Partitioner.java
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- /**
- * 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.hadoop.mapreduce;
- /**
- * Partitions the key space.
- *
- * <p><code>Partitioner</code> controls the partitioning of the keys of the
- * intermediate map-outputs. The key (or a subset of the key) is used to derive
- * the partition, typically by a hash function. The total number of partitions
- * is the same as the number of reduce tasks for the job. Hence this controls
- * which of the <code>m</code> reduce tasks the intermediate key (and hence the
- * record) is sent for reduction.</p>
- *
- * @see Reducer
- */
- public abstract class Partitioner<KEY, VALUE> {
-
- /**
- * Get the partition number for a given key (hence record) given the total
- * number of partitions i.e. number of reduce-tasks for the job.
- *
- * <p>Typically a hash function on a all or a subset of the key.</p>
- *
- * @param key the key to be partioned.
- * @param value the entry value.
- * @param numPartitions the total number of partitions.
- * @return the partition number for the <code>key</code>.
- */
- public abstract int getPartition(KEY key, VALUE value, int numPartitions);
-
- }