Marker Interfaces in Java

Marker Interfaces in Java have special significance because of the fact that they have no methods declared in them which means that the classes implementing these interfaces don't have to override any of the methods. A few of the marker interfaces already exist in the JDK like Serializable and Cloneable.

One can also create their own custom interfaces which doesn't have any method. The purpose of these interfaces is to force some kind of functionality in the classes by providing some functionality to a class if it implements the 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. A very good example of marker interface is Serializable where the class implements can be used with ObjectOutputStream and ObjectInputStream classes.

The Java language specification doesn't itself define the term marker interface and the term has been coined by authors, developers and designers. 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.