[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.

4 files changed, 7 insertions, 7 deletions

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/columnar/ColumnAccessor.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/columnar/ColumnAccessor.scala
index 78664baa56..7cde04b626 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/columnar/ColumnAccessor.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/columnar/ColumnAccessor.scala
@@ -38,7 +38,7 @@ private[columnar] trait ColumnAccessor {
 
   def hasNext: Boolean
 
-  def extractTo(row: MutableRow, ordinal: Int)
+  def extractTo(row: MutableRow, ordinal: Int): Unit
 
   protected def underlyingBuffer: ByteBuffer
 }
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/columnar/ColumnBuilder.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/columnar/ColumnBuilder.scala
index 9a173367f4..d30655e0c4 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/columnar/ColumnBuilder.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/columnar/ColumnBuilder.scala
@@ -28,12 +28,12 @@ private[columnar] trait ColumnBuilder {
   /**
    * Initializes with an approximate lower bound on the expected number of elements in this column.
    */
-  def initialize(initialSize: Int, columnName: String = "", useCompression: Boolean = false)
+  def initialize(initialSize: Int, columnName: String = "", useCompression: Boolean = false): Unit
 
   /**
    * Appends `row(ordinal)` to the column builder.
    */
-  def appendFrom(row: InternalRow, ordinal: Int)
+  def appendFrom(row: InternalRow, ordinal: Int): Unit
 
   /**
    * Column statistics information
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/state/StateStore.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/state/StateStore.scala
index cc5327e0e2..9521506325 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/state/StateStore.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/state/StateStore.scala
@@ -50,7 +50,7 @@ trait StateStore {
   def get(key: UnsafeRow): Option[UnsafeRow]
 
   /** Put a new value for a key. */
-  def put(key: UnsafeRow, value: UnsafeRow)
+  def put(key: UnsafeRow, value: UnsafeRow): Unit
 
   /**
    * Remove keys that match the following condition.
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/util/ContinuousQueryListener.scala b/sql/core/src/main/scala/org/apache/spark/sql/util/ContinuousQueryListener.scala
index bf78be9d9f..ba1facf11b 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/util/ContinuousQueryListener.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/util/ContinuousQueryListener.scala
@@ -37,7 +37,7 @@ abstract class ContinuousQueryListener {
    *       `DataFrameWriter.startStream()` returns the corresponding [[ContinuousQuery]]. Please
    *       don't block this method as it will block your query.
    */
-  def onQueryStarted(queryStarted: QueryStarted)
+  def onQueryStarted(queryStarted: QueryStarted): Unit
 
   /**
    * Called when there is some status update (ingestion rate updated, etc.)
@@ -47,10 +47,10 @@ abstract class ContinuousQueryListener {
    *       may be changed before/when you process the event. E.g., you may find [[ContinuousQuery]]
    *       is terminated when you are processing [[QueryProgress]].
    */
-  def onQueryProgress(queryProgress: QueryProgress)
+  def onQueryProgress(queryProgress: QueryProgress): Unit
 
   /** Called when a query is stopped, with or without error */
-  def onQueryTerminated(queryTerminated: QueryTerminated)
+  def onQueryTerminated(queryTerminated: QueryTerminated): Unit
 }