Adding element to Set (not last and not first)

Hi,
I'm a beginner, so please help me.
I have


Set mySet= new HashSet();
(Let's assume, that here the set has already 20 elements)

int pos=7;

MyClass myClass;


How could i add myClass to mySet on the 7th position?

What would be the easiest way?
The Set must be here.

I don't think you want to use a Set for whatever you're trying to do. Instead, look at ArrayList