Understanding TreeSet when compareto returns 0

Question

I have created a Student class like this:

public class Student implements Comparable {

    private String firstName;
    private String lastN         


        

Answer 1

That is because TreeSet uses compareTo (or Comparator.compare) to test whether two elements are equal. This is what the docs say about it.

Note that the ordering maintained by a set (whether or not an explicit comparator is provided) must be consistent with equals if it is to correctly implement the Set interface. (See Comparable or Comparator for a precise definition of consistent with equals.) This is so because the Set interface is defined in terms of the equals operation, but a TreeSet instance performs all element comparisons using its compareTo (or compare) method, so two elements that are deemed equal by this method are, from the standpoint of the set, equal. The behavior of a set is well-defined even if its ordering is inconsistent with equals; it just fails to obey the general contract of the Set interface.