用java编写一个学生成绩管理系统 ，主要功能： 1、输入一个班级的学生的基本信息（包括学号，姓名，性别，5门课程成绩）。 2、按姓名或者学号查找、修改、删除和保存各个学生的信息。 3、计算每个学生各门功课总分和平均分，按学号或总分排序输出每个学生的基本信息及总分、平均分和名次。 4、计算全班各门功课的平均分，显示每门课程中低于平均分的每一个学生的学号，姓名，性别，科目，成绩。 5、显示每门科目中，成绩在90分以上的学生信息，以及每门科目中不及格的学生信息。 6、用菜单进行管理 7、只有正确输入用户名密码才能使用此系统。 8、所有内容能够保存记事本文件中。下次进入系统是从记事本文件中读取原有信息。

以下是一个简单的学生成绩管理系统的Java代码示例：

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

public class StudentManagementSystem {
    private static final String USERNAME = "admin";
    private static final String PASSWORD = "123456";
    private static final String FILE_NAME = "students.txt";

    private static List<Student> students = new ArrayList<>();
    private static Scanner scanner = new Scanner(System.in);

    public static void main(String[] args) {
        login();
        loadStudentsFromFile();
        boolean running = true;
        while (running) {
            printMenu();
            int choice = scanner.nextInt();
            switch (choice) {
                case 1:
                    addStudent();
                    break;
                case 2:
                    findStudent();
                    break;
                case 3:
                    modifyStudent();
                    break;
                case 4:
                    deleteStudent();
                    break;
                case 5:
                    sortStudents();
                    break;
                case 6:
                    showLowScores();
                    break;
                case 7:
                    showHighScores();
                    break;
                case 8:
                    saveStudentsToFile();
                    running = false;
                    break;
                default:
                    System.out.println("Invalid choice!");
            }
        }
        System.out.println("Goodbye!");
    }

    private static void login() {
        System.out.println("Please enter username:");
        String username = scanner.next();
        System.out.println("Please enter password:");
        String password = scanner.next();
        if (!username.equals(USERNAME) || !password.equals(PASSWORD)) {
            System.out.println("Incorrect username or password!");
            System.exit(0);
        }
    }

    private static void loadStudentsFromFile() {
        try {
            FileReader fileReader = new FileReader(FILE_NAME);
            BufferedReader bufferedReader = new BufferedReader(fileReader);
            String line;
            while ((line = bufferedReader.readLine()) != null) {
                String[] parts = line.split(",");
                String id = parts[0];
                String name = parts[1];
                String gender = parts[2];
                int[] scores = new int[5];
                for (int i = 3; i < 8; i++) {
                    scores[i - 3] = Integer.parseInt(parts[i]);
                }
                Student student = new Student(id, name, gender, scores);
                students.add(student);
            }
            bufferedReader.close();
            fileReader.close();
        } catch (IOException e) {
            System.out.println("Error reading file!");
        }
    }

    private static void saveStudentsToFile() {
        try {
            FileWriter fileWriter = new FileWriter(FILE_NAME);
            BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);
            for (Student student : students) {
                bufferedWriter.write(student.toString());
                bufferedWriter.newLine();
            }
            bufferedWriter.close();
            fileWriter.close();
        } catch (IOException e) {
            System.out.println("Error writing file!");
        }
    }

    private static void addStudent() {
        System.out.println("Please enter student id:");
        String id = scanner.next();
        System.out.println("Please enter student name:");
        String name = scanner.next();
        System.out.println("Please enter student gender:");
        String gender = scanner.next();
        int[] scores = new int[5];
        System.out.println("Please enter scores for 5 courses:");
        for (int i = 0; i < 5; i++) {
            scores[i] = scanner.nextInt();
        }
        Student student = new Student(id, name, gender, scores);
        students.add(student);
        System.out.println("Student added successfully!");
    }

    private static void findStudent() {
        System.out.println("Please enter student id or name:");
        String keyword = scanner.next();
        boolean found = false;
        for (Student student : students) {
            if (student.getId().equals(keyword) || student.getName().equals(keyword)) {
                System.out.println(student);
                found = true;
            }
        }
        if (!found) {
            System.out.println("Student not found!");
        }
    }

    private static void modifyStudent() {
        System.out.println("Please enter student id:");
        String id = scanner.next();
        boolean found = false;
        for (Student student : students) {
            if (student.getId().equals(id)) {
                System.out.println("Please enter new name (press enter to skip):");
                String name = scanner.nextLine();
                if (!name.isEmpty()) {
                    student.setName(name);
                }
                System.out.println("Please enter new gender (press enter to skip):");
                String gender = scanner.nextLine();
                if (!gender.isEmpty()) {
                    student.setGender(gender);
                }
                System.out.println("Please enter new scores for 5 courses (press enter to skip):");
                String scoresStr = scanner.nextLine();
                if (!scoresStr.isEmpty()) {
                    String[] scoresParts = scoresStr.split(",");
                    int[] scores = new int[5];
                    for (int i = 0; i < 5; i++) {
                        scores[i] = Integer.parseInt(scoresParts[i]);
                    }
                    student.setScores(scores);
                }
                System.out.println("Student modified successfully!");
                found = true;
                break;
            }
        }
        if (!found) {
            System.out.println("Student not found!");
        }
    }

    private static void deleteStudent() {
        System.out.println("Please enter student id:");
        String id = scanner.next();
        boolean found = false;
        for (int i = 0; i < students.size(); i++) {
            Student student = students.get(i);
            if (student.getId().equals(id)) {
                students.remove(i);
                System.out.println("Student deleted successfully!");
                found = true;
                break;
            }
        }
        if (!found) {
            System.out.println("Student not found!");
        }
    }

    private static void sortStudents() {
        System.out.println("Sort by (1. id, 2. total score):");
        int choice = scanner.nextInt();
        if (choice == 1) {
            Collections.sort(students, Comparator.comparing(Student::getId));
        } else if (choice == 2) {
            Collections.sort(students, Comparator.comparing(Student::getTotalScore).reversed());
        } else {
            System.out.println("Invalid choice!");
            return;
        }
        int position = 1;
        for (Student student : students) {
            System.out.println(position + ". " + student);
            position++;
        }
    }

    private static void showLowScores() {
        for (int i = 0; i < 5; i++) {
            int sum = 0;
            int count = 0;
            for (Student student : students) {
                int score = student.getScores()[i];
                sum += score;
                count++;
            }
            double average = (double) sum / count;
            System.out.println("Course " + (i + 1) + " average score: " + average);
            System.out.println("Students with score below average:");
            for (Student student : students) {
                int score = student.getScores()[i];
                if (score < average) {
                    System.out.println(student);
                }
            }
        }
    }

    private static void showHighScores() {
        for (int i = 0; i < 5; i++) {
            System.out.println("Students with score above 90 in course " + (i + 1) + ":");
            for (Student student : students) {
                int score = student.getScores()[i];
                if (score > 90) {
                    System.out.println(student);
                }
            }
            System.out.println("Students with score below 60 in course " + (i + 1) + ":");
            for (Student student : students) {
                int score = student.getScores()[i];
                if (score < 60) {
                    System.out.println(student);
                }
            }
        }
    }

    private static void printMenu() {
        System.out.println("1. Add student");
        System.out.println("2. Find student");
        System.out.println("3. Modify student");
        System.out.println("4. Delete student");
        System.out.println("5. Sort students");
        System.out.println("6. Show low scores");
        System.out.println("7. Show high scores");
        System.out.println("8. Save and exit");
        System.out.println("Please enter your choice:");
    }
}

class Student {
    private String id;
    private String name;
    private String gender;
    private int[] scores;

    public Student(String id, String name, String gender, int[] scores) {
        this.id = id;
        this.name = name;
        this.gender = gender;
        this.scores = scores;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }

    public int[] getScores() {
        return scores;
    }

    public void setScores(int[] scores) {
        this.scores = scores;
    }

    public int getTotalScore() {
        int sum = 0;
        for (int score : scores) {
            sum += score;
        }
        return sum;
    }

    @Override
    public String toString() {
        return id + "," + name + "," + gender + "," + scores[0] + "," + scores[1] + "," + scores[2] + "," + scores[3] + "," + scores[4];
    }
}

注意，这只是一个简单的示例，实际应用中可能需要更多的功能和更严格的输入验证。