code: public void merge(BloomKFilter that)
    {
      if (this != that && this.m == that.m && this.k == that.k) {
        this.bitSet.putAll(that.bitSet);
      } else {
        throw new IllegalArgumentException("BloomKFilters are not compatible for merging." +
                                           " this - " + this + " that - " + that);
      }
    }
docs: Merge the specified bloom filter with current bloom filter.
  
  @param that - bloom filter to merge