Why the answere is 169 not 1996?

Question

(java.util.*is imported)
TreeSet set =new TreeSet();
set.add(1);
set.add(9);
set.add(9);
set.add(6);
for(Object O : set){

System.out.println(o);
}
Explain this code?

Answer 1

From java 7 documentation:
"A NavigableSet implementation based on a TreeMap. The elements are ordered using their natural ordering, or by a Comparator provided at set creation time, depending on which constructor is used."

TreeSet automaticly sort the given element in ascending order, thats why 6 appear before 9.

Where's the other 9 ?
Also from documentation
"add (E e): Adds the specified element to this set if it is not already present."

Answer 2

Only unique values are stored on a set, not duplicate values.