[SPARK-14508][BUILD] Add a new ScalaStyle Rule `OmitBracesInCase`

## What changes were proposed in this pull request?

According to the [Spark Code Style Guide](https://cwiki.apache.org/confluence/display/SPARK/Spark+Code+Style+Guide) and [Scala Style Guide](http://docs.scala-lang.org/style/control-structures.html#curlybraces), we had better enforce the following rule.
  ```
  case: Always omit braces in case clauses.
  ```
This PR makes a new ScalaStyle rule, 'OmitBracesInCase', and enforces it to the code.

## How was this patch tested?

Pass the Jenkins tests (including Scala style checking)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12280 from dongjoon-hyun/SPARK-14508.

5 files changed, 7 insertions, 14 deletions

diff --git a/mllib/src/main/scala/org/apache/spark/ml/r/SparkRWrappers.scala b/mllib/src/main/scala/org/apache/spark/ml/r/SparkRWrappers.scala
index 551e75dc0a..fa143715be 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/r/SparkRWrappers.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/r/SparkRWrappers.scala
@@ -53,7 +53,7 @@ private[r] object SparkRWrappers {
 
   def getModelCoefficients(model: PipelineModel): Array[Double] = {
     model.stages.last match {
-      case m: LinearRegressionModel => {
+      case m: LinearRegressionModel =>
         val coefficientStandardErrorsR = Array(m.summary.coefficientStandardErrors.last) ++
           m.summary.coefficientStandardErrors.dropRight(1)
         val tValuesR = Array(m.summary.tValues.last) ++ m.summary.tValues.dropRight(1)
@@ -64,14 +64,12 @@ private[r] object SparkRWrappers {
         } else {
           m.coefficients.toArray ++ coefficientStandardErrorsR ++ tValuesR ++ pValuesR
         }
-      }
-      case m: LogisticRegressionModel => {
+      case m: LogisticRegressionModel =>
         if (m.getFitIntercept) {
           Array(m.intercept) ++ m.coefficients.toArray
         } else {
           m.coefficients.toArray
         }
-      }
     }
   }
 
diff --git a/mllib/src/main/scala/org/apache/spark/mllib/clustering/GaussianMixture.scala b/mllib/src/main/scala/org/apache/spark/mllib/clustering/GaussianMixture.scala
index 03eb903bb8..f04c87259c 100644
--- a/mllib/src/main/scala/org/apache/spark/mllib/clustering/GaussianMixture.scala
+++ b/mllib/src/main/scala/org/apache/spark/mllib/clustering/GaussianMixture.scala
@@ -181,13 +181,12 @@ class GaussianMixture private (
     val (weights, gaussians) = initialModel match {
       case Some(gmm) => (gmm.weights, gmm.gaussians)
 
-      case None => {
+      case None =>
         val samples = breezeData.takeSample(withReplacement = true, k * nSamples, seed)
         (Array.fill(k)(1.0 / k), Array.tabulate(k) { i =>
           val slice = samples.view(i * nSamples, (i + 1) * nSamples)
           new MultivariateGaussian(vectorMean(slice), initCovariance(slice))
         })
-      }
     }
 
     var llh = Double.MinValue // current log-likelihood
diff --git a/mllib/src/main/scala/org/apache/spark/mllib/clustering/GaussianMixtureModel.scala b/mllib/src/main/scala/org/apache/spark/mllib/clustering/GaussianMixtureModel.scala
index 02417b1124..f87613cc72 100644
--- a/mllib/src/main/scala/org/apache/spark/mllib/clustering/GaussianMixtureModel.scala
+++ b/mllib/src/main/scala/org/apache/spark/mllib/clustering/GaussianMixtureModel.scala
@@ -183,7 +183,7 @@ object GaussianMixtureModel extends Loader[GaussianMixtureModel] {
     val k = (metadata \ "k").extract[Int]
     val classNameV1_0 = SaveLoadV1_0.classNameV1_0
     (loadedClassName, version) match {
-      case (classNameV1_0, "1.0") => {
+      case (classNameV1_0, "1.0") =>
         val model = SaveLoadV1_0.load(sc, path)
         require(model.weights.length == k,
           s"GaussianMixtureModel requires weights of length $k " +
@@ -192,7 +192,6 @@ object GaussianMixtureModel extends Loader[GaussianMixtureModel] {
           s"GaussianMixtureModel requires gaussians of length $k" +
           s"got gaussians of length ${model.gaussians.length}")
         model
-      }
       case _ => throw new Exception(
         s"GaussianMixtureModel.load did not recognize model with (className, format version):" +
         s"($loadedClassName, $version).  Supported:\n" +
diff --git a/mllib/src/main/scala/org/apache/spark/mllib/clustering/KMeans.scala b/mllib/src/main/scala/org/apache/spark/mllib/clustering/KMeans.scala
index 37a21cd879..8ff0b83e8b 100644
--- a/mllib/src/main/scala/org/apache/spark/mllib/clustering/KMeans.scala
+++ b/mllib/src/main/scala/org/apache/spark/mllib/clustering/KMeans.scala
@@ -253,16 +253,14 @@ class KMeans private (
     }
 
     val centers = initialModel match {
-      case Some(kMeansCenters) => {
+      case Some(kMeansCenters) =>
         Array(kMeansCenters.clusterCenters.map(s => new VectorWithNorm(s)))
-      }
-      case None => {
+      case None =>
         if (initializationMode == KMeans.RANDOM) {
           initRandom(data)
         } else {
           initKMeansParallel(data)
         }
-      }
     }
     val initTimeInSeconds = (System.nanoTime() - initStartTime) / 1e9
     logInfo(s"Initialization with $initializationMode took " + "%.3f".format(initTimeInSeconds) +
diff --git a/mllib/src/main/scala/org/apache/spark/mllib/stat/test/KolmogorovSmirnovTest.scala b/mllib/src/main/scala/org/apache/spark/mllib/stat/test/KolmogorovSmirnovTest.scala
index 0ec8975fed..ef284531c9 100644
--- a/mllib/src/main/scala/org/apache/spark/mllib/stat/test/KolmogorovSmirnovTest.scala
+++ b/mllib/src/main/scala/org/apache/spark/mllib/stat/test/KolmogorovSmirnovTest.scala
@@ -97,7 +97,7 @@ private[stat] object KolmogorovSmirnovTest extends Logging {
     : KolmogorovSmirnovTestResult = {
     val distObj =
       distName match {
-        case "norm" => {
+        case "norm" =>
           if (params.nonEmpty) {
             // parameters are passed, then can only be 2
             require(params.length == 2, "Normal distribution requires mean and standard " +
@@ -109,7 +109,6 @@ private[stat] object KolmogorovSmirnovTest extends Logging {
               "initialized to standard normal (i.e. N(0, 1))")
             new NormalDistribution(0, 1)
           }
-        }
         case  _ => throw new UnsupportedOperationException(s"$distName not yet supported through" +
           s" convenience method. Current options are:['norm'].")
       }