Expose numPartitions parameter in JavaPairRDD.sortByKey()

This change make Java and Scala API on sortByKey() the same.

core/src/main/scala/org/apache/spark/api/java/JavaPairRDD.scala

@@ -583,12 +583,20 @@ class JavaPairRDD[K, V](val rdd: RDD[(K, V)])(implicit val kManifest: ClassManif
    * (in the `save` case, they will be written to multiple `part-X` files in the filesystem, in
    * order of the keys).
    */
-  def sortByKey(comp: Comparator[K], ascending: Boolean): JavaPairRDD[K, V] = {
+  def sortByKey(comp: Comparator[K], ascending: Boolean): JavaPairRDD[K, V] = sortByKey(comp, ascending, rdd.partitions.size)
+
+  /**
+   * Sort the RDD by key, so that each partition contains a sorted range of the elements. Calling
+   * `collect` or `save` on the resulting RDD will return or output an ordered list of records
+   * (in the `save` case, they will be written to multiple `part-X` files in the filesystem, in
+   * order of the keys).
+   */
+  def sortByKey(comp: Comparator[K], ascending: Boolean, numPartitions: Int): JavaPairRDD[K, V] = {
     class KeyOrdering(val a: K) extends Ordered[K] {
       override def compare(b: K) = comp.compare(a, b)
     }
     implicit def toOrdered(x: K): Ordered[K] = new KeyOrdering(x)
-    fromRDD(new OrderedRDDFunctions[K, V, (K, V)](rdd).sortByKey(ascending))
+    fromRDD(new OrderedRDDFunctions[K, V, (K, V)](rdd).sortByKey(ascending, numPartitions))
   }
 
   /**