測試資料：

file1.txt

102
10
39
109
200
11
2
90
28

file2.txt

5
2
30
838
10005

mapper程式碼：

import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;

import java.io.IOException;

public class MaxMinMapper extends Mapper<LongWritable, Text, Text, IntWritable> {
    private static final Text KEY = new Text("key");
    @Override
    protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
        context.write(KEY, new IntWritable(Integer.valueOf(value.toString().trim())));
    }
}
 

reducer程式碼：

import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;
import java.io.IOException;

public class MaxMinReducer extends Reducer<Text, IntWritable, Text, IntWritable> {
    @Override
    protected void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
        long min = Long.MAX_VALUE;
        long max = Long.MIN_VALUE;

        for(IntWritable val : values) {
            if (min > val.get()) {
                min = val.get();
            }
            if (max < val.get()) {
                max = val.get();
            }
        }

        context.write(new Text("MIN:"), new IntWritable(Integer.valueOf("" + min)));
        context.write(new Text("MAX:"), new IntWritable(Integer.valueOf("" + max)));
    }
}
 

main程式碼：

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

import java.io.IOException;

/**
 * 求最大最小值。資料量不多的情況下，直接把輸入資料做為一個相同key和list<value>形式
 */
public class JobMain {
    public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
        if(args.length != 2) {
            System.err.println("Usage: MinMaxTemperature<input path> <output path>");
            System.exit(-1);
        }

        Configuration conf = new Configuration();
        Job job = Job.getInstance(conf, "max_min_job");
        job.setJarByClass(Job.class);

        job.setMapperClass(MaxMinMapper.class);
        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(IntWritable.class);

        job.setReducerClass(MaxMinReducer.class);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(IntWritable.class);

        FileInputFormat.addInputPath(job, new Path(args[0]));

        Path outDirPath = new Path(args[1]);
        FileSystem fs = FileSystem.get(conf);
        if(fs.exists(outDirPath)) {
            fs.delete(outDirPath, true);
        }
        FileOutputFormat.setOutputPath(job, outDirPath);

        System.exit(job.waitForCompletion(true) ? 0 : 1);
    }
}
 

結果：

MIN:    2
MAX:    10005