[SPARK-2279] Added emptyRDD method to Java API

Added emptyRDD method to Java API with tests.

Author: bpaulin <bob@bobpaulin.com>

Closes #1597 from bobpaulin/SPARK-2279 and squashes the following commits:

5ad57c2 [bpaulin] [SPARK-2279] Added emptyRDD method to Java API

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

@@ -34,7 +34,7 @@ import org.apache.spark._
 import org.apache.spark.SparkContext.{DoubleAccumulatorParam, IntAccumulatorParam}
 import org.apache.spark.api.java.JavaSparkContext.fakeClassTag
 import org.apache.spark.broadcast.Broadcast
-import org.apache.spark.rdd.RDD
+import org.apache.spark.rdd.{EmptyRDD, RDD}
 
 /**
  * A Java-friendly version of [[org.apache.spark.SparkContext]] that returns
@@ -135,6 +135,13 @@ class JavaSparkContext(val sc: SparkContext) extends JavaSparkContextVarargsWork
     sc.parallelize(JavaConversions.asScalaBuffer(list), numSlices)
   }
 
+  /** Get an RDD that has no partitions or elements. */
+  def emptyRDD[T]: JavaRDD[T] = {
+    implicit val ctag: ClassTag[T] = fakeClassTag
+    JavaRDD.fromRDD(new EmptyRDD[T](sc))
+  }
+
+
   /** Distribute a local Scala collection to form an RDD. */
   def parallelize[T](list: java.util.List[T]): JavaRDD[T] =
     parallelize(list, sc.defaultParallelism)

core/src/test/java/org/apache/spark/JavaAPISuite.java

@@ -118,8 +118,7 @@ public class JavaAPISuite implements Serializable {
     JavaRDD<Integer> intersections = s1.intersection(s2);
     Assert.assertEquals(3, intersections.count());
 
-    List<Integer> list = new ArrayList<Integer>();
-    JavaRDD<Integer> empty = sc.parallelize(list);
+    JavaRDD<Integer> empty = sc.emptyRDD();
     JavaRDD<Integer> emptyIntersection = empty.intersection(s2);
     Assert.assertEquals(0, emptyIntersection.count());
 
@@ -184,6 +183,12 @@ public class JavaAPISuite implements Serializable {
     Assert.assertEquals(new Tuple2<Integer, Integer>(3, 2), sortedPairs.get(2));
   }
 
+  @Test
+  public void emptyRDD() {
+    JavaRDD<String> rdd = sc.emptyRDD();
+    Assert.assertEquals("Empty RDD shouldn't have any values", 0, rdd.count());
+  }
+
   @Test
   public void sortBy() {
     List<Tuple2<Integer, Integer>> pairs = new ArrayList<Tuple2<Integer, Integer>>();