public T computeIfAbsent(final Supplier<T> lazy){
  final T val=get();
  if (val == UNSET)   return setOnceFromSupplier(lazy);
  return val;
}

public T computeIfAbsent ( final Supplier < T > lazy ) { final T val = get ( ) ; if ( val == UNSET ) return setOnceFromSupplier ( lazy ) ; return val ; }
get the current value or set if it has not been set yet
