Make private def.

Author: dongjoon-hyun

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -527,7 +527,7 @@ object LikeSimplification extends Rule[LogicalPlan] {
  * Null value propagation from bottom to top of the expression tree.
  */
 object NullPropagation extends Rule[LogicalPlan] {
-  def nonNullLiteral(e: Expression): Boolean = e match {
+  private def nonNullLiteral(e: Expression): Boolean = e match {
     case Literal(null, _) => false
     case _ => true
   }
@@ -773,7 +773,7 @@ object BooleanSimplification extends Rule[LogicalPlan] with PredicateHelper {
  * Simplifies conditional expressions (if / case).
  */
 object SimplifyConditionals extends Rule[LogicalPlan] with PredicateHelper {
-  def falseOrNullLiteral(e: Expression): Boolean = e match {
+  private def falseOrNullLiteral(e: Expression): Boolean = e match {
     case FalseLiteral => true
     case Literal(null, _) => true
     case _ => false