Main.java

import java.util.*;

public class Main {

    static List<Student> students = FileManager.load("students.dat");
    static List<Course> courses = FileManager.load("courses.dat");
    static List<Enrollment> enrollments = FileManager.load("enrollments.dat");

    static Scanner sc = new Scanner(System.in);

    public static void main(String[] args) {

        students.stream().mapToInt(Student::getId).max()
                .ifPresent(Student::setCounter);

        courses.stream().mapToInt(Course::getCourseId).max()
                .ifPresent(Course::setCounter);

        while (true) {
            try {
                menu();
                int choice = Integer.parseInt(sc.nextLine());

                if (choice < 1 || choice > 10)
                    throw new InvalidChoiceException("Invalid choice");

                switch (choice) {
                    case 1 -> addStudent();
                    case 2 -> addCourse();
                    case 3 -> enrollStudent();
                    case 4 -> students.forEach(System.out::println);
                    case 5 -> courses.forEach(System.out::println);
                    case 6 -> viewEnrollments();
                    case 7 -> searchStudent();
                    case 8 -> dropCourse();
                    case 9 -> showStudentCourses();
                    case 10 -> exitSystem();
                }

            } catch (InvalidChoiceException e) {
                System.out.println("❌ " + e.getMessage());
            } catch (NumberFormatException e) {
                System.out.println("❌ Enter numeric value.");
            }
        }
    }

    static void menu() {
        System.out.println("\n===== COURSE ENROLLMENT SYSTEM =====");
        System.out.println("1. Add Student");
        System.out.println("2. Add Course");
        System.out.println("3. Enroll Student");
        System.out.println("4. View Students");
        System.out.println("5. View Courses");
        System.out.println("6. View Enrollments");
        System.out.println("7. Search Student");
        System.out.println("8. Drop Course");
        System.out.println("9. Show Student Courses");
        System.out.println("10. Exit");
    }

    static int getInt(String msg) {
        while (true) {
            try {
                System.out.print(msg);
                return Integer.parseInt(sc.nextLine());
            } catch (Exception e) {
                System.out.println("❌ Invalid number");
            }
        }
    }

    static void addStudent() {
        System.out.print("Enter name: ");
        String name = sc.nextLine();

        Student s = new Student(name);
        students.add(s);

        System.out.println("✅ Student added. ID = " + s.getId());
    }

    static void addCourse() {
        System.out.print("Enter course name: ");
        String name = sc.nextLine();

        int cap = getInt("Enter capacity: ");

        Course c = new Course(name, cap);
        courses.add(c);

        System.out.println("✅ Course added. ID = " + c.getCourseId());
    }

    static void enrollStudent() {

        int sid = getInt("Enter student ID: ");
        int cid = getInt("Enter course ID: ");

        Course course = courses.stream()
                .filter(c -> c.getCourseId() == cid)
                .findFirst().orElse(null);

        if (course == null) {
            System.out.println("Course not found!");
            return;
        }

        if (course.isFull()) {
            System.out.println("Course is full!");
            return;
        }

        boolean exists = enrollments.stream()
                .anyMatch(e -> e.getStudentId() == sid &&
                               e.getCourseId() == cid);

        if (exists) {
            System.out.println("Already enrolled!");
            return;
        }

        enrollments.add(new Enrollment(sid, cid));
        course.incrementEnrolled();

        System.out.println("✅ Enrollment successful!");
    }

    static void viewEnrollments() {
        enrollments.forEach(e ->
                System.out.println("Student " + e.getStudentId()
                        + " → Course " + e.getCourseId()));
    }

    static void searchStudent() {
        int id = getInt("Enter student ID: ");

        students.stream()
                .filter(s -> s.getId() == id)
                .findFirst()
                .ifPresentOrElse(
                        System.out::println,
                        () -> System.out.println("Not found"));
    }

    static void dropCourse() {

        int sid = getInt("Enter student ID: ");
        int cid = getInt("Enter course ID: ");

        Iterator<Enrollment> it = enrollments.iterator();

        while (it.hasNext()) {
            Enrollment e = it.next();

            if (e.getStudentId() == sid && e.getCourseId() == cid) {

                it.remove();

                courses.stream()
                        .filter(c -> c.getCourseId() == cid)
                        .forEach(Course::decrementEnrolled);

                System.out.println("✅ Course dropped");
                return;
            }
        }

        System.out.println("Enrollment not found");
    }

    static void showStudentCourses() {

        int sid = getInt("Enter student ID: ");

        enrollments.stream()
                .filter(e -> e.getStudentId() == sid)
                .forEach(e ->
                        courses.stream()
                                .filter(c -> c.getCourseId() ==
                                             e.getCourseId())
                                .forEach(System.out::println));
    }

    static void exitSystem() {
        FileManager.save("students.dat", students);
        FileManager.save("courses.dat", courses);
        FileManager.save("enrollments.dat", enrollments);

        System.out.println("💾 Data saved. Goodbye!");
        System.exit(0);
    }
}