Marker interface

[praveen kumar gowda]

what is marker interface?

the interface does not have method declare in it? if it so then,
Comparable is also a marker interface though there is a method in it.

please tell me ..clearly about this topic...

[Seetharaman Venkatasamy]

From JLS :

If an interface has no direct superinterfaces, then the interface implicitly declares a public abstract member method m with signature ss, return type rr, and throws clause t corresponding to each public instance method mm with signature ss, return type rr, and throws clause tt declared in Object, unless a method with the same signature, same return type, and a compatible throws clause is explicitly declared by the interface. It is a compile-time error if the interface explicitly declares such a method m in the case where m is declared to be final in Object.

reference : JLS

[Anayonkar Shivalkar]

praveen kumar gowda wrote: what is marker interface?

the interface does not have method declare in it? if it so then,
Comparable is also a marker interface though there is a method in it.

please tell me ..clearly about this topic...

As you have mentioned, yes, marker interface is an interface with no methods.

What exactly do you want to know? I didn't understand what is your question.

[Seetharaman Venkatasamy]

by the way: Comparable is not a Marker Interface.http://docs.oracle.com/javase/1.4.2/docs/api/java/lang/Comparable.html . I assumed that you are asking about how a marker interfaces used to access object methods.

<edit>
I do call it as a Mixin instead of Marker...
</edit>

[Kumaravadivel Subramani]

Marker interface doesn't have any methods in it and it simply tells the JVM what to do. you could also create marker interface to do some process.

[Matthew Brown]

Seetharaman Venkatasamy wrote:I do call it as a Mixin instead of Marker...

That's not standard usage, though. In languages that have proper mixins (and that doesn't include Java) they aren't the equivalent of marker interfaces. In fact, a mixin contains implementation, as well as defined methods.

[Winston Gutkowski]

Kumaravadivel Subramani wrote:Marker interface doesn't have any methods in it and it simply tells the JVM what to do. you could also create marker interface to do some process.

Actually it doesn't do anything, JVM or otherwise. It is exactly what it says: a marker (or flag) that your program can check (using instanceof) to see if an object has a particular characteristic or not.

The best example I know is RandomAccess, which is supposed to be implemented by Lists that exhibit good (usually O(1), but certainly better than O(n)) "get element n" performance.
If you look at ArrayList, you'll notice that it implements it, whereas LinkedList doesn't.

HIH

Winston

[Edit] BTW, java.lang.Cloneable looks like a Marker interface, but actually isn't. In fact, it's a bit of an aberration, and (hopefully) never to be repeated.

[Jeff Verdegan]

Winston Gutkowski wrote:
[Edit] BTW, java.lang.Cloneable looks like a Marker interface, but actually isn't. In fact, it's a bit of an aberration, and (hopefully) never to be repeated.

How do you figure it's not?

[Kumaravadivel Subramani]

Yeap, used a wrong word over there as "JVM" instead of java libraries which does a specific job when any type implements this marker interface type. Thanks for correcting me.

[Winston Gutkowski]

Jeff Verdegan wrote:

Winston Gutkowski wrote:
[Edit] BTW, java.lang.Cloneable looks like a Marker interface, but actually isn't. In fact, it's a bit of an aberration, and (hopefully) never to be repeated.

How do you figure it's not?

Because
(a) It does do something.
(b) It requires the programmer to implement a clone() method in order to work properly.
(c) It is not used like one. When was the last time you wrote
if (someObject instanceof Cloneable) ...

I suppose you could argue semantically that it is one for the Object class, but in my view it was just a bad day at the office for the designer.

I'd argue the same about Serializable: looks like a duck, but don't quack like one.

Winston

[Jeff Verdegan]

Winston Gutkowski wrote:

Jeff Verdegan wrote:

Winston Gutkowski wrote:
[Edit] BTW, java.lang.Cloneable looks like a Marker interface, but actually isn't. In fact, it's a bit of an aberration, and (hopefully) never to be repeated.

How do you figure it's not?

Because
(a) It does do something.

That's a runtime issue.

(b) It requires the programmer to implement a clone() method.

No it doesn't.

public class Clone implements Cloneable {
    int x;

    Clone(int x) {
        this.x = x;
    }

    public static void main(final String[] args) throws Exception {
        Clone c1 = new Clone(123);
        Clone c2 = (Clone)c1.clone();
        System.out.println(c2);
    }

    public String toString() {
        return String.valueOf(x);
    }
}

(c) It is not used like one. When was the last time you wrote
if (someObject instanceof Cloneable) ...

I don't, but the JVM does.

I always figured what makes a marker interface is simply one that doesn't declare any methods, so implementing it is not carries no compile-time requirements, and it's mainly used at runtime in an instanceof check. The fact that I don't personally do the instanceof check in my code, or that the result of it failing is an exception (as opposed to, say, choosing some other object or some other access method, as with RandomAccess), doesn't make it less of a marker interface.

Unless I've missed an official definition somewhere, which is certainly possible.

[Tim Moores]

Cloneable is not such a good example, as it arguably should contain the clone method. Sun's class library designers weren't perfect :-)

[Winston Gutkowski]

Jeff Verdegan wrote:No it doesn't.

OK, you got me

I'd still argue that that's a pretty weird example (in fact, the whole interface is a bit weird), and that Cloneable is not what I'd consider a Marker interface.

Winston

[Winston Gutkowski]

Winston Gutkowski wrote:and that Cloneable is not what I'd consider a Marker interface.

It occurred to me that maybe I should say what I do consider an MI.

To me, it's an interface that can be used to implement a strategy. For example, if I want to sort or shuffle a List, I can be pretty sure that if it implements RandomAccess, I will be able to swap elements pretty quickly, whereas if it doesn't it's probably best to do a copy, sort and reload.

If you know something's Cloneable, or Serializable, what do you know? It's unlikely that you could use the information in any effective way.

My two-penn'orth, FWIW.

Winston

[Jeff Verdegan]

Winston Gutkowski wrote:
If you know something's Cloneable, or Serializable, what do you know? It's unlikely that you could use the information in any effective way.

Sorry to flog and expired equine, but again, just because it's not my code that's using that information to make a decision, I don't see why that makes a difference.

[Matthew Brown]

I'd agree with Jeff. Just because a marker interface was a bad choice of design doesn't mean it's not a marker interface.

[Winston Gutkowski]

Matthew Brown wrote:I'd agree with Jeff. Just because a marker interface was a bad choice of design doesn't mean it's not a marker interface.

Seems I'm outvoted.

BTW, I clicked on the 'Report' icon by accident (I knew it would happen sooner or later). I hope it didn't cause any ripples.
Any chance it could be moved to the left hand side of the window?

Winston