Use null for the implicit Ordering

Author: zsxwing

core/src/main/scala/org/apache/spark/rdd/RDD.scala

@@ -1175,18 +1175,19 @@ abstract class RDD[T: ClassTag](
    */
   def saveAsTextFile(path: String) {
     // https://issues.apache.org/jira/browse/SPARK-2075
-    // NullWritable is a Comparable rather than Comparable[NullWritable] in Hadoop 1.+,
-    // so the compiler cannot find an implicit Ordering for it and will use the default `null`.
-    // It will generate different anonymous classes for `saveAsTextFile` in Hadoop 1.+ and
-    // Hadoop 2.+. Therefore, here we provide an Ordering for NullWritable so that the compiler
-    // will generate same bytecode.
-    val nullWritableOrdering = new Ordering[NullWritable] {
-      override def compare(x: NullWritable, y: NullWritable): Int = 0
-    }
+    //
+    // NullWritable is a `Comparable` in Hadoop 1.+, so the compiler cannot find an implicit
+    // Ordering for it and will use the default `null`. However, it's a `Comparable[NullWritable]`
+    // in Hadoop 2.+, so the compiler will call the implicit `Ordering.ordered` method to create an
+    // Ordering for `NullWritable`. That's why the compiler will generate different anonymous
+    // classes for `saveAsTextFile` in Hadoop 1.+ and Hadoop 2.+.
+    //
+    // Therefore, here we provide an explicit Ordering `null` to make sure the compiler generate
+    // same bytecodes for `saveAsTextFile`.
     val nullWritableClassTag = implicitly[ClassTag[NullWritable]]
     val textClassTag = implicitly[ClassTag[Text]]
     val r = this.map(x => (NullWritable.get(), new Text(x.toString)))
-    RDD.rddToPairRDDFunctions(r)(nullWritableClassTag, textClassTag, nullWritableOrdering)
+    RDD.rddToPairRDDFunctions(r)(nullWritableClassTag, textClassTag, null)
       .saveAsHadoopFile[TextOutputFormat[NullWritable, Text]](path)
   }
 
@@ -1195,13 +1196,10 @@ abstract class RDD[T: ClassTag](
    */
   def saveAsTextFile(path: String, codec: Class[_ <: CompressionCodec]) {
     // https://issues.apache.org/jira/browse/SPARK-2075
-    val nullWritableOrdering = new Ordering[NullWritable] {
-      override def compare(x: NullWritable, y: NullWritable): Int = 0
-    }
     val nullWritableClassTag = implicitly[ClassTag[NullWritable]]
     val textClassTag = implicitly[ClassTag[Text]]
     val r = this.map(x => (NullWritable.get(), new Text(x.toString)))
-    RDD.rddToPairRDDFunctions(r)(nullWritableClassTag, textClassTag, nullWritableOrdering)
+    RDD.rddToPairRDDFunctions(r)(nullWritableClassTag, textClassTag, null)
       .saveAsHadoopFile[TextOutputFormat[NullWritable, Text]](path, codec)
   }