Add test.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/ArrayData.scala

@@ -188,10 +188,15 @@ class ArrayDataIndexedSeq[T](arrayData: ArrayData, dataType: DataType) extends I
     case _ => (idx: Int) => arrayData.get(idx, dataType)
   }
 
-  override def apply(idx: Int): T = if (idx < arrayData.numElements()) {
-    accessor(idx).asInstanceOf[T]
+  override def apply(idx: Int): T = if (0 <= idx && idx < arrayData.numElements()) {
+    if (arrayData.isNullAt(idx)) {
+      null.asInstanceOf[T]
+    } else {
+      accessor(idx).asInstanceOf[T]
+    }
   } else {
-    throw new IndexOutOfBoundsException(s"Index $idx is greater than the length of the ArrayData.")
+    throw new IndexOutOfBoundsException(
+      s"Index $idx must be between 0 and the length of the ArrayData.")
   }
 
   override def length: Int = arrayData.numElements()

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/ArrayDataIndexedSeqSuite.scala

@@ -0,0 +1,69 @@
+/*
+ * 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.catalyst.util
+
+import org.apache.spark.SparkFunSuite
+import org.apache.spark.sql.catalyst.encoders.ExpressionEncoder
+import org.apache.spark.sql.catalyst.expressions.UnsafeArrayData
+import org.apache.spark.sql.types.StringType
+import org.apache.spark.unsafe.types.UTF8String
+
+class ArrayDataIndexedSeqSuite extends SparkFunSuite {
+  def utf8(str: String): UTF8String = UTF8String.fromString(str)
+  val stringArray = Array("1", "10", "100", null)
+
+  private def testArrayData(arrayData: ArrayData): Unit = {
+    assert(arrayData.numElements == stringArray.length)
+    stringArray.zipWithIndex.map { case (e, i) =>
+      if (e != null) {
+        assert(arrayData.getUTF8String(i).toString().equals(e))
+      } else {
+        assert(arrayData.isNullAt(i))
+      }
+    }
+
+    val seq = arrayData.toSeq[UTF8String](StringType)
+    stringArray.zipWithIndex.map { case (e, i) =>
+      if (e != null) {
+        assert(seq(i).toString().equals(e))
+      } else {
+        assert(seq(i) == null)
+      }
+    }
+
+    intercept[IndexOutOfBoundsException] {
+      seq(-1)
+    }.getMessage().contains("must be between 0 and the length of the ArrayData.")
+
+    intercept[IndexOutOfBoundsException] {
+      seq(seq.length)
+    }.getMessage().contains("must be between 0 and the length of the ArrayData.")
+  }
+
+  test("ArrayDataIndexedSeq can work on GenericArrayData") {
+    val arrayData = new GenericArrayData(stringArray.map(utf8(_)))
+    testArrayData(arrayData)
+  }
+
+  test("ArrayDataIndexedSeq can work on UnsafeArrayData") {
+    val unsafeArrayData = ExpressionEncoder[Array[String]].resolveAndBind().
+      toRow(stringArray).getArray(0)
+    assert(unsafeArrayData.isInstanceOf[UnsafeArrayData])
+    testArrayData(unsafeArrayData)
+  }
+}