Add unit tests.

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

@@ -0,0 +1,49 @@
+/*
+* Licensed to the Apache Software Foundation (ASF) under one or more
+* contributor license agreements.  See the NOTICE file distributed with
+* this work for additional information regarding copyright ownership.
+* The ASF licenses this file to You under the Apache License, Version 2.0
+* (the "License"); you may not use this file except in compliance with
+* the License.  You may obtain a copy of the License at
+*
+*    http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+package org.apache.spark.sql
+
+import org.scalatest.FunSuite
+
+class SchemaSuite extends FunSuite {
+
+  test("constructing an ArrayType") {
+    val array = ArrayType(StringType)
+
+    assert(ArrayType(StringType, false) === array)
+  }
+
+  test("extracting fields from a StructType") {
+    val struct = StructType(
+      StructField("a", IntegerType, true) ::
+      StructField("b", LongType, false) ::
+      StructField("c", StringType, true) ::
+      StructField("d", FloatType, true) :: Nil)
+
+    assert(StructField("b", LongType, false) === struct("b"))
+
+    assert(struct("e") === null)
+
+    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")))
+  }
+}