update the code as comments

Author: chenghao-intel

sql/core/src/main/scala/org/apache/spark/sql/GroupedData.scala

@@ -26,11 +26,6 @@ import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.plans.logical.{Rollup, Cube, Aggregate}
 import org.apache.spark.sql.types.NumericType
 
-/**
- * The Grouping Type
- */
-sealed private[sql] trait GroupType
-
 /**
  * Companion object for GroupedData
  */
@@ -39,9 +34,14 @@ private[sql] object GroupedData {
       df: DataFrame,
       groupingExprs: Seq[Expression],
       groupType: GroupType): GroupedData = {
-    new GroupedData(df, groupingExprs).withNewGroupType(groupType)
+    new GroupedData(df, groupingExprs, groupType: GroupType)
   }
 
+  /**
+   * The Grouping Type
+   */
+  private[sql] trait GroupType
+
   /**
    * To indicate it's the GroupBy
    */
@@ -65,15 +65,10 @@ private[sql] object GroupedData {
  * @since 1.3.0
  */
 @Experimental
-class GroupedData protected[sql](df: DataFrame, groupingExprs: Seq[Expression]) {
-
-  private var groupType: GroupType = _
-
-  private[sql] def withNewGroupType(groupType: GroupType): GroupedData = {
-    this.groupType = groupType
-    this
-  }
-
+class GroupedData protected[sql](
+    df: DataFrame,
+    groupingExprs: Seq[Expression],
+    private val groupType: GroupedData.GroupType) {
   private[sql] implicit def toDF(aggExprs: Seq[NamedExpression]): DataFrame = {
     val aggregates = if (df.sqlContext.conf.dataFrameRetainGroupColumns) {
         val retainedExprs = groupingExprs.map {