Why does TreeSet throw a ClassCastException?

Question

I am trying to add two \'Employee\' objects to a TreeSet:

Set s = new TreeSet();
s.add(new Employee(1001));
s.add(new Employe         


        

Answer 1

From TreeSet#add(E) JavaDoc:

Throws: ClassCastException - if the specified object cannot be compared with the elements currently in this set

Basically what you need is to let Employee implement Comparable or provide a Comparator to the TreeSet object.

If you check TreeMap code you will see that if the comparator wasn't found within the Map object, it will try to cast the key (your Employee object) directly to Comparator:

...
Comparable k = (Comparable) key;
...