Best example of abstract class without any abstract method is HttpServlet

edited Aug 11 '16 at 11:02

answered Aug 08 '16 at 15:08

Maulik Kakadiya

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score 2 · Answer 4 · answered Jul 28 '11 at 08:38

2

If this class extends another abstract class and don't have implementation of inherited abstract methods.

answered Jul 28 '11 at 08:38

jmj

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In this scenario, it means that class **does have** abstract methods, inherited or not, so it's actually mandatory to declare it as `abstract`. The question is about the scenario when the compiler would not complain if you take the `abstract` keyword out of the class declaration. – Costi Ciudatu Jul 28 '11 at 08:40
@Costi Ciudatu yes. but not declared. – jmj Jul 28 '11 at 08:40

score 2 · Answer 5 · answered Jul 28 '11 at 08:41

2

This class contains some common logic for all its inheritors, but itself does not represent usable entity (in terms of particular application)

answered Jul 28 '11 at 08:41

phoenix.ru.smr

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score 1 · Answer 6 · answered Feb 27 '18 at 12:10

Another example for having such class is when you implement creation helpers. These classes are used to ease the client in the creation of objects, which are related in topic but decoupled depending on the need. By nature, the methods of this creator classes are all static and they can be seen as utility classes as well.Obviously, instatntation of this classes is futile and hence the abstractkeyword.

To mention a recent example I met was the Sftpclass from org.springframework.integration.dsl.sftp which is basically an easy way to require objects (e.g: adapters, gateways) from the sftp api.

score 1 · Answer 7 · answered Jul 28 '11 at 08:45

1

These type of classes are used for a implement a general logic which can be implemented by other classes. Making it abstract prevents from instantiating it. But other classes can inherit the class and its methods.

answered Jul 28 '11 at 08:45

Sumit

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Kumar Manish · Answer 8 · 2012-03-08T14:02:09.253

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Say you have a set of related classes, but no related (shared) code, yet. If we make all of these classes extend a base class with no abstract methods, that then if we wan't all of these classes to have an identical method/feature in the future, that can be done in one shot by putting it in the base class. So code is not repeated and it reflects in all child classes by including it in just one place.

edited Mar 08 '12 at 14:02

answered Mar 08 '12 at 13:36

Kumar Manish

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score 0 · Answer 9 · answered Jul 20 '15 at 09:09

I develop a abstract class to prevent instantiation of that class and use it only as a base class. because, These type of classes are used for a implement a general logic which can be implemented by other classes. Sometimes, I have a default implementation for every method in abstract class. In the manner, it doesn't force the sub-class to override all of method, but also it implement everyone that is need.It means implicitly you have to override at least one method to make scene using this abstract class.

score -1 · Answer 10 · answered Jul 28 '11 at 08:37

-1

I can't think of any good reason to use it. It could be used as "marker" but an interface would be a better choice.

answered Jul 28 '11 at 08:37

Bart van Heukelom

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Since it's about "no abstract methods", it means the class provides default implementation for its entire behaviour, so an interface is not at all a replacement, quite the contrary. – Costi Ciudatu Jul 28 '11 at 08:48
@Costi Ciudatu: Of course. I'm afraid I misinterpreted it as "no methods" – Bart van Heukelom Jul 28 '11 at 08:50
It's utterly wrong design choice to have a class declared as abstract with actually no abstract methods. It's confusing to start with. why would anyone do that ? – panzerschreck Jul 28 '11 at 08:50
@panzerschreck: Have a look at Bohemian's answer (or my comment on Bozho's answer) for an example. – Costi Ciudatu Jul 28 '11 at 08:53
An useless class throwing exceptions on calling any of its methods is "useless" - an interface should be the way to go :). Isnt it ? – panzerschreck Jul 28 '11 at 09:44
@panzerschreck They probably don't throws exceptions in *all* methods. – Bart van Heukelom Jul 28 '11 at 15:19
there are dozens of examples of usage - HttpServlet for example, and of course no one use it as a marker – Ilya Buziuk May 22 '15 at 14:01

score -4 · Answer 11 · answered Dec 21 '13 at 09:22

-4

Abstract class without abstract method means you can create object of that abstract class. See my Example.

abstract class Example{
void display(){
 System.out.println("Hi I am Abstract Class.");
}
}
class ExampleDemo 
{
public static void main(String[] args) 
{
Example ob = new Example(){};
ob.display();
}
}

If you write one abstract method inside abstract class then it will not compile. Which means if you create abstract class without abstract method then you can create Object of that Abstract Class.

answered Dec 21 '13 at 09:22

lal

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Your example does not create an instance of the abstract `Example` class. It creates an instance of a non-abstract, anonymous inner class that `extends Example`. Compile your program and look at the .class files that the compiler generates. You'll see what I mean. – Solomon Slow Jan 28 '15 at 15:14
Besides, that would be an unstable side-effect at best. To say that it means that "...you can create [an] object of that abstract class" would be a dangerous proposition because what would happen to the implementation if you later added an abstract method to your abstract class? – Draghon May 30 '15 at 17:07
you can't create an instance of an abstract class, your example is mistaken – jucardi Oct 05 '16 at 21:57

Why use an abstract class without abstract methods?

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