Github user StefanRRichter commented on a diff in the pull request:
https://github.com/apache/flink/pull/6228#discussion_r199812541
--- Diff:
flink-runtime/src/main/java/org/apache/flink/runtime/state/KeyExtractorFunction.java
---
@@ -0,0 +1,36 @@
+/*
+ * 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.runtime.state;
+
+import javax.annotation.Nonnull;
+
+/**
+ * Function to extract a key from a given object.
+ *
+ * @param <T> type of the element from which we extract the key.
+ */
+@FunctionalInterface
+public interface KeyExtractorFunction<T> {
--- End diff --
I find it useful when concepts have names attached to it and some form of
typing otherwise, you end up with a lot of `Function<>` objects and have to
think twice about their concrete use-case.
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