用Hadoop分析学生成绩的代码

以下是一个简单的用Hadoop分析学生成绩的代码示例，假设输入文件格式为每行包含一个学生的姓名和分数，以逗号分隔。

Mapper类：

import java.io.IOException;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;

public class ScoreMapper extends Mapper<LongWritable, Text, Text, LongWritable> {
  private Text name = new Text();
  private LongWritable score = new LongWritable();

  public void map(LongWritable key, Text value, Context context)
      throws IOException, InterruptedException {
    String[] parts = value.toString().split(",");
    name.set(parts[0]);
    score.set(Long.parseLong(parts[1]));
    context.write(name, score);
  }
}

Reducer类：

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

public class ScoreReducer extends Reducer<Text, LongWritable, Text, LongWritable> {
  private LongWritable result = new LongWritable();

  public void reduce(Text key, Iterable<LongWritable> values, Context context)
      throws IOException, InterruptedException {
    long sum = 0;
    int count = 0;
    for (LongWritable val : values) {
      sum += val.get();
      count++;
    }
    long avg = sum / count;
    result.set(avg);
    context.write(key, result);
  }
}

Driver类：

import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.LongWritable;
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;

public class ScoreAnalyzer {
  public static void main(String[] args) throws Exception {
    if (args.length != 2) {
      System.err.println("Usage: ScoreAnalyzer <input path> <output path>");
      System.exit(-1);
    }
    Job job = new Job();
    job.setJarByClass(ScoreAnalyzer.class);
    job.setJobName("Score Analyzer");

    FileInputFormat.addInputPath(job, new Path(args[0]));
    FileOutputFormat.setOutputPath(job, new Path(args[1]));

    job.setMapperClass(ScoreMapper.class);
    job.setReducerClass(ScoreReducer.class);

    job.setOutputKeyClass(Text.class);
    job.setOutputValueClass(LongWritable.class);

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

在以上示例中，Mapper类将输入文件中的每行转换为一个键值对，其中键为学生姓名，值为分数。Reducer类将相同姓名的所有分数求平均值后输出。Driver类设置了输入输出路径、Mapper和Reducer类以及输出键值对类型等相关参数，并运行MapReduce作业。