@Override
  public ExtendedCell peek() { // sanity check, the current should be always valid
    if (closed) {
      return null;
    }
    if (current != null && current.getSequenceId() > readPoint) {
      throw new RuntimeException("current is invalid: read point is " + readPoint + ", "
        + "while current sequence id is " + current.getSequenceId());
    }
    return current;
  }
Look at the next Cell in this scanner, but do not iterate the scanner
@return the currently observed Cell
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/SegmentScanner.java