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.
marker interfaces java example tutorialThis 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 a 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.
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 gives thread-safe guarantee and any modification should not violate that. Marker interface can also help 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 ThraedSafe 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.