Question

What are tagging interfaces and what are they used for?

Answer 1

Because sometimes, it really makes sense if some property of a type can be used as a type itself – Serializable comes to mind. If I make a method like this:

public void save(Object data){ ... }

… you don’t really know how that data will be saved. VM serialization? Bean property serialization? Some homebrewed scheme? Whereas if you write it like this:

public void save(Serializable data){ ... }

… it is quite clear (if only the designer of ObjectOutputStream had used this possibility!). Sometimes it makes sense to use annotations when you want to add meta-data to types, but in this case, I’d argue for a tagging interface.