Major Description
I believe this issue is essentially the same as SPARK-668.
Original error:
[ERROR] /Users/saldaal1/workspace/JavaSparkSimpleApp/src/main/java/SimpleApp.java:[29,105] no suitable method found for groupBy(org.apache.spark.api.java.function.Function<scala.Tuple2<java.lang.String,java.lang.Long>,java.lang.Long>) [ERROR] method org.apache.spark.api.java.JavaPairRDD.<K>groupBy(org.apache.spark.api.java.function.Function<scala.Tuple2<K,java.lang.Long>,K>) is not applicable [ERROR] (inferred type does not conform to equality constraint(s)
from core/src/main/scala/org/apache/spark/api/java/JavaRDDLike.scala
211 /**
212 * Return an RDD of grouped elements. Each group consists of a key and a sequence of elements
213 * mapping to that key.
214 */
215 def groupBy[K](f: JFunction[T, K]): JavaPairRDD[K, JIterable[T]] = {
216 implicit val ctagK: ClassTag[K] = fakeClassTag
217 implicit val ctagV: ClassTag[JList[T]] = fakeClassTag
218 JavaPairRDD.fromRDD(groupByResultToJava(rdd.groupBy(f)(fakeClassTag)))
219 }
Then in core/src/main/scala/org/apache/spark/api/java/JavaPairRDD.scala:
45 class JavaPairRDD[K, V](val rdd: RDD[(K, V)]) 46 (implicit val kClassTag: ClassTag[K], implicit val vClassTag: ClassTag[V]) 47 extends JavaRDDLike[(K, V), JavaPairRDD[K, V]] {
The problem is that the type parameter T in JavaRDDLike is Tuple2[K,V], which means the combined signature for groupBy in the JavaPairRDD is
groupBy[K](f: JFunction[Tuple2[K,V], K])
which imposes an unfortunate correlation between the Tuple2 and the return type of the grouping function, namely that the return type of the grouping function must be the same as the first type of the JavaPairRDD.
If we compare the method signature to flatMap:
105 /** 106 * Return a new RDD by first applying a function to all elements of this 107 * RDD, and then flattening the results. 108 */ 109 def flatMap[U](f: FlatMapFunction[T, U]): JavaRDD[U] = { 110 import scala.collection.JavaConverters._ 111 def fn = (x: T) => f.call(x).asScala 112 JavaRDD.fromRDD(rdd.flatMap(fn)(fakeClassTag[U]))(fakeClassTag[U]) 113 }
we see there should be an easy fix by changing the type parameter of the groupBy function from K to U.
