Java tutorial
/* * 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.mahout.cf.taste.hadoop.similarity.item; import com.google.common.primitives.Longs; import org.apache.hadoop.io.WritableComparable; import org.apache.hadoop.io.WritableComparator; import org.apache.hadoop.mapreduce.Partitioner; import org.apache.mahout.math.VarLongWritable; import org.apache.mahout.math.Varint; import java.io.DataInput; import java.io.DataOutput; import java.io.IOException; import java.io.Serializable; /** * a writable key that is used by {@link CountUsersMapper} and {@link CountUsersReducer} to * count unique users by sending all userIDs to the same reducer and have them sorted in * ascending order so that there's no buffering necessary when counting them */ public class CountUsersKeyWritable implements WritableComparable<CountUsersKeyWritable> { private long userID; public CountUsersKeyWritable() { } public CountUsersKeyWritable(long userID) { this.userID = userID; } public long getUserID() { return userID; } @Override public void readFields(DataInput in) throws IOException { userID = Varint.readSignedVarLong(in); } @Override public void write(DataOutput out) throws IOException { Varint.writeSignedVarLong(userID, out); } @Override public int compareTo(CountUsersKeyWritable other) { return userID == other.userID ? 0 : userID < other.userID ? -1 : 1; } @Override public boolean equals(Object other) { return other instanceof CountUsersKeyWritable && userID == ((CountUsersKeyWritable) other).userID; } @Override public int hashCode() { return Longs.hashCode(userID); } /** * all userIDs go to the same partition */ public static class CountUsersPartitioner extends Partitioner<CountUsersKeyWritable, VarLongWritable> { @Override public int getPartition(CountUsersKeyWritable key, VarLongWritable value, int numPartitions) { return 0; } } /** * all userIDs go to the same reducer */ public static class CountUsersGroupComparator extends WritableComparator implements Serializable { public CountUsersGroupComparator() { super(CountUsersKeyWritable.class, true); } @Override public int compare(WritableComparable a, WritableComparable b) { return 0; } } }