@Override
  public WatermarkFunction getWatermarkFn() {
    return joinFn instanceof WatermarkFunction ? (WatermarkFunction) joinFn : null;
  }
Constructor for {@link StreamTableJoinOperatorSpec}.

@param tableId  the Id of the table on the right side of the join
@param joinFn  the user-defined join function to get join keys and results
@param opId  the unique ID for this operator
@param args additional arguments passed to the table
samza-core/src/main/java/org/apache/samza/operators/spec/StreamTableJoinOperatorSpec.java