Github user kl0u commented on a diff in the pull request:

    https://github.com/apache/flink/pull/5481#discussion_r171235356
  
    --- Diff: 
flink-streaming-java/src/main/java/org/apache/flink/streaming/api/functions/KeyedProcessFunction.java
 ---
    @@ -0,0 +1,130 @@
    +/*
    + * 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.flink.streaming.api.functions;
    +
    +import org.apache.flink.annotation.PublicEvolving;
    +import org.apache.flink.api.common.functions.AbstractRichFunction;
    +import org.apache.flink.streaming.api.TimeDomain;
    +import org.apache.flink.streaming.api.TimerService;
    +import org.apache.flink.util.Collector;
    +import org.apache.flink.util.OutputTag;
    +
    +/**
    + * A keyed function that processes elements of a stream.
    + *
    + * <p>For every element in the input stream {@link #processElement(Object, 
Context, Collector)}
    + * is invoked. This can produce zero or more elements as output. 
Implementations can also
    + * query the time and set timers through the provided {@link Context}. For 
firing timers
    + * {@link #onTimer(long, OnTimerContext, Collector)} will be invoked. This 
can again produce
    + * zero or more elements as output and register further timers.
    + *
    + * <p><b>NOTE:</b> Access to keyed state and timers (which are also scoped 
to a key) is only
    + * available if the {@code KeyedProcessFunction} is applied on a {@code 
KeyedStream}.
    + *
    + * <p><b>NOTE:</b> A {@code KeyedProcessFunction} is always a
    + * {@link org.apache.flink.api.common.functions.RichFunction}. Therefore, 
access to the
    + * {@link org.apache.flink.api.common.functions.RuntimeContext} is always 
available and setup and
    + * teardown methods can be implemented. See
    + * {@link 
org.apache.flink.api.common.functions.RichFunction#open(org.apache.flink.configuration.Configuration)}
    + * and {@link org.apache.flink.api.common.functions.RichFunction#close()}.
    + *
    + * @param <K> Type of the key.
    + * @param <I> Type of the input elements.
    + * @param <O> Type of the output elements.
    + */
    +@PublicEvolving
    +public abstract class KeyedProcessFunction<K, I, O> extends 
AbstractRichFunction {
    +
    +   private static final long serialVersionUID = 1L;
    +
    +   /**
    +    * Process one element from the input stream.
    +    *
    +    * <p>This function can output zero or more elements using the {@link 
Collector} parameter
    +    * and also update internal state or set timers using the {@link 
Context} parameter.
    +    *
    +    * @param value The input value.
    +    * @param ctx A {@link Context} that allows querying the timestamp of 
the element and getting
    +    *            a {@link TimerService} for registering timers and 
querying the time. The
    +    *            context is only valid during the invocation of this 
method, do not store it.
    +    * @param out The collector for returning result values.
    +    *
    +    * @throws Exception This method may throw exceptions. Throwing an 
exception will cause the operation
    +    *                   to fail and may trigger recovery.
    +    */
    +   public abstract void processElement(I value, Context ctx, Collector<O> 
out) throws Exception;
    +
    +   /**
    +    * Called when a timer set using {@link TimerService} fires.
    +    *
    +    * @param timestamp The timestamp of the firing timer.
    +    * @param ctx An {@link OnTimerContext} that allows querying the 
timestamp, the {@link TimeDomain}, and the key
    +    *            of the firing timer and getting a {@link TimerService} 
for registering timers and querying the time.
    +    *            The context is only valid during the invocation of this 
method, do not store it.
    +    * @param out The collector for returning result values.
    +    *
    +    * @throws Exception This method may throw exceptions. Throwing an 
exception will cause the operation
    +    *                   to fail and may trigger recovery.
    +    */
    +   public void onTimer(long timestamp, OnTimerContext<K> ctx, Collector<O> 
out) throws Exception {}
    +
    --- End diff --
    
    Remove the `<K>` (see comment below).


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