Wednesday, 31 May 2017

What is marker Interface in java?

Marker Interface doesn't have any method or field or constant declared within it. So a marker interface is an empty interface.

Example :

  • Serializable
  • Cloneable, 
  • RandomAccess interface and 
  • Remote interface

Now one question that would immediately pop-up in your mind is,

Why would one declare an empty interface? 
What good is it of?

In general a common belief is that a marker interface is generally used to send some kind of signal to the JVM and instructs JVM to  treat the class implementing it in a special way.

Like, when a class implements Cloneable marker interface, it signals the compiler that the class allows creation of clones of its objects.

Generally, a marker interface is used with the instanceof operator to check if a reference type variable refers to an object whose class implements the marker interface.

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  1. instanceof is used to identify a base type irrespective of whether the interface is a marker or any other special type you may want to call it.

    The above interface or the Java specified interfaces are meant to provide a uniform way to interact with the class. If you know how for each loop works you would understand what I mean.

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