[SPARK-15875] Try to use Seq.isEmpty and Seq.nonEmpty instead of Seq.length == 0 and Seq.length > 0

## What changes were proposed in this pull request?

In scala, immutable.List.length is an expensive operation so we should
avoid using Seq.length == 0 or Seq.lenth > 0, and use Seq.isEmpty and Seq.nonEmpty instead.

## How was this patch tested?
existing tests

Author: wangyang <wangyang@haizhi.com>

Closes #13601 from yangw1234/isEmpty.

3 files changed, 5 insertions, 5 deletions

diff --git a/core/src/main/scala/org/apache/spark/api/python/PythonWorkerFactory.scala b/core/src/main/scala/org/apache/spark/api/python/PythonWorkerFactory.scala
index 3df87f62f2..6a5e6f7c5a 100644
--- a/core/src/main/scala/org/apache/spark/api/python/PythonWorkerFactory.scala
+++ b/core/src/main/scala/org/apache/spark/api/python/PythonWorkerFactory.scala
@@ -235,7 +235,7 @@ private[spark] class PythonWorkerFactory(pythonExec: String, envVars: Map[String
   }
 
   private def cleanupIdleWorkers() {
-    while (idleWorkers.length > 0) {
+    while (idleWorkers.nonEmpty) {
       val worker = idleWorkers.dequeue()
       try {
         // the worker will exit after closing the socket
diff --git a/core/src/main/scala/org/apache/spark/rdd/PartitionerAwareUnionRDD.scala b/core/src/main/scala/org/apache/spark/rdd/PartitionerAwareUnionRDD.scala
index b6366f3e68..d744d67592 100644
--- a/core/src/main/scala/org/apache/spark/rdd/PartitionerAwareUnionRDD.scala
+++ b/core/src/main/scala/org/apache/spark/rdd/PartitionerAwareUnionRDD.scala
@@ -60,7 +60,7 @@ class PartitionerAwareUnionRDD[T: ClassTag](
     sc: SparkContext,
     var rdds: Seq[RDD[T]]
   ) extends RDD[T](sc, rdds.map(x => new OneToOneDependency(x))) {
-  require(rdds.length > 0)
+  require(rdds.nonEmpty)
   require(rdds.forall(_.partitioner.isDefined))
   require(rdds.flatMap(_.partitioner).toSet.size == 1,
     "Parent RDDs have different partitioners: " + rdds.flatMap(_.partitioner))
diff --git a/core/src/main/scala/org/apache/spark/util/collection/ExternalAppendOnlyMap.scala b/core/src/main/scala/org/apache/spark/util/collection/ExternalAppendOnlyMap.scala
index fc71f8365c..6ddc72afde 100644
--- a/core/src/main/scala/org/apache/spark/util/collection/ExternalAppendOnlyMap.scala
+++ b/core/src/main/scala/org/apache/spark/util/collection/ExternalAppendOnlyMap.scala
@@ -375,14 +375,14 @@ class ExternalAppendOnlyMap[K, V, C](
     /**
      * Return true if there exists an input stream that still has unvisited pairs.
      */
-    override def hasNext: Boolean = mergeHeap.length > 0
+    override def hasNext: Boolean = mergeHeap.nonEmpty
 
     /**
      * Select a key with the minimum hash, then combine all values with the same key from all
      * input streams.
      */
     override def next(): (K, C) = {
-      if (mergeHeap.length == 0) {
+      if (mergeHeap.isEmpty) {
         throw new NoSuchElementException
       }
       // Select a key from the StreamBuffer that holds the lowest key hash
@@ -397,7 +397,7 @@ class ExternalAppendOnlyMap[K, V, C](
       // For all other streams that may have this key (i.e. have the same minimum key hash),
       // merge in the corresponding value (if any) from that stream
       val mergedBuffers = ArrayBuffer[StreamBuffer](minBuffer)
-      while (mergeHeap.length > 0 && mergeHeap.head.minKeyHash == minHash) {
+      while (mergeHeap.nonEmpty && mergeHeap.head.minKeyHash == minHash) {
         val newBuffer = mergeHeap.dequeue()
         minCombiner = mergeIfKeyExists(minKey, minCombiner, newBuffer)
         mergedBuffers += newBuffer