@Override
      public boolean accept(final Path fileName) {
        // The path should start with dir/<prefix> and end with our suffix
        final String fileNameString = fileName.toString();
        if (!fileNameString.startsWith(prefixPathStr)) {
          return false;
        }
        if (walFileSuffix.isEmpty()) {
          // in the case of the null suffix, we need to ensure the filename ends with a timestamp.
          return org.apache.commons.lang3.StringUtils
            .isNumeric(fileNameString.substring(prefixPathStr.length()));
        } else if (!fileNameString.endsWith(walFileSuffix)) {
          return false;
        }
        return true;
      }
A log file has a creation timestamp (in ms) in its file name ({@link #filenum}. This helper
method returns the creation timestamp from a given log file. It extracts the timestamp assuming
the filename is created with the {@link #computeFilename(long filenum)} method.
@return timestamp, as in the log file name.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/AbstractFSWAL.java