eu.stratosphere.api.java.functions.ReduceFunction Maven / Gradle / Ivy
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*
* Copyright (C) 2010-2013 by the Stratosphere project (http://stratosphere.eu)
*
* Licensed 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
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package eu.stratosphere.api.java.functions;
import eu.stratosphere.api.common.functions.AbstractFunction;
import eu.stratosphere.api.common.functions.GenericReduce;
/**
* The abstract base class for Reduce functions. Reduce functions combine groups of elements to
* a single value, by taking always two elements and combining them into one. Reduce functions
* may be used on entire data sets, or on grouped data sets. In the latter case, each group is reduced
* individually.
*
* For a reduce functions that work on an entire group at the same time (such as the
* MapReduce/Hadoop-style reduce), see {@link GroupReduceFunction}, called via
* {@link eu.stratosphere.api.java.DataSet#reduceGroup(GroupReduceFunction)}. In the general case,
* ReduceFunctions are considered faster, because they allow the system to use hash-based
* execution strategies.
*
* The basic syntax for using a grouped ReduceFunction is as follows:
*
* DataSet input = ...;
*
* DataSet result = input.groupBy().reduce(new MyReduceFunction());
*
*
* Like all functions, the ReduceFunction needs to be serializable, as defined in {@link java.io.Serializable}.
*
* @param Type of the elements that this function processes.
*/
public abstract class ReduceFunction extends AbstractFunction implements GenericReduce {
private static final long serialVersionUID = 1L;
/**
* The core method of the ReduceFunction, combining two values into one value of the same type.
* The reduce function is consecutively applied to all values of a group until only a single value remains.
*
* @param value1 The first value to combine.
* @param value2 The second value to combine.
* @return The combined value of both input values.
*
* @throws Exception This method may throw exceptions. Throwing an exception will cause the operation
* to fail and may trigger recovery.
*/
public abstract T reduce(T value1, T value2) throws Exception;
}