code: def array_join(col, delimiter, null_replacement=None):
      
      sc = SparkContext._active_spark_context
      if null_replacement is None:
          return Column(sc._jvm.functions.array_join(_to_java_column(col), delimiter))
      else:
          return Column(sc._jvm.functions.array_join(
              _to_java_column(col), delimiter, null_replacement))
docs: Concatenates the elements of `column` using the `delimiter`. Null values are replaced with
      `null_replacement` if set, otherwise they are ignored.
  
      >>> df = spark.createDataFrame([(["a", "b", "c"],), (["a", None],)], ['data'])
      >>> df.select(array_join(df.data, ",").alias("joined")).collect()
      [Row(joined=u'a,b,c'), Row(joined=u'a')]
      >>> df.select(array_join(df.data, ",", "NULL").alias("joined")).collect()
      [Row(joined=u'a,b,c'), Row(joined=u'a,NULL')]