takeoff student manager

import java.io.File;
import java.io.FileNotFoundException;
import java.math.RoundingMode;
import java.text.DecimalFormat;
import java.util.*;

class UtilMath {
    static public double roundDown(double nr, int prec) {
        final String format = "." + "#".repeat(prec);
        var decimalFormat = new DecimalFormat(format);
        decimalFormat.setRoundingMode(RoundingMode.DOWN);
        return Double.parseDouble(decimalFormat.format(nr));
    }
}

class Student implements Comparable<Student> {
    private int id;
    private String name;
    private List<Double> grades;

    public ArrayList<Double> getGrades() {
        return (ArrayList<Double>) grades;
    }

    public Student(int id, String name) {
        this.id = id;
        this.name = name;
        grades = new ArrayList<>();
    }

    public void addGrade(double grade) {
        grades.add(grade);
    }
    public double getGPA() {
        double[] primDoubles = grades.stream().mapToDouble(d -> d).toArray();
        double mean = Arrays.stream(primDoubles).average().orElse(Double.NaN);
        return UtilMath.roundDown(mean, 1);
    }

    public String getName() {
        return name;
    }

    public int getId() {
        return id;
    }



    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", grades=" + grades +
                '}';
    }

    @Override
    public int compareTo(Student other) {
        return name.compareTo(other.name);
    }
    static class GpaComp implements Comparator<Student> {
        @Override
        public int compare(Student student1, Student student2) {
            if (student1.getGPA() > student2.getGPA()) {
                return -1;
            }
            return 1;
        }
    }
}


class University implements Comparable<University> {
    final static int SCHOOLARSHIP_NR = 2;

    private String name;
    private List<Student> students;

    public String getName() {
        return name;
    }

    public University(String name) {
        this.name = name;
        students = new ArrayList<>();
    }
    void addStud(final Student student) {
        students.add(student);
    }
    public double getGPA() {
        double mean = 0;
        int nr = 0;
        for (var student : students) {
            for (double grade : student.getGrades()) {
                mean += grade;
            }
            nr += student.getGrades().size();
        }
        mean /= 1.0 * nr;
        return UtilMath.roundDown(mean, 1);
    }

    public List<Student> getStudents() {
        return students;
    }

    public Student findStudById(int id) {
        for (var student : students) {
            if (student.getId() == id) {
                return student;
            }
        }
        return null;
    }

    @Override
    public String toString() {
        return "University{" +
                "name='" + name + '\'' +
                ", students=" + students +
                '}';
    }

    public void sortStudents() {
        Collections.sort(students);
    }

    @Override
    public int compareTo(University other) {
        if (getGPA() > other.getGPA()) {
            return -1;
        }
        return 1;
    }

    List<String> findStudentsNameWithSchoolarship() {

        Set<Student> set = new TreeSet<>(new Student.GpaComp());
        for (var student : students) {
            set.add(student);
        }
        List<String> ans = new ArrayList<>();
        Iterator<Student> iterator = set.iterator();
        while (iterator.hasNext()) {
            if (ans.size() == SCHOOLARSHIP_NR) {
                break;
            }
            ans.add(iterator.next().getName());
        }
        return ans;
    }
    public double getTOpGPA() {
        double ans = Double.NEGATIVE_INFINITY;
        for (var student : students) {
            ans = Math.max(ans, student.getGPA());
        }
        return ans;
    }
}

class UniversitiesManager {
    private static UniversitiesManager instance = null;
    List<University> universities;
    private UniversitiesManager() {
        universities = new ArrayList<>();
    }
    public static synchronized UniversitiesManager getInstance() {
        if (instance == null) {
            instance = new UniversitiesManager();
        }
        return instance;
    }

    private void readStudents(Scanner scanner, List<University> universities) {
        int n = scanner.nextInt();
        for (int i = 0; i < n; i++) {
            int studId = scanner.nextInt();;
            String name = scanner.next();
            String uniName = scanner.next();
            int idx = -1;
            for (int j = 0; j < universities.size() && idx == -1; j++) {
                if (universities.get(j).getName().equals(uniName)) {
                    idx = j;
                }
            }
            var student = new Student(studId, name);
            if (idx != -1) {
                universities.get(idx).addStud(student);
            } else {
                var university = new University(uniName);
                university.addStud(student);
                universities.add(university);
            }
        }
    }
    private void readGrades(Scanner scanner, List<University> universities) {
        int m = scanner.nextInt();
        for (int i = 0; i < m; i++) {
            double grade = scanner.nextDouble();
            int studId = scanner.nextInt();
            for (var university : universities) {
                var student = university.findStudById(studId);
                if (student != null) {
                    student.addGrade(grade);
                    break;
                }
            }
        }
    }
    public void readData(final String FILE_PATH) throws FileNotFoundException {
        var file = new File(FILE_PATH);
        universities = new ArrayList<>();
        var scanner = new Scanner(file);
        readStudents(scanner, universities);
        readGrades(scanner, universities);
        scanner.close();
    }

    public void task1() {
        System.out.println("TASK1: ");

        for (var university : universities) {
            university.sortStudents();
        }
        Collections.sort(universities);

        for (var university : universities) {
            System.out.print(university.getName() + ": ");
            for (var student : university.getStudents()) {
                System.out.print(student.getName() + " ");
            }
            System.out.println();
        }
        System.out.println();
    }

    public void task2() {
        System.out.println("TASK2: ");
        for (var university : universities) {
            String schoolarshipsStr = String.join(" ", university.findStudentsNameWithSchoolarship());
            System.out.println(university.getName() + ": " + schoolarshipsStr);
        }
        System.out.println();
    }

    public void task3() {
        System.out.println("TASK3: ");

        List<Student> students = new ArrayList<>();
        for (var university : universities) {
            students.addAll(university.getStudents());
            university.getStudents().clear();
        }

        Collections.sort(students, new Student.GpaComp());
        for (int i = universities.size() - 1, j = 0; i >= 0; i--, j++) {
            universities.get(i).addStud(students.get(j));
        }
        for (int i = students.size() - 1; i >= universities.size(); i--) {
            universities.get(0).addStud(students.get(i));
        }

        for (var university : universities) {
            System.out.print(university.getName() + ": ");
            for (var student : university.getStudents()) {
                System.out.print(student.getName() + " ");
            }
            System.out.println();
        }
        //if you want to check the difference uncomment the next line
        //System.out.println(universities.get(universities.size() - 1).getTOpGPA() - universities.get(0).getTOpGPA());
    }
}

public class Main{
    final static String FILE_PATH = "data.txt";
    public static void main(String args[]) throws FileNotFoundException {
        var universitiesManager = UniversitiesManager.getInstance();
        universitiesManager.readData(FILE_PATH);
        universitiesManager.task1();
        universitiesManager.task2();
        universitiesManager.task3();
    }
}