@Override
  public boolean equals(Object other) {
    if (this == other) {
      return true;
    }
    if (!(other instanceof UpdateMessage)) {
      return false;
    }
    UpdateMessage<?, ?> otherPair = (UpdateMessage<?, ?>) other;
    return Objects.deepEquals(this.update, otherPair.getUpdate())
        && Objects.deepEquals(this.defaultValue, otherPair.getDefault());
  }
Represents an update and an optional default record to be inserted for a key,
if the update is applied to a non-existent record.

@param <U> type of the update record
@param <V> type of the default record
samza-api/src/main/java/org/apache/samza/operators/UpdateMessage.java