val pairRDD1 = sc.parallelize(List( ("cat",2), ("girl", 5), ("book", 4),("Tom", 12)))
val pairRDD2 = sc.parallelize(List( ("cat",2), ("cup", 5), ("mouse", 4),("girl", 12)))

val kk = pairRDD1.fullOuterJoin(pairRDD2).collect

kk: Array[(String, (Option[Int], Option[Int]))] = Array((book,(Some(4),None)), (Tom,(Some(12),None)), (girl,(Some(5),Some(12))), (mouse,(None,Some(4))), (cup,(None,Some(5))), (cat,(Some(2),Some(2))))

pairRDD1.fullOuterJoin(pairRDD2).mapValues(pair => (pair._1.getOrElse(0), pair._2.getOrElse(0)))

kk.map { case (k, pair) => (k, (pair._1.getOrElse(0), pair._2.getOrElse(0))) }

val ss = SparkSession.builder().master("local[*]").getOrCreate()
    val sc = ss.sparkContext

    import ss.implicits._

    val pairRDD1 = sc.parallelize(List(("cat", 2,9999), ("girl", 5,8888), ("book", 4,9999), ("Tom", 12,6666)))
    val pairRDD2 = sc.parallelize(List(("cat", 2,9999), ("cup", 5,7777), ("mouse", 4,3333), ("girl", 12,1111)))

    val df1 = pairRDD1.toDF
    val df2 = pairRDD2.toDF

    val joined = df1.join(df2, df1.col("_1") === df2.col("_1"),"fullouter")
    joined.show()

+----+----+----+-----+----+----+
|  _1|  _2|  _3|   _1|  _2|  _3|
+----+----+----+-----+----+----+
|girl|   5|8888| girl|  12|1111|
| Tom|  12|6666| null|null|null|
| cat|   2|9999|  cat|   2|9999|
|null|null|null|  cup|   5|7777|
|null|null|null|mouse|   4|3333|
|book|   4|9999| null|null|null|
+----+----+----+-----+----+----+