@Override
  public WatermarkFunction getWatermarkFn() {
    return joinFn instanceof WatermarkFunction ? (WatermarkFunction) joinFn : null;
  }
Default constructor for a {@link JoinOperatorSpec}.

@param leftInputOpSpec  the operator spec for the stream on the left side of the join
@param rightInputOpSpec  the operator spec for the stream on the right side of the join
@param joinFn  the user-defined join function to get join keys and results
@param ttlMs  the ttl in ms for retaining messages in each stream
@param opId  the unique ID for this operator
samza-core/src/main/java/org/apache/samza/operators/spec/JoinOperatorSpec.java