give separated buffer for each hex method

Author: zhichao-li

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

@@ -295,8 +295,6 @@ case class Pow(left: Expression, right: Expression)
 case class Hex(child: Expression)
   extends UnaryExpression with Serializable  {
 
-  private var value = new Array[Byte](16)
-
   override def dataType: DataType = StringType
 
   override def checkInputDataTypes(): TypeCheckResult = {
@@ -337,9 +335,7 @@ case class Hex(child: Expression)
   }
 
   private def doHex(bytes: Array[Byte], length: Int): UTF8String = {
-    if (value.length < length * 2) {
-      value = new Array[Byte](length * 2)
-    }
+    val value = new Array[Byte](length * 2)
     var i = 0
     while(i < length) {
       value(i * 2) = Character.toUpperCase(Character.forDigit(
@@ -348,11 +344,12 @@ case class Hex(child: Expression)
         bytes(i) & 0x0F, 16)).toByte
       i += 1
     }
-    UTF8String.fromBytes(Arrays.copyOfRange(value, 0, length*2))
+    UTF8String.fromBytes(value)
   }
 
   private def hex(num: Long): UTF8String = {
     // Extract the hex digits of num into value[] from right to left
+    val value = new Array[Byte](16)
     var numBuf = num
     var len = 0
     do {