/*
* 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.java.functions;
import org.apache.flink.annotation.Public;
import org.apache.flink.api.common.functions.Function;
import java.io.Serializable;
/**
* The {@link KeySelector} allows to use arbitrary objects for operations such as
* reduce, reduceGroup, join, coGoup, etc.
*
* The extractor takes an object and returns the key for that object.
*
* @param <IN> Type of objects to extract the key from.
* @param <KEY> Type of key.
*/
@Public
public interface KeySelector<IN, KEY> extends Function, Serializable {
/**
* User-defined function that extracts the key from an arbitrary object.
*
* For example for a class:
* <pre>
* public class Word {
* String word;
* int count;
* }
* </pre>
* The key extractor could return the word as
* a key to group all Word objects by the String they contain.
*
* The code would look like this
* <pre>
* public String getKey(Word w) {
* return w.word;
* }
* </pre>
*
* @param value The object to get the key from.
* @return The extracted key.
*
* @throws Exception Throwing an exception will cause the execution of the respective task to fail,
* and trigger recovery or cancellation of the program.
*/
KEY getKey(IN value) throws Exception;
}