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编程

好的，这是一个比较复杂的问题，需要一步步来实现。首先，我们需要安装和配置Hadoop和HBase集群，并将学生成绩数据导入到HBase中。这里假设这些步骤已经完成。

接下来，我们需要编写Java程序来读取HBase中的数据，并进行平均分计算。以下是实现的步骤：

1. 定义HBase的配置信息和表名

Configuration conf = HBaseConfiguration.create();
TableName tableName = TableName.valueOf("student_scores");

2. 创建HBase连接和表对象

Connection connection = ConnectionFactory.createConnection(conf);
Table table = connection.getTable(tableName);

3. 定义MapReduce作业

Job job = Job.getInstance(conf, "CalculateScores");
job.setJarByClass(CalculateScores.class);
job.setMapperClass(ScoreMapper.class);
job.setReducerClass(ScoreReducer.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(FloatWritable.class);

4. 实现Mapper类，读取HBase中的数据，并将每个学生的成绩和课程作为键值对传递给Reducer类

public static class ScoreMapper extends TableMapper<Text, FloatWritable> {
  private Text outKey = new Text();
  private FloatWritable outValue = new FloatWritable();

  public void map(ImmutableBytesWritable row, Result value, Context context) throws IOException, InterruptedException {
    String studentId = new String(value.getValue(Bytes.toBytes("info"), Bytes.toBytes("student_id")));
    String course = new String(value.getValue(Bytes.toBytes("info"), Bytes.toBytes("course")));
    float score = Float.parseFloat(new String(value.getValue(Bytes.toBytes("info"), Bytes.toBytes("score"))));
    outKey.set(studentId + "," + course);
    outValue.set(score);
    context.write(outKey, outValue);
  }
}

5. 实现Reducer类，计算每个学生的平均分和每门课程的平均分

public static class ScoreReducer extends Reducer<Text, FloatWritable, Text, FloatWritable> {
  private FloatWritable outValue = new FloatWritable();

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

6. 运行MapReduce作业并输出结果

if (job.waitForCompletion(true)) {
  Scan scan = new Scan();
  ResultScanner scanner = table.getScanner(scan);
  float sum = 0;
  int count = 0;
  float mathSum = 0;
  int mathCount = 0;
  float englishSum = 0;
  int englishCount = 0;
  for (Result result : scanner) {
    String studentId = new String(result.getValue(Bytes.toBytes("info"), Bytes.toBytes("student_id")));
    String course = new String(result.getValue(Bytes.toBytes("info"), Bytes.toBytes("course")));
    float score = Float.parseFloat(new String(result.getValue(Bytes.toBytes("info"), Bytes.toBytes("score"))));
    sum += score;
    count++;
    if (course.equals("math")) {
      mathSum += score;
      mathCount++;
    } else if (course.equals("english")) {
      englishSum += score;
      englishCount++;
    }
  }
  System.out.println("Average score: " + sum / count);
  System.out.println("Math average score: " + mathSum / mathCount);
  System.out.println("English average score: " + englishSum / englishCount);
}

以上就是利用Java API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的步骤。