Задание по ООП

public class Student {
    private String firstName;
    private String lastName;
    private String middleName;
    private double grade;
 
    public Student(String firstName, String lastName, String middleName, int grade) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.middleName = middleName;
        this.grade = grade;
    }
 
    public double getGrade() {
        return grade;
    }
 
    public String getLastName() {
        return lastName;
    }
 
    @Override
    public String toString() {
        return "Student{" +
                "firstName='" + firstName + '\'' +
                ", lastName='" + lastName + '\'' +
                ", middleName='" + middleName + '\'' +
                ", grade=" + grade +
                '}';
    }
}
 
 
public class StudentRepository {
    private List<Student> students;
 
    public StudentRepository() {
        this.students = new ArrayList<Student>();
    }
 
    public void addStudent(Student student) {
        students.add(student);
    }
 
    public List<Student> getStudents() {
        return students;
    }
}
 
 
public class UserService {
    public static List<Student> getStudentsByFirstLetter(List<Student> students, char letter) {
        List<Student> suitableStudents = new ArrayList<Student>();
 
        for (Student student : students) {
            char[] chars = student.getLastName().toCharArray();
            if (chars[0] == letter)
                suitableStudents.add(student);
        }
        return suitableStudents;
    }
 
    public static double getAverageGrade(List<Student> students) {
        if (students.isEmpty())
            return 0;
 
        double sum = 0;
        for (Student student : students) {
            sum += student.getGrade();
        }
        return sum/students.size();
    }
}
 
 
public class Main {
    public static void main(String[] args) {
        char letter = 'A';
        StudentRepository studentRepository = new StudentRepository();
        studentRepository.addStudent(new Student("Georgiy", "Sidorov", "Nikolaevich", 5));
        studentRepository.addStudent(new Student("Vasya", "Arsenjev", "Petrovich", 4));
        studentRepository.addStudent(new Student("Kirill", "Abramov", "Semenych", 3));
        studentRepository.addStudent(new Student("Andrey", "Avseev", "Dmitrievich", 3));
        studentRepository.addStudent(new Student("Tolya", "Avseev", "Egorjevich", 3));
 
        List<Student> suitableStudents = UserService.getStudentsByFirstLetter(studentRepository.getStudents(), letter);
 
        if (suitableStudents.isEmpty())
            System.out.println("No students with first letter " + letter + " in last name found!");
        else {
            System.out.println("The students whose last name starts with letter " + letter + " :");
 
            for (Student student : suitableStudents) {
                System.out.println(student.toString());
            }
 
            double averageGrade = UserService.getAverageGrade(suitableStudents);
            System.out.println("An average grade of these students is: " + averageGrade);
        }
    }
}

import java.util.ArrayList;
import java.util.List;
 
class StudentProcesser {
 
    public static void main(String[] args) {
        List<Student> group123 = new ArrayList<>();
        group123.add(new Student("Georgiy", "Sidorov", "Nikolaevich", 5));
        group123.add(new Student("Georgiy", "Aidorov", "Nikolaevich", 4));
        group123.add(new Student("Eorgiya", "Arova", "Nikolaevich", 3));
        group123.add(new Student("Kirill", "Abramov", "Semenych", 3));
        group123.add(new Student("Nika", "Abramova", "Ivanovna", 4));
        double averageGroup = group123.
                stream().
                filter(p -> p.getLastName().toLowerCase().charAt(0) == 'a').
                mapToInt(Student::getGrade).average().getAsDouble();
        System.out.printf("Cредний бал на \'A\' = %.2f%n",averageGroup);
        averageGroup = group123.
                stream().
                mapToInt(Student::getGrade).average().getAsDouble();
        System.out.printf("Cредний бал по группе = %.2f%n",averageGroup);
    }
}
//####
class Student {
    private String firstName;
    private String lastName;
    private String middleName;
    private int grade;
 
    Student(String firstName, String lastName, String middleName, int grade) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.middleName = middleName;
        this.grade = grade;
    }
 
    public int getGrade() {
        return grade;
    }
 
    public String getLastName() {
        return lastName;
    }
 
    @Override
    public String toString() {
        return "Student{" +
                "firstName='" + firstName + '\'' +
                ", lastName='" + lastName + '\'' +
                ", middleName='" + middleName + '\'' +
                ", grade=" + grade +
                '}';
    }
}