[pedantic]Set doesn't use any methods, it's just an interface[/pedantic]
Lukas is right and that's why if you override equals or hashCode,
you should really override the other one as well. If you don't, you can get some very, very strange behaviour in your sets (and maps). Like being able to add something to a set or map, but never able to find out later on if it is in there.
You appear to have Sierra&Bates - read chapter 7 (Objective 6.2), it's excellent.