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 the equals method of the User class.
But according with the sources of HashSet and HashMap they have to call it. The equals method of the User simply calls equals on user's name. The hashCode method returns the hashcode of the user's name.
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.