[SPARK-15866] Rename listAccumulator collectionAccumulator

## What changes were proposed in this pull request?
SparkContext.listAccumulator, by Spark's convention, makes it sound like "list" is a verb and the method should return a list of accumulators. This patch renames the method and the class collection accumulator.

## How was this patch tested?
Updated test case to reflect the names.

Author: Reynold Xin <rxin@databricks.com>

Closes #13594 from rxin/SPARK-15866.

3 files changed, 19 insertions, 14 deletions

diff --git a/core/src/main/scala/org/apache/spark/SparkContext.scala b/core/src/main/scala/org/apache/spark/SparkContext.scala
index 33b11ed2e6..230fabd211 100644
--- a/core/src/main/scala/org/apache/spark/SparkContext.scala
+++ b/core/src/main/scala/org/apache/spark/SparkContext.scala
@@ -1340,21 +1340,21 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
   }
 
   /**
-   * Create and register a list accumulator, which starts with empty list and accumulates inputs
-   * by adding them into the inner list.
+   * Create and register a [[CollectionAccumulator]], which starts with empty list and accumulates
+   * inputs by adding them into the list.
    */
-  def listAccumulator[T]: ListAccumulator[T] = {
-    val acc = new ListAccumulator[T]
+  def collectionAccumulator[T]: CollectionAccumulator[T] = {
+    val acc = new CollectionAccumulator[T]
     register(acc)
     acc
   }
 
   /**
-   * Create and register a list accumulator, which starts with empty list and accumulates inputs
-   * by adding them into the inner list.
+   * Create and register a [[CollectionAccumulator]], which starts with empty list and accumulates
+   * inputs by adding them into the list.
    */
-  def listAccumulator[T](name: String): ListAccumulator[T] = {
-    val acc = new ListAccumulator[T]
+  def collectionAccumulator[T](name: String): CollectionAccumulator[T] = {
+    val acc = new CollectionAccumulator[T]
     register(acc, name)
     acc
   }
diff --git a/core/src/main/scala/org/apache/spark/util/AccumulatorV2.scala b/core/src/main/scala/org/apache/spark/util/AccumulatorV2.scala
index 0b9a47c112..044dd69cc9 100644
--- a/core/src/main/scala/org/apache/spark/util/AccumulatorV2.scala
+++ b/core/src/main/scala/org/apache/spark/util/AccumulatorV2.scala
@@ -415,15 +415,20 @@ class DoubleAccumulator extends AccumulatorV2[jl.Double, jl.Double] {
 }
 
 
-class ListAccumulator[T] extends AccumulatorV2[T, java.util.List[T]] {
+/**
+ * An [[AccumulatorV2 accumulator]] for collecting a list of elements.
+ *
+ * @since 2.0.0
+ */
+class CollectionAccumulator[T] extends AccumulatorV2[T, java.util.List[T]] {
   private val _list: java.util.List[T] = new ArrayList[T]
 
   override def isZero: Boolean = _list.isEmpty
 
-  override def copyAndReset(): ListAccumulator[T] = new ListAccumulator
+  override def copyAndReset(): CollectionAccumulator[T] = new CollectionAccumulator
 
-  override def copy(): ListAccumulator[T] = {
-    val newAcc = new ListAccumulator[T]
+  override def copy(): CollectionAccumulator[T] = {
+    val newAcc = new CollectionAccumulator[T]
     newAcc._list.addAll(_list)
     newAcc
   }
@@ -433,7 +438,7 @@ class ListAccumulator[T] extends AccumulatorV2[T, java.util.List[T]] {
   override def add(v: T): Unit = _list.add(v)
 
   override def merge(other: AccumulatorV2[T, java.util.List[T]]): Unit = other match {
-    case o: ListAccumulator[T] => _list.addAll(o.value)
+    case o: CollectionAccumulator[T] => _list.addAll(o.value)
     case _ => throw new UnsupportedOperationException(
       s"Cannot merge ${this.getClass.getName} with ${other.getClass.getName}")
   }
diff --git a/core/src/test/scala/org/apache/spark/util/AccumulatorV2Suite.scala b/core/src/test/scala/org/apache/spark/util/AccumulatorV2Suite.scala
index 439da1306f..a04644d57e 100644
--- a/core/src/test/scala/org/apache/spark/util/AccumulatorV2Suite.scala
+++ b/core/src/test/scala/org/apache/spark/util/AccumulatorV2Suite.scala
@@ -88,7 +88,7 @@ class AccumulatorV2Suite extends SparkFunSuite {
   }
 
   test("ListAccumulator") {
-    val acc = new ListAccumulator[Double]
+    val acc = new CollectionAccumulator[Double]
     assert(acc.value.isEmpty)
     assert(acc.isZero)