@Override
  public int compareTo(EventHandler o) {
    if (o == null) {
      return 1;
    }
    if (getPriority() != o.getPriority()) {
      return (getPriority() < o.getPriority()) ? -1 : 1;
    }
    return (this.seqid < o.seqid) ? -1 : 1;
  }
Default prioritized runnable comparator which implements a FIFO ordering.
<p>
Subclasses should not override this. Instead, if they want to implement priority beyond FIFO,
they should override {@link #getPriority()}.
hbase-server/src/main/java/org/apache/hadoop/hbase/executor/EventHandler.java