diff --git a/core/src/main/scala/org/apache/spark/MapOutputTracker.scala b/core/src/main/scala/org/apache/spark/MapOutputTracker.scala
index b749d7e862..ea9e641101 100644
--- a/core/src/main/scala/org/apache/spark/MapOutputTracker.scala
+++ b/core/src/main/scala/org/apache/spark/MapOutputTracker.scala
@@ -720,7 +720,7 @@ private[spark] class MapOutputTrackerMaster(
     }
   }
 
-  def registerMergeResult(shuffleId: Int, reduceId: Int, status: MergeStatus) {
+  def registerMergeResult(shuffleId: Int, reduceId: Int, status: MergeStatus): Unit = {
     shuffleStatuses(shuffleId).addMergeResult(reduceId, status)
   }
 
@@ -745,7 +745,7 @@ private[spark] class MapOutputTrackerMaster(
     shuffleId: Int,
     reduceId: Int,
     bmAddress: BlockManagerId,
-    mapId: Option[Int] = None) {
+    mapId: Option[Int] = None): Unit = {
     shuffleStatuses.get(shuffleId) match {
       case Some(shuffleStatus) =>
         val mergeStatus = shuffleStatus.mergeStatuses(reduceId)
diff --git a/core/src/test/scala/org/apache/spark/util/collection/ExternalAppendOnlyMapSuite.scala b/core/src/test/scala/org/apache/spark/util/collection/ExternalAppendOnlyMapSuite.scala
index 81a145906d..3f6cb2475a 100644
--- a/core/src/test/scala/org/apache/spark/util/collection/ExternalAppendOnlyMapSuite.scala
+++ b/core/src/test/scala/org/apache/spark/util/collection/ExternalAppendOnlyMapSuite.scala
@@ -220,7 +220,7 @@ class ExternalAppendOnlyMapSuite extends SparkFunSuite
     testSimpleSpilling()
   }
 
-  private def testSimpleSpillingForAllCodecs(encrypt: Boolean) {
+  private def testSimpleSpillingForAllCodecs(encrypt: Boolean): Unit = {
     // Keep track of which compression codec we're using to report in test failure messages
     var lastCompressionCodec: Option[String] = None
     try {
diff --git a/mllib-local/src/test/scala/org/apache/spark/ml/linalg/BLASBenchmark.scala b/mllib-local/src/test/scala/org/apache/spark/ml/linalg/BLASBenchmark.scala
index 86a576826b..699d841e06 100644
--- a/mllib-local/src/test/scala/org/apache/spark/ml/linalg/BLASBenchmark.scala
+++ b/mllib-local/src/test/scala/org/apache/spark/ml/linalg/BLASBenchmark.scala
@@ -50,7 +50,7 @@ object BLASBenchmark extends BenchmarkBase {
     println("nativeBLAS = " + nativeBLAS.getClass.getName)
     // scalastyle:on println
 
-    def runBLASBenchmark(name: String, n: Int)(bench: NetlibBLAS => Unit) {
+    def runBLASBenchmark(name: String, n: Int)(bench: NetlibBLAS => Unit): Unit = {
       val benchmark = new Benchmark(name, n, iters,
                                     warmupTime = 30.seconds,
                                     minTime = 30.seconds,
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/command/DataWritingCommand.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/command/DataWritingCommand.scala
index ca71395fc5..ca51b88de0 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/command/DataWritingCommand.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/command/DataWritingCommand.scala
@@ -107,7 +107,7 @@ object DataWritingCommand {
    * @param saveMode  Save mode of the table.
    * @param hadoopConf Configuration.
    */
-  def assertEmptyRootPath(tablePath: URI, saveMode: SaveMode, hadoopConf: Configuration) {
+  def assertEmptyRootPath(tablePath: URI, saveMode: SaveMode, hadoopConf: Configuration): Unit = {
     if (saveMode == SaveMode.ErrorIfExists && !SQLConf.get.allowNonEmptyLocationInCTAS) {
       val filePath = new org.apache.hadoop.fs.Path(tablePath)
       val fs = filePath.getFileSystem(hadoopConf)
diff --git a/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/SQLQuerySuite.scala b/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/SQLQuerySuite.scala
index 462d3f3423..684b2f22ff 100644
--- a/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/SQLQuerySuite.scala
+++ b/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/SQLQuerySuite.scala
@@ -599,7 +599,7 @@ abstract class SQLQuerySuiteBase extends QueryTest with SQLTestUtils with TestHi
   }
 
   test("SPARK-28551: CTAS Hive Table should be with non-existent or empty location") {
-    def executeCTASWithNonEmptyLocation(tempLocation: String) {
+    def executeCTASWithNonEmptyLocation(tempLocation: String): Unit = {
       sql(s"CREATE TABLE ctas1(id string) stored as rcfile LOCATION '$tempLocation/ctas1'")
       sql("INSERT INTO TABLE ctas1 SELECT 'A' ")
       sql(s"""CREATE TABLE ctas_with_existing_location stored as rcfile LOCATION