public static long extractFileNumFromWAL(final WAL wal) {
    final Path walPath = ((AbstractFSWAL<?>) wal).getCurrentFileName();
    if (walPath == null) {
      throw new IllegalArgumentException("The WAL path couldn't be null");
    }
    String name = walPath.getName();
    long timestamp = getTimestamp(name);
    if (timestamp == NO_TIMESTAMP) {
      throw new IllegalArgumentException(name + " is not a valid wal file name");
    }
    return timestamp;
  }
It returns the file create timestamp (the 'FileNum') from the file name. For name format see
{@link #validateWALFilename(String)} public until remaining tests move to o.a.h.h.wal
@param wal must not be null
@return the file number that is part of the WAL file name
hbase-server/src/main/java/org/apache/hadoop/hbase/wal/AbstractFSWALProvider.java