 <p>The add function returns a boolean which you can check to determine if the item was already in the Set. This is of course based on your needs and isn't a best practice. Its good to know that it will not remove an item that is already there so it can't be depended on to update the existing value with new information if you are defining equals based on surrogate keys from your database. This is opposite the way Maps work as a map will return any existing value and replace it with the new value.</p>