Monday, 16 June 2014

What is marker interface in Java? Why need marker interface?

   Marker interface in Java is interfaces with no field or methods or in simple word empty interface in java is called marker interface. Example of market interface is Serializable, Clonnable and Remote interface. Now if marker interface doesn't have any field or method or behavior they why would Java needs it? 

      Why Marker or Tag interface do in Java:--

1) Looking carefully on marker interface in Java e.g Serializable,Clonnable and Remote it looks they are used to indicate something to compiler or JVM.
So if JVM sees a Class is Serializable it done some special operation on it,similar way if JVM sees one Class is implement Clonnable it performs some operation to support cloning. Same is true for RMI and Remote interface. So in short Marker interface indicate signal or a command to Compiler or JVM.
          This is pretty standard answer of question about marker interface and once you give this answer most of the time interviewee definitely asked "Why this indication can not be done using flag inside a class?" this make sense right? Yes this can be done by using a boolean flag or a String but doesn't marking a class like Serializable or Clonnable makes it more readable and it also allows to take advantage of Polymorphism in Java.

   Where Should I use Marker interface in Java :--

           Apart from using built in marker interface for making a class Serializable or Clonnable. One can also develop his own marker interface. Marker interface is a good way to classify code. You can create marker interface to logically divide your code and if you have your own tool than you can perform some pre-processing operation on those classes. Particularly useful for developing API and framework like Spring or Struts.
         After introduction of Annotation on Java5, Annotation is better choice than marker interface and JUnit is a perfect example of using Annotation e.g @Test for specifying a Test class. Same can also be achieved by using Test marker interface.

      Another use Of Marker Interface in Java:

           One more use of marker interface in Java can be commenting. A marker interface called Thread Safe can be used to communicate other developers that classes implementing this marker interface thread-safe guarantee and any modification should not violate that.  Marker interface can also code coverage or code review tool to find bugs based on specified behavior of marker interfaces. Again Annotations  are better choice @ThreadSafe looks lot better than implementing ThreadSafe marker interface.

A common question asked very frequently is about Runnable interface being marker or not.?
       Runnable interface is not marker because Runnable interface has the public void run() method declared inside it.

   One common question asked is if we can create a marker interface or not and the answer is yes because of following reason:
           We can't create marker interface similar to Serializable or Cloneable but we can simulate the functionality by writing extra code around the custom marker interface.

            In summary marker interface in Java is used to indicate something to compiler, JVM or any other tool but Annotation is better way of doing same thing.

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