[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.

2 files changed, 2 insertions, 2 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/SpecificMutableRow.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/SpecificMutableRow.scala
index 4615c55d67..61ca7272df 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/SpecificMutableRow.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/SpecificMutableRow.scala
@@ -62,7 +62,7 @@ import org.apache.spark.sql.types._
 abstract class MutableValue extends Serializable {
   var isNull: Boolean = true
   def boxed: Any
-  def update(v: Any)
+  def update(v: Any): Unit
   def copy(): MutableValue
 }
 
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/rows.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/rows.scala
index be6b2530ef..93a8278528 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/rows.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/rows.scala
@@ -164,7 +164,7 @@ trait BaseGenericInternalRow extends InternalRow {
 abstract class MutableRow extends InternalRow {
   def setNullAt(i: Int): Unit
 
-  def update(i: Int, value: Any)
+  def update(i: Int, value: Any): Unit
 
   // default implementation (slow)
   def setBoolean(i: Int, value: Boolean): Unit = { update(i, value) }