code: public void ensureCapacity (int minCapacity)
      {
          int size = _buckets.length;
          while (minCapacity > (int) (size * _loadFactor)) {
              size *= 2;
          }
          if (size != _buckets.length) {
              resizeBuckets(size);
          }
      }
docs: Ensure that the hash can comfortably hold the specified number of elements. Calling this
  method is not necessary, but can improve performance if done prior to adding many elements.