[SPARK-14630][BUILD][CORE][SQL][STREAMING] Code style: public abstract methods should have explicit return types

## What changes were proposed in this pull request?

Currently many public abstract methods (in abstract classes as well as traits) don't declare return types explicitly, such as in [o.a.s.streaming.dstream.InputDStream](https://github.com/apache/spark/blob/master/streaming/src/main/scala/org/apache/spark/streaming/dstream/InputDStream.scala#L110):
```scala
def start() // should be: def start(): Unit
def stop()  // should be: def stop(): Unit
```

These methods exist in core, sql, streaming; this PR fixes them.

## How was this patch tested?

N/A

## Which piece of scala style rule led to the changes?

the rule was added separately in https://github.com/apache/spark/pull/12396

Author: Liwei Lin <lwlin7@gmail.com>

Closes #12389 from lw-lin/public-abstract-methods.

16 files changed, 27 insertions, 26 deletions

diff --git a/core/src/main/scala/org/apache/spark/ContextCleaner.scala b/core/src/main/scala/org/apache/spark/ContextCleaner.scala
index 8fc657c5eb..76692ccec8 100644
--- a/core/src/main/scala/org/apache/spark/ContextCleaner.scala
+++ b/core/src/main/scala/org/apache/spark/ContextCleaner.scala
@@ -278,9 +278,9 @@ private object ContextCleaner {
  * Listener class used for testing when any item has been cleaned by the Cleaner class.
  */
 private[spark] trait CleanerListener {
-  def rddCleaned(rddId: Int)
-  def shuffleCleaned(shuffleId: Int)
-  def broadcastCleaned(broadcastId: Long)
-  def accumCleaned(accId: Long)
-  def checkpointCleaned(rddId: Long)
+  def rddCleaned(rddId: Int): Unit
+  def shuffleCleaned(shuffleId: Int): Unit
+  def broadcastCleaned(broadcastId: Long): Unit
+  def accumCleaned(accId: Long): Unit
+  def checkpointCleaned(rddId: Long): Unit
 }
diff --git a/core/src/main/scala/org/apache/spark/FutureAction.scala b/core/src/main/scala/org/apache/spark/FutureAction.scala
index ce11772a6d..339266a5d4 100644
--- a/core/src/main/scala/org/apache/spark/FutureAction.scala
+++ b/core/src/main/scala/org/apache/spark/FutureAction.scala
@@ -41,7 +41,7 @@ trait FutureAction[T] extends Future[T] {
   /**
    * Cancels the execution of this action.
    */
-  def cancel()
+  def cancel(): Unit
 
   /**
    * Blocks until this action completes.
@@ -65,7 +65,7 @@ trait FutureAction[T] extends Future[T] {
    * When this action is completed, either through an exception, or a value, applies the provided
    * function.
    */
-  def onComplete[U](func: (Try[T]) => U)(implicit executor: ExecutionContext)
+  def onComplete[U](func: (Try[T]) => U)(implicit executor: ExecutionContext): Unit
 
   /**
    * Returns whether the action has already been completed with a value or an exception.
diff --git a/core/src/main/scala/org/apache/spark/deploy/client/AppClientListener.scala b/core/src/main/scala/org/apache/spark/deploy/client/AppClientListener.scala
index e584952a9a..94506a0cbb 100644
--- a/core/src/main/scala/org/apache/spark/deploy/client/AppClientListener.scala
+++ b/core/src/main/scala/org/apache/spark/deploy/client/AppClientListener.scala
@@ -33,7 +33,8 @@ private[spark] trait AppClientListener {
   /** An application death is an unrecoverable failure condition. */
   def dead(reason: String): Unit
 
-  def executorAdded(fullId: String, workerId: String, hostPort: String, cores: Int, memory: Int)
+  def executorAdded(
+      fullId: String, workerId: String, hostPort: String, cores: Int, memory: Int): Unit
 
   def executorRemoved(fullId: String, message: String, exitStatus: Option[Int]): Unit
 }
diff --git a/core/src/main/scala/org/apache/spark/deploy/master/LeaderElectionAgent.scala b/core/src/main/scala/org/apache/spark/deploy/master/LeaderElectionAgent.scala
index 70f21fbe0d..52e2854961 100644
--- a/core/src/main/scala/org/apache/spark/deploy/master/LeaderElectionAgent.scala
+++ b/core/src/main/scala/org/apache/spark/deploy/master/LeaderElectionAgent.scala
@@ -32,8 +32,8 @@ trait LeaderElectionAgent {
 
 @DeveloperApi
 trait LeaderElectable {
-  def electedLeader()
-  def revokedLeadership()
+  def electedLeader(): Unit
+  def revokedLeadership(): Unit
 }
 
 /** Single-node implementation of LeaderElectionAgent -- we're initially and always the leader. */
diff --git a/core/src/main/scala/org/apache/spark/deploy/master/PersistenceEngine.scala b/core/src/main/scala/org/apache/spark/deploy/master/PersistenceEngine.scala
index dddf2be57e..b30bc821b7 100644
--- a/core/src/main/scala/org/apache/spark/deploy/master/PersistenceEngine.scala
+++ b/core/src/main/scala/org/apache/spark/deploy/master/PersistenceEngine.scala
@@ -40,12 +40,12 @@ abstract class PersistenceEngine {
    * Defines how the object is serialized and persisted. Implementation will
    * depend on the store used.
    */
-  def persist(name: String, obj: Object)
+  def persist(name: String, obj: Object): Unit
 
   /**
    * Defines how the object referred by its name is removed from the store.
    */
-  def unpersist(name: String)
+  def unpersist(name: String): Unit
 
   /**
    * Gives all objects, matching a prefix. This defines how objects are
diff --git a/core/src/main/scala/org/apache/spark/deploy/worker/DriverRunner.scala b/core/src/main/scala/org/apache/spark/deploy/worker/DriverRunner.scala
index 9c6bc5c62f..aad2e91b25 100644
--- a/core/src/main/scala/org/apache/spark/deploy/worker/DriverRunner.scala
+++ b/core/src/main/scala/org/apache/spark/deploy/worker/DriverRunner.scala
@@ -218,7 +218,7 @@ private[deploy] class DriverRunner(
 }
 
 private[deploy] trait Sleeper {
-  def sleep(seconds: Int)
+  def sleep(seconds: Int): Unit
 }
 
 // Needed because ProcessBuilder is a final class and cannot be mocked
diff --git a/core/src/main/scala/org/apache/spark/executor/ExecutorBackend.scala b/core/src/main/scala/org/apache/spark/executor/ExecutorBackend.scala
index e07cb31cbe..7153323d01 100644
--- a/core/src/main/scala/org/apache/spark/executor/ExecutorBackend.scala
+++ b/core/src/main/scala/org/apache/spark/executor/ExecutorBackend.scala
@@ -25,6 +25,6 @@ import org.apache.spark.TaskState.TaskState
  * A pluggable interface used by the Executor to send updates to the cluster scheduler.
  */
 private[spark] trait ExecutorBackend {
-  def statusUpdate(taskId: Long, state: TaskState, data: ByteBuffer)
+  def statusUpdate(taskId: Long, state: TaskState, data: ByteBuffer): Unit
 }
 
diff --git a/core/src/main/scala/org/apache/spark/network/BlockTransferService.scala b/core/src/main/scala/org/apache/spark/network/BlockTransferService.scala
index e43e3a2de2..09ce012e4e 100644
--- a/core/src/main/scala/org/apache/spark/network/BlockTransferService.scala
+++ b/core/src/main/scala/org/apache/spark/network/BlockTransferService.scala
@@ -36,7 +36,7 @@ abstract class BlockTransferService extends ShuffleClient with Closeable with Lo
    * Initialize the transfer service by giving it the BlockDataManager that can be used to fetch
    * local blocks or put local blocks.
    */
-  def init(blockDataManager: BlockDataManager)
+  def init(blockDataManager: BlockDataManager): Unit
 
   /**
    * Tear down the transfer service.
diff --git a/core/src/main/scala/org/apache/spark/scheduler/JobListener.scala b/core/src/main/scala/org/apache/spark/scheduler/JobListener.scala
index 50c2b9acd6..e0f7c8f021 100644
--- a/core/src/main/scala/org/apache/spark/scheduler/JobListener.scala
+++ b/core/src/main/scala/org/apache/spark/scheduler/JobListener.scala
@@ -23,6 +23,6 @@ package org.apache.spark.scheduler
  * job fails (and no further taskSucceeded events will happen).
  */
 private[spark] trait JobListener {
-  def taskSucceeded(index: Int, result: Any)
-  def jobFailed(exception: Exception)
+  def taskSucceeded(index: Int, result: Any): Unit
+  def jobFailed(exception: Exception): Unit
 }
diff --git a/core/src/main/scala/org/apache/spark/scheduler/SchedulableBuilder.scala b/core/src/main/scala/org/apache/spark/scheduler/SchedulableBuilder.scala
index 5baebe8c1f..100ed76ecb 100644
--- a/core/src/main/scala/org/apache/spark/scheduler/SchedulableBuilder.scala
+++ b/core/src/main/scala/org/apache/spark/scheduler/SchedulableBuilder.scala
@@ -34,9 +34,9 @@ import org.apache.spark.util.Utils
 private[spark] trait SchedulableBuilder {
   def rootPool: Pool
 
-  def buildPools()
+  def buildPools(): Unit
 
-  def addTaskSetManager(manager: Schedulable, properties: Properties)
+  def addTaskSetManager(manager: Schedulable, properties: Properties): Unit
 }
 
 private[spark] class FIFOSchedulableBuilder(val rootPool: Pool)
diff --git a/core/src/main/scala/org/apache/spark/scheduler/TaskScheduler.scala b/core/src/main/scala/org/apache/spark/scheduler/TaskScheduler.scala
index 8477a66b39..647d44a0f0 100644
--- a/core/src/main/scala/org/apache/spark/scheduler/TaskScheduler.scala
+++ b/core/src/main/scala/org/apache/spark/scheduler/TaskScheduler.scala
@@ -51,7 +51,7 @@ private[spark] trait TaskScheduler {
   def submitTasks(taskSet: TaskSet): Unit
 
   // Cancel a stage.
-  def cancelTasks(stageId: Int, interruptThread: Boolean)
+  def cancelTasks(stageId: Int, interruptThread: Boolean): Unit
 
   // Set the DAG scheduler for upcalls. This is guaranteed to be set before submitTasks is called.
   def setDAGScheduler(dagScheduler: DAGScheduler): Unit
diff --git a/core/src/main/scala/org/apache/spark/serializer/KryoSerializer.scala b/core/src/main/scala/org/apache/spark/serializer/KryoSerializer.scala
index 3d090a4353..918ae376f6 100644
--- a/core/src/main/scala/org/apache/spark/serializer/KryoSerializer.scala
+++ b/core/src/main/scala/org/apache/spark/serializer/KryoSerializer.scala
@@ -357,7 +357,7 @@ private[spark] class KryoSerializerInstance(ks: KryoSerializer) extends Serializ
  * serialization.
  */
 trait KryoRegistrator {
-  def registerClasses(kryo: Kryo)
+  def registerClasses(kryo: Kryo): Unit
 }
 
 private[serializer] object KryoSerializer {
diff --git a/core/src/main/scala/org/apache/spark/shuffle/FileShuffleBlockResolver.scala b/core/src/main/scala/org/apache/spark/shuffle/FileShuffleBlockResolver.scala
index 6cd7d69518..be1e84a2ba 100644
--- a/core/src/main/scala/org/apache/spark/shuffle/FileShuffleBlockResolver.scala
+++ b/core/src/main/scala/org/apache/spark/shuffle/FileShuffleBlockResolver.scala
@@ -35,7 +35,7 @@ private[spark] trait ShuffleWriterGroup {
   val writers: Array[DiskBlockObjectWriter]
 
   /** @param success Indicates all writes were successful. If false, no blocks will be recorded. */
-  def releaseWriters(success: Boolean)
+  def releaseWriters(success: Boolean): Unit
 }
 
 /**
diff --git a/core/src/main/scala/org/apache/spark/ui/WebUI.scala b/core/src/main/scala/org/apache/spark/ui/WebUI.scala
index 3939b111b5..2b0bc32cf6 100644
--- a/core/src/main/scala/org/apache/spark/ui/WebUI.scala
+++ b/core/src/main/scala/org/apache/spark/ui/WebUI.scala
@@ -129,7 +129,7 @@ private[spark] abstract class WebUI(
   }
 
   /** Initialize all components of the server. */
-  def initialize()
+  def initialize(): Unit
 
   /** Bind to the HTTP server behind this web interface. */
   def bind() {
diff --git a/core/src/main/scala/org/apache/spark/util/logging/RollingPolicy.scala b/core/src/main/scala/org/apache/spark/util/logging/RollingPolicy.scala
index b34880d3a7..6e80db2f51 100644
--- a/core/src/main/scala/org/apache/spark/util/logging/RollingPolicy.scala
+++ b/core/src/main/scala/org/apache/spark/util/logging/RollingPolicy.scala
@@ -32,10 +32,10 @@ private[spark] trait RollingPolicy {
   def shouldRollover(bytesToBeWritten: Long): Boolean
 
   /** Notify that rollover has occurred */
-  def rolledOver()
+  def rolledOver(): Unit
 
   /** Notify that bytes have been written */
-  def bytesWritten(bytes: Long)
+  def bytesWritten(bytes: Long): Unit
 
   /** Get the desired name of the rollover file */
   def generateRolledOverFileSuffix(): String
diff --git a/core/src/main/scala/org/apache/spark/util/random/Pseudorandom.scala b/core/src/main/scala/org/apache/spark/util/random/Pseudorandom.scala
index 70f3dd62b9..41f28f6e51 100644
--- a/core/src/main/scala/org/apache/spark/util/random/Pseudorandom.scala
+++ b/core/src/main/scala/org/apache/spark/util/random/Pseudorandom.scala
@@ -26,5 +26,5 @@ import org.apache.spark.annotation.DeveloperApi
 @DeveloperApi
 trait Pseudorandom {
   /** Set random seed. */
-  def setSeed(seed: Long)
+  def setSeed(seed: Long): Unit
 }