Why don't I get an error when adding a duplicate item to a Set?

Question

I added the same integer twice to a Set, but it will not give any error despite Sets not allowing duplicates. Why is this?

 Set<Integer> set = new HashSet<Integer>();
 set.add(1);
 set.add(1);

Answer 1

Set:add is not supposed to give you an error when you try to add a value already in the Set. It will just return false and not add the value to the Set.

Check the JavaDoc:

boolean add(E e)

Adds the specified element to this set if it is not already present (optional operation). More formally, adds the specified element e to this set if the set contains no element e2 such that (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false. In combination with the restriction on constructors, this ensures that sets never contain duplicate elements.