fix style and add comment

Author: Davies Liu

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregates.scala

@@ -505,7 +505,8 @@ case class AverageFunction(expr: Expression, base: AggregateExpression)
   private var count: Long = _
   private val sum = MutableLiteral(zero.eval(null), calcType)
 
-  private def addFunction(value: Any) = Add(sum, Cast(Literal.create(value, expr.dataType), calcType))
+  private def addFunction(value: Any) = Add(sum,
+    Cast(Literal.create(value, expr.dataType), calcType))
 
   override def eval(input: Row): Any = {
     if (count == 0L) {

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/literals.scala

@@ -64,6 +64,9 @@ object IntegerLiteral {
   }
 }
 
+/**
+ * In order to do type checking, use Literal.create() instead of constructor
+ */
 case class Literal protected (value: Any, dataType: DataType) extends LeafExpression {
 
   override def foldable: Boolean = true

sql/core/src/main/scala/org/apache/spark/sql/parquet/newParquet.scala

@@ -961,7 +961,8 @@ private[sql] object ParquetRelation2 extends Logging {
       .orElse(Try(Literal.create(new JBigDecimal(raw), DecimalType.Unlimited)))
       // Then falls back to string
       .getOrElse {
-        if (raw == defaultPartitionName) Literal.create(null, NullType) else Literal.create(raw, StringType)
+        if (raw == defaultPartitionName) Literal.create(null, NullType)
+        else Literal.create(raw, StringType)
       }
   }