Java docs says the following about Set interface, can someone please help me understand how these additional stipulations are forced? I mean you cannot throw an extra exception that is not thrown by super interface (in this case Collection). 

> 
> The Set interface places additional
>  stipulations, beyond those inherited
>  from the Collection interface, on the
>  contracts of all constructors and on
>  the contracts of the add, equals and
>  hashCode methods.
> 
> 
> 

Thanks

-Abidi