Github user zsxwing commented on a diff in the pull request:
https://github.com/apache/spark/pull/9256#discussion_r44091460
--- Diff: streaming/src/main/scala/org/apache/spark/streaming/State.scala
---
@@ -0,0 +1,141 @@
+/*
+ * 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
+
+/**
+ * Abstract class for getting and updating the tracked state in the
`trackStateByKey` operation of
+ * [[org.apache.spark.streaming.dstream.PairDStreamFunctions pair
DStream]] and
+ * [[org.apache.spark.streaming.api.java.JavaPairDStream]].
+ * {{{
+ *
+ * }}}
+ */
+sealed abstract class State[S] {
+
+ /** Whether the state already exists */
+ def exists(): Boolean
+
+ /**
+ * Get the state if it exists, otherwise wise it will throw an exception.
+ * Check with `exists()` whether the state exists or not before calling
`get()`.
+ */
+ def get(): S
+
+ /**
+ * Update the state with a new value. Note that you cannot update the
state if the state is
+ * timing out (that is, `isTimingOut() return true`, or if the state has
already been removed by
+ * `remove()`.
+ */
+ def update(newState: S): Unit
+
+ /** Remove the state if it exists. */
+ def remove(): Unit
+
+ /** Is the state going to be timed out by the system after this batch
interval */
+ def isTimingOut(): Boolean
+
+ @inline final def getOption(): Option[S] = Option(get())
+
+ /** Get the state if it exists, otherwise return the default value */
+ @inline final def getOrElse[S1 >: S](default: => S1): S1 = {
+ if (exists) this.get else default
+ }
+
+ @inline final override def toString(): String = {
+ getOption.map { _.toString }.getOrElse("<state not set>")
+ }
+}
+
+/** Internal implementation of the [[State]] interface */
+private[streaming] class StateImpl[S] extends State[S] {
+
+ private var state: S = null.asInstanceOf[S]
+ private var defined: Boolean = true
+ private var timingOut: Boolean = false
+ private var updated: Boolean = false
+ private var removed: Boolean = false
+
+ // ========= Public API =========
+ def exists(): Boolean = {
+ defined
+ }
+
+ def get(): S = {
+ state
--- End diff --
`State.get` says `Get the state if it exists, otherwise wise it will throw
an exception.` but here it won't throw an exception.
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