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diff --git a/core/src/main/scala/org/apache/spark/util/collection/PrimitiveVector.scala b/core/src/main/scala/org/apache/spark/util/collection/PrimitiveVector.scala
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+/*
+ * 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.util.collection
+
+import scala.reflect.ClassTag
+
+/**
+ * An append-only, non-threadsafe, array-backed vector that is optimized for primitive types.
+ */
+private[spark]
+class PrimitiveVector[@specialized(Long, Int, Double) V: ClassTag](initialSize: Int = 64) {
+  private var _numElements = 0
+  private var _array: Array[V] = _
+
+  // NB: This must be separate from the declaration, otherwise the specialized parent class
+  // will get its own array with the same initial size.
+  _array = new Array[V](initialSize)
+
+  def apply(index: Int): V = {
+    require(index < _numElements)
+    _array(index)
+  }
+
+  def +=(value: V) {
+    if (_numElements == _array.length) {
+      resize(_array.length * 2)
+    }
+    _array(_numElements) = value
+    _numElements += 1
+  }
+
+  def capacity: Int = _array.length
+
+  def length: Int = _numElements
+
+  def size: Int = _numElements
+
+  /** Gets the underlying array backing this vector. */
+  def array: Array[V] = _array
+
+  /** Trims this vector so that the capacity is equal to the size. */
+  def trim(): PrimitiveVector[V] = resize(size)
+
+  /** Resizes the array, dropping elements if the total length decreases. */
+  def resize(newLength: Int): PrimitiveVector[V] = {
+    val newArray = new Array[V](newLength)
+    _array.copyToArray(newArray)
+    _array = newArray
+    if (newLength < _numElements) {
+      _numElements = newLength
+    }
+    this
+  }
+}