@Override
  public int hashCode() {
    final int prime = 31;
    int result = super.hashCode();
    result = prime * result + Long.hashCode(timestamp);
    return result;
  }
The WatermarkMessage is a control message that is sent out to next stage
 with a watermark timestamp and the task that produces the watermark.
samza-api/src/main/java/org/apache/samza/system/WatermarkMessage.java