Tagging Interfaces in Java

Question

What are tagging interfaces and what are they used for?

Answer 1

The question of marker interfaces vs annotations is discussed in Bloch's "Effective Java", and part of that section is available on google books here

Answer 2

I would also add you can use tagging interfaces to restrict ownership of an instance:

interface IFlumThing;
interface IFlooThing;

class BaseThing {...}

class FlumThing extends BaseThing implements IFlumThing {};
class FlooThing extends BaseThing implements IFlooThing {};

class Flum {
   addThing(IFlumThing thing){...};
}

class Floo {
   addThing(IFlooThing thing){...};
}