StudentService.java

package Student_Management_System;

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

public class StudentService {

          private static final String FILE_NAME = "students.dat";
          static Scanner sc = new Scanner(System.in);

          
          public static ArrayList<Student> readStudents() {
                    ArrayList<Student> list = new ArrayList<>();

                    try (ObjectInputStream ois = new ObjectInputStream(new FileInputStream(FILE_NAME))) {
                              list = (ArrayList<Student>) ois.readObject();
                    } catch (Exception e) {
                              // first run
                    }

                    return list;
          }

          // WRITE
          public static void writeStudents(ArrayList<Student> list) {
                    try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(FILE_NAME))) {
                              oos.writeObject(list);
                    } catch (Exception e) {
                              System.out.println("Error saving file");
                    }
          }

          // ADD
          public static void addStudent() {
                    ArrayList<Student> list = readStudents();

                    int id;
                    while (true) {
                              System.out.print("Enter ID: ");
                              id = sc.nextInt();

                              boolean exists = false;
                              for (Student s : list) {
                                        if (s.getId() == id) {
                                                  exists = true;
                                                  System.out.println("ID already exists!");
                                                  break;
                                        }
                              }

                              if (!exists)
                                        break;
                    }

                    sc.nextLine();

                    String name;
                    while (true) {
                              System.out.print("Enter Name: ");
                              name = sc.nextLine();
                              if (!name.trim().isEmpty())
                                        break;
                              System.out.println("Invalid name!");
                    }

                    float marks;
                    while (true) {
                              System.out.print("Enter Marks: ");
                              marks = sc.nextFloat();
                              if (marks >= 0 && marks <= 100)
                                        break;
                              System.out.println("Invalid marks!");
                    }

                    list.add(new Student(id, name, marks));
                    writeStudents(list);

                    System.out.println("✅ Student Added!");
          }

          // VIEW
          public static void viewStudents() {
                    ArrayList<Student> list = readStudents();

                    if (list.isEmpty()) {
                              System.out.println("No records found!");
                              return;
                    }

                    
                    System.out.printf("%-10s %-20s %-10s\n", "ID", "NAME", "MARKS");
                    

                    for (Student s : list) {
                              System.out.println(s);
                    }

                    System.out.println("--------------------------------------------------");
          }

          // SEARCH
          public static void searchStudent() {
                    ArrayList<Student> list = readStudents();

                    System.out.print("Enter ID: ");
                    int id = sc.nextInt();

                    for (Student s : list) {
                              if (s.getId() == id) {
                                        System.out.println("Found: " + s);
                                        return;
                              }
                    }

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

          // UPDATE (temp logic)
          public static void updateStudent() {
                    ArrayList<Student> list = readStudents();

                    System.out.print("Enter ID: ");
                    int id = sc.nextInt();
                    sc.nextLine();

                    for (Student s : list) {
                              if (s.getId() == id) {
                                        System.out.print("New Name: ");
                                        s.setName(sc.nextLine());

                                        System.out.print("New Marks: ");
                                        s.setMarks(sc.nextFloat());

                                        writeStudents(list);
                                        System.out.println("Updated!");
                                        return;
                              }
                    }

                    System.out.println("Not found");
          }

          // DELETE
          public static void deleteStudent() {
                    ArrayList<Student> list = readStudents();

                    System.out.print("Enter ID: ");
                    int id = sc.nextInt();

                    Iterator<Student> it = list.iterator();

                    while (it.hasNext()) {
                              Student s = it.next();
                              if (s.getId() == id) {
                                        it.remove();
                                        writeStudents(list);
                                        System.out.println("Deleted!");
                                        return;
                              }
                    }

                    System.out.println("Not found");
          }

          // SORT
          public static void sortStudents() {
                    ArrayList<Student> list = readStudents();

                    list.sort((a, b) -> Float.compare(b.getMarks(), a.getMarks()));
                    viewStudents();
          }

          // TOPPER
          public static void showTopper() {
                    ArrayList<Student> list = readStudents();

                    if (list.isEmpty())
                              return;

                    Student top = Collections.max(list, Comparator.comparing(Student::getMarks));
                    System.out.println("Topper: " + top);
          }

          // COUNT
          public static void countStudents() {
                    System.out.println("Total Students: " + readStudents().size());
          }

          // EXPORT
          public static void exportData() {
                    ArrayList<Student> list = readStudents();

                    try (BufferedWriter bw = new BufferedWriter(new FileWriter("students.txt"))) {
                              for (Student s : list) {
                                        bw.write(s.toString());
                                        bw.newLine();
                              }
                              System.out.println("Exported!");
                    } catch (Exception e) {
                              System.out.println("Export error");
                    }
          }
}