[SPARK-14375][ML] Unit test for spark.ml KMeansSummary

## What changes were proposed in this pull request?
* Modify ```KMeansSummary.clusterSizes``` method to make it robust to empty clusters.
* Add unit test for spark.ml ```KMeansSummary```.
* Add Since tag.

## How was this patch tested?
unit tests.

cc jkbradley

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #12254 from yanboliang/spark-14375.

1 files changed, 17 insertions, 1 deletions

diff --git a/mllib/src/test/scala/org/apache/spark/ml/clustering/KMeansSuite.scala b/mllib/src/test/scala/org/apache/spark/ml/clustering/KMeansSuite.scala
index 2076c745e2..2ca386e422 100644
--- a/mllib/src/test/scala/org/apache/spark/ml/clustering/KMeansSuite.scala
+++ b/mllib/src/test/scala/org/apache/spark/ml/clustering/KMeansSuite.scala
@@ -82,7 +82,7 @@ class KMeansSuite extends SparkFunSuite with MLlibTestSparkContext with DefaultR
     }
   }
 
-  test("fit & transform") {
+  test("fit, transform, and summary") {
     val predictionColName = "kmeans_prediction"
     val kmeans = new KMeans().setK(k).setPredictionCol(predictionColName).setSeed(1)
     val model = kmeans.fit(dataset)
@@ -99,6 +99,22 @@ class KMeansSuite extends SparkFunSuite with MLlibTestSparkContext with DefaultR
     assert(clusters === Set(0, 1, 2, 3, 4))
     assert(model.computeCost(dataset) < 0.1)
     assert(model.hasParent)
+
+    // Check validity of model summary
+    val numRows = dataset.count()
+    assert(model.hasSummary)
+    val summary: KMeansSummary = model.summary
+    assert(summary.predictionCol === predictionColName)
+    assert(summary.featuresCol === "features")
+    assert(summary.predictions.count() === numRows)
+    for (c <- Array(predictionColName, "features")) {
+      assert(summary.predictions.columns.contains(c))
+    }
+    assert(summary.cluster.columns === Array(predictionColName))
+    val clusterSizes = summary.clusterSizes
+    assert(clusterSizes.length === k)
+    assert(clusterSizes.sum === numRows)
+    assert(clusterSizes.forall(_ >= 0))
   }
 
   test("read/write") {