restrict V type of foldByKey in order to retain ClassManifest; added foldByKey to Java API and test

core/src/main/scala/spark/PairRDDFunctions.scala

@@ -91,8 +91,8 @@ class PairRDDFunctions[K: ClassManifest, V: ClassManifest](
   /**
    * Merge the values for each key using an associative function and a neutral "zero value".
    */
-  def foldByKey[V1 >: V](zeroValue: V1)(op: (V1, V1) => V1): RDD[(K, V1)] = {
-    groupByKey.mapValues(seq => seq.fold(zeroValue)(op))
+  def foldByKey(zeroValue: V)(op: (V, V) => V): RDD[(K, V)] = {
+    groupByKey.mapValues(seq => seq.fold[V](zeroValue)(op))
   }
 
   /**

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

@@ -160,6 +160,12 @@ class JavaPairRDD[K, V](val rdd: RDD[(K, V)])(implicit val kManifest: ClassManif
   : PartialResult[java.util.Map[K, BoundedDouble]] =
     rdd.countByKeyApprox(timeout, confidence).map(mapAsJavaMap)
 
+  /**
+   * Merge the values for each key using an associative function and a neutral "zero value".
+   */
+  def foldByKey(zeroValue: V, func: JFunction2[V, V, V]): JavaPairRDD[K, V] =
+    fromRDD(rdd.foldByKey(zeroValue)(func))
+
   /**
    * Merge the values for each key using an associative reduce function. This will also perform
    * the merging locally on each mapper before sending results to a reducer, similarly to a

core/src/test/scala/spark/JavaAPISuite.java

@@ -196,6 +196,28 @@ public class JavaAPISuite implements Serializable {
     Assert.assertEquals(33, sum);
   }
 
+    @Test
+    public void foldByKey() {
+        List<Tuple2<Integer, Integer>> pairs = Arrays.asList(
+                new Tuple2<Integer, Integer>(2, 1),
+                new Tuple2<Integer, Integer>(2, 1),
+                new Tuple2<Integer, Integer>(1, 1),
+                new Tuple2<Integer, Integer>(3, 2),
+                new Tuple2<Integer, Integer>(3, 1)
+        );
+        JavaPairRDD<Integer, Integer> rdd = sc.parallelizePairs(pairs);
+        JavaPairRDD<Integer, Integer> sums = rdd.foldByKey(0,
+          new Function2<Integer, Integer, Integer>() {
+                    @Override
+                    public Integer call(Integer a, Integer b) {
+                        return a + b;
+                    }
+                });
+        Assert.assertEquals(1, sums.lookup(1).get(0).intValue());
+        Assert.assertEquals(2, sums.lookup(2).get(0).intValue());
+        Assert.assertEquals(3, sums.lookup(3).get(0).intValue());
+    }
+
     @Test
   public void reduceByKey() {
     List<Tuple2<Integer, Integer>> pairs = Arrays.asList(