SPARK-24863: address review comments

external/kafka-0-10-sql/src/main/scala/org/apache/spark/sql/kafka010/JsonUtils.scala

@@ -56,7 +56,7 @@ private object JsonUtils {
    * Write TopicPartitions as json string
    */
   def partitions(partitions: Iterable[TopicPartition]): String = {
-    val result = new HashMap[String, List[Int]]
+    val result = HashMap.empty[String, List[Int]]
     partitions.foreach { tp =>
       val parts: List[Int] = result.getOrElse(tp.topic, Nil)
       result += tp.topic -> (tp.partition::parts)
@@ -85,11 +85,11 @@ private object JsonUtils {
    * Write per-TopicPartition offsets as json string
    */
   def partitionOffsets(partitionOffsets: Map[TopicPartition, Long]): String = {
-    val result = new HashMap[String, HashMap[Int, Long]]()
+    val result = HashMap.empty[String, HashMap[Int, Long]]
     val partitions = partitionOffsets.keySet.toSeq.sorted  // sort for more determinism
     partitions.foreach { tp =>
         val off = partitionOffsets(tp)
-        val parts = result.getOrElse(tp.topic, new HashMap[Int, Long])
+        val parts = result.getOrElse(tp.topic, HashMap.empty[Int, Long])
         parts += tp.partition -> off
         result += tp.topic -> parts
     }
@@ -99,13 +99,14 @@ private object JsonUtils {
   /**
    * Write per-topic partition lag as json string
    */
-  def partitionLags(latestOffsets: Map[TopicPartition, Long],
-                    processedOffsets: Map[TopicPartition, Long]): String = {
-    val result = new HashMap[String, HashMap[Int, Long]]()
+  def partitionLags(
+      latestOffsets: Map[TopicPartition, Long],
+      processedOffsets: Map[TopicPartition, Long]): String = {
+    val result = HashMap.empty[String, HashMap[Int, Long]]
     val partitions = latestOffsets.keySet.toSeq.sorted
     partitions.foreach { tp =>
       val lag = latestOffsets(tp) - processedOffsets.getOrElse(tp, 0L)
-      val parts = result.getOrElse(tp.topic, new HashMap[Int, Long])
+      val parts = result.getOrElse(tp.topic, HashMap.empty[Int, Long])
       parts += tp.partition -> lag
       result += tp.topic -> parts
     }