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Null Interface

Prashanth Paul
Greenhorn

Joined: Jul 24, 2002
Posts: 4
Hi All ,
I am curious know something about the Null interface . What are they ? Can Cloneable , Serializable be called as Null interfaces ? Please let me know.
Thanks.
Valentin Crettaz
Gold Digger
Sheriff

Joined: Aug 26, 2001
Posts: 7610
...
A null interface is an interface with no method declaration. I don't really like this term since "null" has a special meaning in Java. I would prefer calling such an interface an "empty interface" or better a "tag interface" but not a "null interface". Maybe someone else has an idea as to why such interfaces have been called "null interfaces".
I don't know who invented that but "null interface" does not exist in the official Java terminology.
[ December 10, 2002: Message edited by: Valentin Crettaz ]

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Manish Hatwalne
Ranch Hand

Joined: Sep 22, 2001
Posts: 2578

Aren't they called Marker Interfaces?
- Manish
Valentin Crettaz
Gold Digger
Sheriff

Joined: Aug 26, 2001
Posts: 7610
Marker and Tag are synonym in my opinion.
Wiki provides some good explanations:
Tag Interface
Marker Interface
Gustavo Adolpho Bonesso
Ranch Hand

Joined: May 11, 2002
Posts: 103
Would somebody give some examples of null interfaces ?


Gustavo Adolpho Bonesso<br />SCJP1.4
shweta mathur
Ranch Hand

Joined: Sep 23, 2002
Posts: 109
RandomAccess Interface implemented by Vector class & ArrayList is a Marker Interface


--Shweta<br />SCJP 1.4 <br />SCWCD
Ashik Uzzaman
Ranch Hand

Joined: Jul 05, 2001
Posts: 2370

Marker interface, also known as Tag interface, are like directives to the compiler without any method declaration. It is implemented in a class so that other java codes and JVM can test an object of it with "instanceof interface". java.io.Serializable, java.lang.Cloneable, java.util.EventListener, java.util.RandomAccess are marker interfaces from J2SE API that I can recall quickly.

To support with an example from the J2SE API Documentation, you'll see that Serializable is a marker interface, in its comment its written that All subtypes of a serializable class are themselves serializable.

BTW shewta, ArrayList is a concrete final class, its not an interface rather an implementation of RandomAccess marker interface.

Regards,


Ashik Uzzaman
Senior Member of Technical Staff, Salesforce.com, San Francisco, CA, USA.
Pradeep bhatt
Ranch Hand

Joined: Feb 27, 2002
Posts: 8919

BTW shewta, ArrayList is a concrete final class, its not an interface rather an implementation of RandomAccess marker interface.


ArrayList is not a final class.


Groovy
Tony Morris
Ranch Hand

Joined: Sep 24, 2003
Posts: 1608
An interface containing no methods (a tag/marker interface) is poor form, however, the core API has done it, and many other, more disastrous, atrocities. You need not be so concerned with it.


Tony Morris
Java Q&A (FAQ, Trivia)
 
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