[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.

5 files changed, 14 insertions, 14 deletions

diff --git a/streaming/src/main/scala/org/apache/spark/streaming/dstream/InputDStream.scala b/streaming/src/main/scala/org/apache/spark/streaming/dstream/InputDStream.scala
index dc88349db5..a3c125c306 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/dstream/InputDStream.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/dstream/InputDStream.scala
@@ -107,8 +107,8 @@ abstract class InputDStream[T: ClassTag](_ssc: StreamingContext)
   }
 
   /** Method called to start receiving data. Subclasses must implement this method. */
-  def start()
+  def start(): Unit
 
   /** Method called to stop receiving data. Subclasses must implement this method. */
-  def stop()
+  def stop(): Unit
 }
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/receiver/BlockGenerator.scala b/streaming/src/main/scala/org/apache/spark/streaming/receiver/BlockGenerator.scala
index e42bea6ec6..4592e015ed 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/receiver/BlockGenerator.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/receiver/BlockGenerator.scala
@@ -37,7 +37,7 @@ private[streaming] trait BlockGeneratorListener {
    * that will be useful when a block is generated. Any long blocking operation in this callback
    * will hurt the throughput.
    */
-  def onAddData(data: Any, metadata: Any)
+  def onAddData(data: Any, metadata: Any): Unit
 
   /**
    * Called when a new block of data is generated by the block generator. The block generation
@@ -47,7 +47,7 @@ private[streaming] trait BlockGeneratorListener {
    * be useful when the block has been successfully stored. Any long blocking operation in this
    * callback will hurt the throughput.
    */
-  def onGenerateBlock(blockId: StreamBlockId)
+  def onGenerateBlock(blockId: StreamBlockId): Unit
 
   /**
    * Called when a new block is ready to be pushed. Callers are supposed to store the block into
@@ -55,13 +55,13 @@ private[streaming] trait BlockGeneratorListener {
    * thread, that is not synchronized with any other callbacks. Hence it is okay to do long
    * blocking operation in this callback.
    */
-  def onPushBlock(blockId: StreamBlockId, arrayBuffer: ArrayBuffer[_])
+  def onPushBlock(blockId: StreamBlockId, arrayBuffer: ArrayBuffer[_]): Unit
 
   /**
    * Called when an error has occurred in the BlockGenerator. Can be called form many places
    * so better to not do any long block operation in this callback.
    */
-  def onError(message: String, throwable: Throwable)
+  def onError(message: String, throwable: Throwable): Unit
 }
 
 /**
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/receiver/ReceivedBlockHandler.scala b/streaming/src/main/scala/org/apache/spark/streaming/receiver/ReceivedBlockHandler.scala
index 85350ff658..7aea1c9b64 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/receiver/ReceivedBlockHandler.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/receiver/ReceivedBlockHandler.scala
@@ -48,7 +48,7 @@ private[streaming] trait ReceivedBlockHandler {
   def storeBlock(blockId: StreamBlockId, receivedBlock: ReceivedBlock): ReceivedBlockStoreResult
 
   /** Cleanup old blocks older than the given threshold time */
-  def cleanupOldBlocks(threshTime: Long)
+  def cleanupOldBlocks(threshTime: Long): Unit
 }
 
 
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/receiver/Receiver.scala b/streaming/src/main/scala/org/apache/spark/streaming/receiver/Receiver.scala
index 3376cd557d..5157ca62dc 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/receiver/Receiver.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/receiver/Receiver.scala
@@ -99,13 +99,13 @@ abstract class Receiver[T](val storageLevel: StorageLevel) extends Serializable
    * (iii) `restart(...)` can be called to restart the receiver. This will call `onStop()`
    * immediately, and then `onStart()` after a delay.
    */
-  def onStart()
+  def onStart(): Unit
 
   /**
    * This method is called by the system when the receiver is stopped. All resources
    * (threads, buffers, etc.) set up in `onStart()` must be cleaned up in this method.
    */
-  def onStop()
+  def onStop(): Unit
 
   /** Override this to specify a preferred location (hostname). */
   def preferredLocation: Option[String] = None
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/receiver/ReceiverSupervisor.scala b/streaming/src/main/scala/org/apache/spark/streaming/receiver/ReceiverSupervisor.scala
index e0fe8d2206..42fc84c19b 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/receiver/ReceiverSupervisor.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/receiver/ReceiverSupervisor.scala
@@ -70,28 +70,28 @@ private[streaming] abstract class ReceiverSupervisor(
   @volatile private[streaming] var receiverState = Initialized
 
   /** Push a single data item to backend data store. */
-  def pushSingle(data: Any)
+  def pushSingle(data: Any): Unit
 
   /** Store the bytes of received data as a data block into Spark's memory. */
   def pushBytes(
       bytes: ByteBuffer,
       optionalMetadata: Option[Any],
       optionalBlockId: Option[StreamBlockId]
-    )
+    ): Unit
 
   /** Store a iterator of received data as a data block into Spark's memory. */
   def pushIterator(
       iterator: Iterator[_],
       optionalMetadata: Option[Any],
       optionalBlockId: Option[StreamBlockId]
-    )
+    ): Unit
 
   /** Store an ArrayBuffer of received data as a data block into Spark's memory. */
   def pushArrayBuffer(
       arrayBuffer: ArrayBuffer[_],
       optionalMetadata: Option[Any],
       optionalBlockId: Option[StreamBlockId]
-    )
+    ): Unit
 
   /**
    * Create a custom [[BlockGenerator]] that the receiver implementation can directly control
@@ -103,7 +103,7 @@ private[streaming] abstract class ReceiverSupervisor(
   def createBlockGenerator(blockGeneratorListener: BlockGeneratorListener): BlockGenerator
 
   /** Report errors. */
-  def reportError(message: String, throwable: Throwable)
+  def reportError(message: String, throwable: Throwable): Unit
 
   /**
    * Called when supervisor is started.