Understanding contains method of Java HashSet

Question

Newbie question about java HashSet

Set<User> s = new HashSet<User>();
User u = new User();
u.setName("name1");
s.add(u);
u.setName("name3");
System.out.println(s.contains(u));

Can someone explain why this code output false ? Moreover this code does not even call equals method of User. But according to the sources of HashSet and HashMap it have to call it. Method equals of User simply calls equals on user's name. Method hashCode return hashCode of user's name

Answer 1

If the hash code method is based on the name field, and you then change it after adding the object, then the second contains check will use the new hash value, and won't find the object you were looking for. This is because HashSets first search by hash code, so they won't bother calling equals if that search fails.

The only way this would work is if you hadn't overridden equals (and so the default reference equality was used) and you got lucky and the hash codes of the two objects were equal. But this is a really unlikely scenario, and you shouldn't rely on it.

In general, you should never update an object after you have added it to a HashSet if that change will also change its hashcode.

Answer 2

Since your new User has a different hashcode, the HashSet knows that it isn't equal.

HashSets store their items according to their hashcodes.
The HashSet will only call equals if it finds an item with the same hashcode, to make sure that the two items are actually equal (as opposed to a hash collision)