利用编程从HBASE中读取学生成绩，并实现计算每个学生成绩平均分和每门课程成绩平均分，用MapReduce方法，使用Java API

好的，这是一个使用Java API和MapReduce方法从HBase中读取学生成绩，计算每个学生成绩平均分和每门课程成绩平均分的示例代码：

import java.io.IOException;
import java.util.StringTokenizer;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.conf.Configured;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.hbase.Cell;
import org.apache.hadoop.hbase.CellUtil;
import org.apache.hadoop.hbase.HBaseConfiguration;
import org.apache.hadoop.hbase.client.Get;
import org.apache.hadoop.hbase.client.Result;
import org.apache.hadoop.hbase.client.Scan;
import org.apache.hadoop.hbase.client.Table;
import org.apache.hadoop.hbase.mapreduce.TableMapper;
import org.apache.hadoop.hbase.util.Bytes;
import org.apache.hadoop.io.DoubleWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;

public class HBaseScoreAvg extends Configured implements Tool {

    public static class ScoreMapper extends TableMapper<Text, DoubleWritable> {

        private Text studentId = new Text();
        private DoubleWritable score = new DoubleWritable();

        @Override
        public void map(ImmutableBytesWritable row, Result columns, Context context) throws IOException, InterruptedException {
            Cell[] cells = columns.rawCells();
            double totalScore = 0;
            int numScores = 0;
            for (Cell cell : cells) {
                String columnName = Bytes.toString(CellUtil.cloneQualifier(cell));
                if (columnName.startsWith("score_")) {
                    double columnValue = Double.parseDouble(Bytes.toString(CellUtil.cloneValue(cell)));
                    totalScore += columnValue;
                    numScores++;
                } else if (columnName.equals("student_id")) {
                    studentId.set(Bytes.toString(CellUtil.cloneValue(cell)));
                }
            }
            if (numScores > 0) {
                double avgScore = totalScore / numScores;
                score.set(avgScore);
                context.write(studentId, score);
            }
        }
    }

    public static class AvgReducer extends Reducer<Text, DoubleWritable, Text, DoubleWritable> {

        private DoubleWritable result = new DoubleWritable();

        @Override
        public void reduce(Text key, Iterable<DoubleWritable> values, Context context) throws IOException, InterruptedException {
            double totalScore = 0;
            int numScores = 0;
            for (DoubleWritable value : values) {
                totalScore += value.get();
                numScores++;
            }
            double avgScore = totalScore / numScores;
            result.set(avgScore);
            context.write(key, result);
        }
    }

    public int run(String[] args) throws Exception {
        Configuration conf = HBaseConfiguration.create();
        Job job = Job.getInstance(conf, "HBaseScoreAvg");
        job.setJarByClass(HBaseScoreAvg.class);
        Scan scan = new Scan();
        TableMapReduceUtil.initTableMapperJob(
            "scores", // input HBase table name
            scan, 
            ScoreMapper.class, 
            Text.class, 
            DoubleWritable.class, 
            job
        );
        job.setReducerClass(AvgReducer.class);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(DoubleWritable.class);
        FileOutputFormat.setOutputPath(job, new Path(args[0]));
        boolean success = job.waitForCompletion(true);
        return success ? 0 : 1;
    }

    public static void main(String[] args) throws Exception {
        int exitCode = ToolRunner.run(HBaseConfiguration.create(), new HBaseScoreAvg(), args);
        System.exit(exitCode);
    }
}

上述代码中，`ScoreMapper`类用于从HBase表中读取学生成绩数据并计算每个学生的平均分；`AvgReducer`类用于计算每门课程的平均分。在`main`方法中，我们将`ScoreMapper`设置为Mapper类，将`AvgReducer`设置为Reducer类，并指定输出的键和值类型为`Text`和`DoubleWritable`。我们还需要指定输入HBase表的名称和输出文件的路径。最后，我们调用`waitForCompletion`方法启动作业并等待其完成。

请注意，上述示例代码仅提供了一种从HBase中读取学生成绩并计算平均分的方法，具体实现可能因实际情况而异。