Github user rxin commented on a diff in the pull request:
https://github.com/apache/spark/pull/9256#discussion_r44205466
--- Diff:
streaming/src/main/scala/org/apache/spark/streaming/StateSpec.scala ---
@@ -0,0 +1,181 @@
+/*
+ * 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.streaming
+
+import scala.reflect.ClassTag
+
+import org.apache.spark.annotation.Experimental
+import org.apache.spark.api.java.JavaPairRDD
+import org.apache.spark.rdd.RDD
+import org.apache.spark.{HashPartitioner, Partitioner}
+
+
+/**
+ * :: Experimental ::
+ * Abstract class representing all the specifications of the DStream
transformation
+ * `trackStateByKey` operation of a
+ * [[org.apache.spark.streaming.dstream.PairDStreamFunctions pair
DStream]] (Scala) or a
+ * [[org.apache.spark.streaming.api.java.JavaPairDStream JavaPairDStream]]
(Java).
+ * Use the [[org.apache.spark.streaming.StateSpec StateSpec.apply()]] or
+ * [[org.apache.spark.streaming.StateSpec StateSpec.create()]] to create
instances of
+ * this class.
+ *
+ * Example in Scala:
+ * {{{
+ * val spec = StateSpec(trackingFunction).numPartitions(10)
+ *
+ * val emittedRecordDStream =
keyValueDStream.trackStateByKey[StateType, EmittedDataType](spec)
+ * }}}
+ *
+ * Example in Java:
+ * {{{
+ * StateStateSpec[StateType, EmittedDataType] spec =
+ * StateStateSpec.create[StateType,
EmittedDataType](trackingFunction).numPartition(10);
+ *
+ * JavaDStream[EmittedDataType] emittedRecordDStream =
+ * javaPairDStream.trackStateByKey[StateType, EmittedDataType](spec);
+ * }}}
+ */
+@Experimental
+sealed abstract class StateSpec[K, V, S, T] extends Serializable {
+
+ /** Set the RDD containing the initial states that will be used by
`trackStateByKey`*/
--- End diff --
I really don't like apply on an object to create everything. I like
```
StateSpec.function(...)
```
as the only constructor, which also makes it obvious it is setting a
function.
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