[SPARK-11901][SQL] API audit for Aggregator.

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

@@ -22,7 +22,6 @@ import scala.collection.JavaConverters._
 import org.apache.spark.annotation.Experimental
 import org.apache.spark.rdd.RDD
 import org.apache.spark.api.java.function._
-import org.apache.spark.sql.catalyst.InternalRow
 
 import org.apache.spark.sql.catalyst.encoders._
 import org.apache.spark.sql.catalyst.expressions._

sql/core/src/main/scala/org/apache/spark/sql/expressions/Aggregator.scala

@@ -17,11 +17,10 @@
 
 package org.apache.spark.sql.expressions
 
-import org.apache.spark.sql.Encoder
 import org.apache.spark.sql.catalyst.encoders.encoderFor
 import org.apache.spark.sql.catalyst.expressions.aggregate.{AggregateExpression, Complete}
 import org.apache.spark.sql.execution.aggregate.TypedAggregateExpression
-import org.apache.spark.sql.{Dataset, DataFrame, TypedColumn}
+import org.apache.spark.sql.{DataFrame, Dataset, Encoder, TypedColumn}
 
 /**
  * A base class for user-defined aggregations, which can be used in [[DataFrame]] and [[Dataset]]
@@ -32,55 +31,65 @@ import org.apache.spark.sql.{Dataset, DataFrame, TypedColumn}
  *   case class Data(i: Int)
  *
  *   val customSummer =  new Aggregator[Data, Int, Int] {
- *     def zero = 0
- *     def reduce(b: Int, a: Data) = b + a.i
- *     def present(r: Int) = r
+ *     def zero: Int = 0
+ *     def reduce(b: Int, a: Data): Int = b + a.i
+ *     def merge(b1: Int, b2: Int): Int = b1 + b2
+ *     def present(r: Int): Int = r
  *   }.toColumn()
  *
- *   val ds: Dataset[Data]
+ *   val ds: Dataset[Data] = ...
  *   val aggregated = ds.select(customSummer)
  * }}}
  *
  * Based loosely on Aggregator from Algebird: https://github.com/twitter/algebird
  *
- * @tparam A The input type for the aggregation.
+ * @tparam I The input type for the aggregation.
  * @tparam B The type of the intermediate value of the reduction.
- * @tparam C The type of the final result.
+ * @tparam O The type of the final output result.
+ *
+ * @since 1.6.0
  */
-abstract class Aggregator[-A, B, C] extends Serializable {
+abstract class Aggregator[-I, B, O] extends Serializable {
 
-  /** A zero value for this aggregation. Should satisfy the property that any b + zero = b */
+  /**
+   * A zero value for this aggregation. Should satisfy the property that any b + zero = b.
+   * @since 1.6.0
+   */
   def zero: B
 
   /**
    * Combine two values to produce a new value.  For performance, the function may modify `b` and
    * return it instead of constructing new object for b.
+   * @since 1.6.0
    */
-  def reduce(b: B, a: A): B
+  def reduce(b: B, a: I): B
 
   /**
-   * Merge two intermediate values
+   * Merge two intermediate values.
+   * @since 1.6.0
    */
   def merge(b1: B, b2: B): B
 
   /**
    * Transform the output of the reduction.
+   * @since 1.6.0
    */
-  def finish(reduction: B): C
+  def finish(reduction: B): O
 
   /**
    * Returns this `Aggregator` as a [[TypedColumn]] that can be used in [[Dataset]] or [[DataFrame]]
    * operations.
+   * @since 1.6.0
    */
   def toColumn(
       implicit bEncoder: Encoder[B],
-      cEncoder: Encoder[C]): TypedColumn[A, C] = {
+      cEncoder: Encoder[O]): TypedColumn[I, O] = {
     val expr =
       new AggregateExpression(
         TypedAggregateExpression(this),
         Complete,
         false)
 
-    new TypedColumn[A, C](expr, encoderFor[C])
+    new TypedColumn[I, O](expr, encoderFor[O])
   }
 }