What is marker or tagged interfaces in Java

The most common use of extending interfaces occurs when the parent interface does not contain any methods. For example, the MouseListener interface in the java.awt.event package extended java.util.EventListener, which is defined as −

Example 

package java.util;
public interface EventListener {}

An interface with no methods in it is referred to as a tagging interface. There are two basic design purposes of tagging interfaces − 

Creates a common parent

As with the EventListener interface, which is extended by dozens of other interfaces in the Java API, you can use a tagging interface to create a common parent among a group of interfaces. For example, when an interface extends EventListener, the JVM knows that this particular interface is going to be used in an event delegation scenario. 

Adds a data type to a class

This situation is where the term, tagging comes from. A class that implements a tagging interface does not need to define any methods (since the interface does not have any), but the class becomes an interface type through polymorphism.