Question

Set<Type> union = new HashSet<Type>(s1);
union.addAll(s2);

AND

Set <Type> union = new HashSet<Type>();
union.addAll(s1);
union.addAll(s2);

Answer 1

The difference here is much less than your previous question.
The first way should be faster because the original Set will not have to grow as much.

The way you may want to do this is:

Set<Type> union = new HashSet<Type>(s1.size() + s2.size());
union.addAll(s1);
union.addAll(s2);

That way you won’t have to resize the new Set at all. (Even though you may have some extra space)