code: public static DataChecksum newDataChecksum( int type, int bytesPerChecksum, Checksum sum ) {
      if ( bytesPerChecksum <= 0 ) {
        return null;
      }
  
      int checksumSize = getChecksumSizeByType(type);
      
      switch ( type ) {
      case CHECKSUM_NULL :
        return new DataChecksum( CHECKSUM_NULL, new ChecksumNull(), 
            checksumSize, bytesPerChecksum );
      case CHECKSUM_CRC32 :
        return new DataChecksum( CHECKSUM_CRC32, sum, 
            checksumSize, bytesPerChecksum );
      default:
        return null;  
      }
    }
docs: This constructor uses the specified summer instance