java代码（2）编程实现以下功能： 1）从键盘输入姓名、学号、成绩，并保存到文本文档中。重复进行，直到输入空字符串为止。 2）从文件中读取各学生的成绩，并计算所有学生成绩的平均值、最大值和最小值。

以下是实现该功能的 Java 代码：

import java.io.*;
import java.util.ArrayList;

public class StudentGrade {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        ArrayList<Student> students = new ArrayList<>();

        // 从键盘输入学生信息并保存到文件中
        while (true) {
            System.out.print("请输入学生姓名：");
            String name = reader.readLine();
            if (name.equals("")) break;

            System.out.print("请输入学生学号：");
            String id = reader.readLine();

            System.out.print("请输入学生成绩：");
            double grade = Double.parseDouble(reader.readLine());

            Student student = new Student(name, id, grade);
            students.add(student);

            FileWriter writer = new FileWriter("students.txt", true);
            writer.write(student.toString() + "\n");
            writer.close();
        }

        // 从文件中读取学生信息并计算平均值、最大值和最小值
        FileReader fileReader = new FileReader("students.txt");
        BufferedReader bufferedReader = new BufferedReader(fileReader);

        double sum = 0;
        double max = Double.MIN_VALUE;
        double min = Double.MAX_VALUE;
        int count = 0;

        String line;
        while ((line = bufferedReader.readLine()) != null) {
            String[] parts = line.split(",");
            String name = parts[0];
            String id = parts[1];
            double grade = Double.parseDouble(parts[2]);

            Student student = new Student(name, id, grade);
            students.add(student);

            sum += grade;
            max = Math.max(max, grade);
            min = Math.min(min, grade);
            count++;
        }

        double average = sum / count;
        System.out.println("学生成绩平均值：" + average);
        System.out.println("学生成绩最大值：" + max);
        System.out.println("学生成绩最小值：" + min);

        bufferedReader.close();
        fileReader.close();
    }
}

class Student {
    private String name;
    private String id;
    private double grade;

    public Student(String name, String id, double grade) {
        this.name = name;
        this.id = id;
        this.grade = grade;
    }

    public String getName() {
        return name;
    }

    public String getId() {
        return id;
    }

    public double getGrade() {
        return grade;
    }

    @Override
    public String toString() {
        return name + "," + id + "," + grade;
    }
}

该代码使用了 `BufferedReader` 和 `FileReader` 读取和写入文件，将输入的学生信息保存到 `ArrayList` 中，并在每次输入后将学生信息写入到文件中。然后从文件中读取学生信息，计算平均值、最大值和最小值，并输出结果。