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https://issues.apache.org/jira/browse/FLINK-3254?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15126319#comment-15126319
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ASF GitHub Bot commented on FLINK-3254:
---------------------------------------
Github user fhueske commented on a diff in the pull request:
https://github.com/apache/flink/pull/1568#discussion_r51425852
--- Diff:
flink-core/src/main/java/org/apache/flink/api/common/functions/util/CombineToGroupCombineWrapper.java
---
@@ -0,0 +1,49 @@
+/*
+ * 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.api.common.functions.util;
+
+import com.google.common.base.Preconditions;
+import org.apache.flink.api.common.functions.CombineFunction;
+import org.apache.flink.api.common.functions.GroupCombineFunction;
+import org.apache.flink.api.common.functions.GroupReduceFunction;
+import org.apache.flink.util.Collector;
+
+/**
+ * A wrapper the wraps a function that implements both {@link
CombineFunction} and {@link GroupReduceFunction} interfaces
+ * and makes it look like a function that implements {@link
GroupCombineFunction} and {@link GroupReduceFunction} to the runtime.
+ */
+public class CombineToGroupCombineWrapper<IN, OUT, F extends
CombineFunction<IN, OUT> & GroupReduceFunction<IN, OUT>>
+ implements GroupCombineFunction<IN, OUT>, GroupReduceFunction<IN, OUT> {
+
+ private final F wrappedFunction;
+
+ public CombineToGroupCombineWrapper(F wrappedFunction) {
+ this.wrappedFunction =
Preconditions.checkNotNull(wrappedFunction);
+ }
+
+ @Override
+ public void combine(Iterable<IN> values, Collector<OUT> out) throws
Exception {
--- End diff --
Should be `Collector<IN>`. A combine function (if applied in the context of
a reduce function) must preserve the input type.
> CombineFunction interface not respected
> ---------------------------------------
>
> Key: FLINK-3254
> URL: https://issues.apache.org/jira/browse/FLINK-3254
> Project: Flink
> Issue Type: Bug
> Components: DataSet API
> Affects Versions: 1.0.0, 0.10.1
> Reporter: Fabian Hueske
> Assignee: Kostas
> Priority: Blocker
> Fix For: 1.0.0
>
>
> The DataSet API offers a {{CombineFunction}} interface, which differs from
> the {{GroupCombineFunction}} interface by being restricted to return a single
> value instead of returning arbitrary many values through a {{Collector}}.
> The JavaDocs of the {{GroupCombineFunction}} point to the {{CombineFunction}}
> interface, advertising it as more efficient.
> However, the {{CombineFunction}} interface is nor respected by Flink, i.e., a
> {{GroupReduceFunction}} that implements this interface is executed without
> leveraging the combine method.
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