@Override
  public boolean seekToPreviousRow(ExtendedCell key) throws IOException {
    throw new NotImplementedException(
      "seekToPreviousRow must not be called on a " + "non-reversed scanner");
  }
A "non-reversed &amp; non-lazy" scanner which does not support backward scanning and always does
a real seek operation. Most scanners are inherited from this class.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/NonReversedNonLazyKeyValueScanner.java