Tagging Interfaces in Java

Question

What are tagging interfaces and what are they used for?

Answer 1

You can tag your class with a tagging interface to say to your fellow developer and consumer of your class that you explicitely support that functionality. Think of Serializable; someone who needs to persist a Session and uses serialization to do that can safely use an object of your class.

It can be further used in reflection; nowadays it is common to use annotations to do this but in the olden days you can inspect a class, check whether it implements a certain interface (like DAO) and if so, process the object further (I'm thinking about the Entity annotation here).