Update data type tests.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/types/dataTypes.scala

@@ -295,19 +295,31 @@ object StructType {
 case class StructType(fields: Seq[StructField]) extends DataType {
   require(StructType.validateFields(fields), "Found fields with the same name.")
 
+  /**
+   * Returns all field names in a [[Seq]].
+   */
+  lazy val fieldNames: Seq[String] = fields.map(_.name)
+  private lazy val fieldNamesSet: Set[String] = fieldNames.toSet
+
   /**
    * Extracts a [[StructField]] of the given name. If the [[StructType]] object does not
    * have a name matching the given name, `null` will be returned.
    */
   def apply(name: String): StructField = {
-    fields.find(f => f.name == name).orNull
+    fields.find(f => f.name == name).getOrElse(
+      throw new IllegalArgumentException(s"Field ${name} does not exist."))
   }
 
   /**
    * Returns a [[StructType]] containing [[StructField]]s of the given names.
    * Those names which do not have matching fields will be ignored.
    */
   def apply(names: Set[String]): StructType = {
+    val nonExistFields = names -- fieldNamesSet
+    if (!nonExistFields.isEmpty) {
+      throw new IllegalArgumentException(
+        s"Field ${nonExistFields.mkString(",")} does not exist.")
+    }
     StructType(fields.filter(f => names.contains(f.name)))
   }
 

sql/core/src/test/scala/org/apache/spark/sql/SchemaSuite.scala → sql/core/src/test/scala/org/apache/spark/sql/DataTypeSuite.scala

@@ -19,15 +19,15 @@ package org.apache.spark.sql
 
 import org.scalatest.FunSuite
 
-class SchemaSuite extends FunSuite {
+class DataTypeSuite extends FunSuite {
 
-  test("constructing an ArrayType") {
+  test("construct an ArrayType") {
     val array = ArrayType(StringType)
 
     assert(ArrayType(StringType, false) === array)
   }
 
-  test("extracting fields from a StructType") {
+  test("extract fields from a StructType") {
     val struct = StructType(
       StructField("a", IntegerType, true) ::
       StructField("b", LongType, false) ::
@@ -36,14 +36,17 @@ class SchemaSuite extends FunSuite {
 
     assert(StructField("b", LongType, false) === struct("b"))
 
-    assert(struct("e") === null)
+    intercept[IllegalArgumentException] {
+      struct("e")
+    }
 
     val expectedStruct = StructType(
       StructField("b", LongType, false) ::
       StructField("d", FloatType, true) :: Nil)
 
     assert(expectedStruct === struct(Set("b", "d")))
-    // struct does not have a field called e. So e is ignored.
-    assert(expectedStruct === struct(Set("b", "d", "e")))
+    intercept[IllegalArgumentException] {
+      struct(Set("b", "d", "e", "f"))
+    }
   }
 }