/** * Copyright 2005 The Apache Software Foundation * * 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 specific language governing permissions and * limitations under the License. */package org.apache.hadoop.mapred;import java.io.IOException;import java.util.Iterator;import org.apache.hadoop.io.Closeable;import org.apache.hadoop.io.Writable;import org.apache.hadoop.io.WritableComparable;/** Reduces a set of intermediate values which share a key to a smaller set of * values.  Input values are the grouped output of a {@link Mapper}. */public interface Reducer extends JobConfigurable, Closeable {  /** Combines values for a given key.  Output values must be of the same type   * as input values.  Input keys must not be altered.  Typically all values   * are combined into zero or one value.  Output pairs are collected with   * calls to {@link OutputCollector#collect(WritableComparable,Writable)}.   *   * @param key the key   * @param values the values to combine   * @param output to collect combined values   */  void reduce(WritableComparable key, Iterator values,              OutputCollector output, Reporter reporter)    throws IOException;}