public boolean add ( final Integer i ) {
    return add(i.intValue());
  }
Add {@code i} to the set.  Uppercase {@link Integer} version of add,
requires auto-unboxing.  When possible use the {@code int} version of
{@link #add(int)} for efficiency.
@throws IllegalArgumentException if i is negative.
@return <tt>true</tt> if i was added to the set.
jctools-core/src/main/java/org/jctools/maps/NonBlockingSetInt.java