SPARK-6993 : Add default min, max methods for JavaDoubleRDD

The default method will use Guava's Ordering instead of
java.util.Comparator.naturalOrder() because it's not available
in Java 7, only in Java 8.

Author: Olivier Girardot <o.girardot@lateral-thoughts.com>

Closes apache#5571 from ogirardot/master and squashes the following commits:

7fe2e9e [Olivier Girardot] SPARK-6993 : Add default min, max methods for JavaDoubleRDD

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

@@ -163,6 +163,20 @@ class JavaDoubleRDD(val srdd: RDD[scala.Double])
   /** Add up the elements in this RDD. */
   def sum(): JDouble = srdd.sum()
 
+  /**
+   * Returns the minimum element from this RDD as defined by
+   * the default comparator natural order.
+   * @return the minimum of the RDD
+   */
+  def min(): JDouble = min(com.google.common.collect.Ordering.natural())
+
+  /**
+   * Returns the maximum element from this RDD as defined by
+   * the default comparator natural order.
+   * @return the maximum of the RDD
+   */
+  def max(): JDouble = max(com.google.common.collect.Ordering.natural())
+
   /**
    * Return a [[org.apache.spark.util.StatCounter]] object that captures the mean, variance and
    * count of the RDD's elements in one operation.

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

@@ -761,6 +761,20 @@ public void min() {
     Assert.assertEquals(1.0, max, 0.001);
   }
 
+  @Test
+  public void naturalMax() {
+    JavaDoubleRDD rdd = sc.parallelizeDoubles(Arrays.asList(1.0, 2.0, 3.0, 4.0));
+    double max = rdd.max();
+    Assert.assertTrue(4.0 == max);
+  }
+
+  @Test
+  public void naturalMin() {
+    JavaDoubleRDD rdd = sc.parallelizeDoubles(Arrays.asList(1.0, 2.0, 3.0, 4.0));
+    double max = rdd.min();
+    Assert.assertTrue(1.0 == max);
+  }
+
   @Test
   public void takeOrdered() {
     JavaDoubleRDD rdd = sc.parallelizeDoubles(Arrays.asList(1.0, 2.0, 3.0, 4.0));