[SPARK-17331][CORE][MLLIB] Avoid allocating 0-length arrays

## What changes were proposed in this pull request?

Avoid allocating some 0-length arrays, esp. in UTF8String, and by using Array.empty in Scala over Array[T]()

## How was this patch tested?

Jenkins

Author: Sean Owen <sowen@cloudera.com>

Closes #14895 from srowen/SPARK-17331.

1 files changed, 3 insertions, 3 deletions

diff --git a/mllib-local/src/main/scala/org/apache/spark/ml/linalg/Matrices.scala b/mllib-local/src/main/scala/org/apache/spark/ml/linalg/Matrices.scala
index f1ecc65af1..98080bb71a 100644
--- a/mllib-local/src/main/scala/org/apache/spark/ml/linalg/Matrices.scala
+++ b/mllib-local/src/main/scala/org/apache/spark/ml/linalg/Matrices.scala
@@ -713,7 +713,7 @@ object SparseMatrix {
       "The expected number of nonzeros cannot be greater than Int.MaxValue.")
     val nnz = math.ceil(expected).toInt
     if (density == 0.0) {
-      new SparseMatrix(numRows, numCols, new Array[Int](numCols + 1), Array[Int](), Array[Double]())
+      new SparseMatrix(numRows, numCols, new Array[Int](numCols + 1), Array.empty, Array.empty)
     } else if (density == 1.0) {
       val colPtrs = Array.tabulate(numCols + 1)(j => j * numRows)
       val rowIndices = Array.tabulate(size.toInt)(idx => idx % numRows)
@@ -961,7 +961,7 @@ object Matrices {
   @Since("2.0.0")
   def horzcat(matrices: Array[Matrix]): Matrix = {
     if (matrices.isEmpty) {
-      return new DenseMatrix(0, 0, Array[Double]())
+      return new DenseMatrix(0, 0, Array.empty)
     } else if (matrices.length == 1) {
       return matrices(0)
     }
@@ -1020,7 +1020,7 @@ object Matrices {
   @Since("2.0.0")
   def vertcat(matrices: Array[Matrix]): Matrix = {
     if (matrices.isEmpty) {
-      return new DenseMatrix(0, 0, Array[Double]())
+      return new DenseMatrix(0, 0, Array.empty)
     } else if (matrices.length == 1) {
       return matrices(0)
     }