给出该题的Java代码：学生成绩存储在文件中，每行一个学生成绩，形如： 20151201 85 84 91.5 20151202 70 85 65 … 请编程读入学生成绩，并计算各门课程的均值和方差，写入文件的最后一行，如： 均值和方差 71/10 81.2/11.3 76/15.3

以下是该题的Java代码：

import java.io.*;

public class StudentGrades {
    public static void main(String[] args) {
        try {
            // 读取文件
            BufferedReader br = new BufferedReader(new FileReader("grades.txt"));

            // 初始化变量
            int count = 0;
            double sum1 = 0, sum2 = 0, sum3 = 0;

            // 读取每一行并计算各门课程的总成绩
            String line;
            while ((line = br.readLine()) != null) {
                String[] grades = line.split(" ");
                sum1 += Double.parseDouble(grades[1]);
                sum2 += Double.parseDouble(grades[2]);
                sum3 += Double.parseDouble(grades[3]);
                count++;
            }

            // 计算均值和方差
            double avg1 = sum1 / count;
            double avg2 = sum2 / count;
            double avg3 = sum3 / count;
            double var1 = 0, var2 = 0, var3 = 0;
            br = new BufferedReader(new FileReader("grades.txt"));
            while ((line = br.readLine()) != null) {
                String[] grades = line.split(" ");
                var1 += Math.pow((Double.parseDouble(grades[1]) - avg1), 2);
                var2 += Math.pow((Double.parseDouble(grades[2]) - avg2), 2);
                var3 += Math.pow((Double.parseDouble(grades[3]) - avg3), 2);
            }
            var1 /= count;
            var2 /= count;
            var3 /= count;

            // 写入文件
            BufferedWriter bw = new BufferedWriter(new FileWriter("grades.txt", true));
            bw.write(String.format("\n均值和方差 %.1f/%.1f %.1f/%.1f %.1f/%.1f", avg1, var1, avg2, var2, avg3, var3));
            bw.close();
            br.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

假设成绩文件名为`grades.txt`，每行格式为`学号 课程1成绩 课程2成绩 课程3成绩`，如下所示：

20151201 85 84 91.5
20151202 70 85 65
20151203 92 88 95
...

程序会读取该文件，并计算每门课程的均值和方差，然后将结果写入文件的最后一行。结果格式为`均值和方差 课程1均值/课程1方差 课程2均值/课程2方差 课程3均值/课程3方差`，如下所示：

20151201 85 84 91.5
20151202 70 85 65
20151203 92 88 95
...
均值和方差 82.3/67.6 85.7/21.4 83.5/48.2