@Nullable private ConcurrentMap<SwapKey,SwapValue> partition(int part,boolean create){
  ConcurrentMap<SwapKey,SwapValue> map=parts.get(part);
  if (map == null && create) {
    ConcurrentMap<SwapKey,SwapValue> old=parts.putIfAbsent(part,map=new ConcurrentHashMap<>());
    if (old != null)     map=old;
  }
  return map;
}

@ Nullable private ConcurrentMap < SwapKey , SwapValue > partition ( int part , boolean create ) { ConcurrentMap < SwapKey , SwapValue > map = parts . get ( part ) ; if ( map == null && create ) { ConcurrentMap < SwapKey , SwapValue > old = parts . putIfAbsent ( part , map = new ConcurrentHashMap < > ( ) ) ; if ( old != null ) map = old ; } return map ; }
gets partition map by its number .
