protected Path computeFilename(final long filenum) {
    if (filenum < 0) {
      throw new RuntimeException("WAL file number can't be < 0");
    }
    String child = walFilePrefix + WAL_FILE_NAME_DELIMITER + filenum + walFileSuffix;
    return new Path(walDir, child);
  }
This is a convenience method that computes a new filename with a given file-number.
@param filenum to use
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/AbstractFSWAL.java