Student Management System

/**
 * STUDENT MANAGEMENT SYSTEM (ENHANCED)
 * 
 * Features:
 * - CRUD Operations (Create, Read, Update, Delete)
 * - Input validation
 * - Error handling
 * - Clean console interface
 * - Detailed comments
 */

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

// Model class representing a Student entity
class Student {
    private int id;
    private String name;
    private String grade;

    public Student(int id, String name, String grade) {
        this.id = id;
        this.name = name;
        this.grade = grade;
    }

    // Getters and setters with validation
    public int getId() { return id; }
    
    public String getName() { return name; }
    public void setName(String name) { 
        if (!name.isEmpty()) this.name = name; 
    }

    public String getGrade() { return grade; }
    public void setGrade(String grade) { 
        if (isValidGrade(grade)) this.grade = grade.toUpperCase(); 
    }

    // Validate grade format (A-F with optional + or -)
    private boolean isValidGrade(String grade) {
        return grade.matches("[A-Fa-f][+-]?");
    }

    @Override
    public String toString() {
        return String.format("| %-4d | %-20s | %-6s |", id, name, grade.toUpperCase());
    }
}

// Service class handling business logic
class StudentService {
    private List<Student> students = new ArrayList<>();
    private int nextId = 1;
    private Scanner scanner = new Scanner(System.in);

    // Add new student with input validation
    public void addStudent() {
        System.out.println("\n--- ADD NEW STUDENT ---");
        
        String name = getValidInput("Enter student name: ", 
            "Name cannot be empty!", 
            input -> !input.trim().isEmpty());

        String grade = getValidInput("Enter student grade (A-F with +/-): ", 
            "Invalid grade format! Use A-F with optional + or -", 
            input -> input.matches("[A-Fa-f][+-]?"));

        students.add(new Student(nextId++, name, grade));
        System.out.println("\n✅ Student added successfully!");
    }

    // Display all students in tabular format
    public void viewStudents() {
        System.out.println("\n--- STUDENT LIST ---");
        
        if (students.isEmpty()) {
            System.out.println("No students found.");
            return;
        }

        printTableHeader();
        students.forEach(System.out::println);
        printTableFooter();
    }

    // Update existing student record
    public void updateStudent() {
        System.out.println("\n--- UPDATE STUDENT ---");
        Student student = findStudentById();
        
        if (student == null) return;

        String newName = getValidInput("Enter new name (press Enter to keep current): ", 
            "",  // No error message for empty input
            input -> true); // Accept any input

        if (!newName.isEmpty()) {
            student.setName(newName);
        }

        String newGrade = getValidInput("Enter new grade (press Enter to keep current): ", 
            "Invalid grade format! Use A-F with optional + or -", 
            input -> input.isEmpty() || input.matches("[A-Fa-f][+-]?"));

        if (!newGrade.isEmpty()) {
            student.setGrade(newGrade);
        }

        System.out.println("\n✅ Student updated successfully!");
    }

    // Delete student by ID
    public void deleteStudent() {
        System.out.println("\n--- DELETE STUDENT ---");
        Student student = findStudentById();
        
        if (student == null) return;

        students.remove(student);
        System.out.println("\n✅ Student deleted successfully!");
    }

    // Helper method to find student by ID
    private Student findStudentById() {
        try {
            System.out.print("Enter student ID: ");
            int id = Integer.parseInt(scanner.nextLine());
            
            return students.stream()
                .filter(s -> s.getId() == id)
                .findFirst()
                .orElseThrow(() -> new Exception());
                
        } catch (Exception e) {
            System.out.println("\n❌ Student not found or invalid ID!");
            return null;
        }
    }

    // Generic input validation method
    private String getValidInput(String prompt, String errorMessage, Validator validator) {
        String input;
        do {
            System.out.print(prompt);
            input = scanner.nextLine().trim();
            if (!validator.validate(input)) {
                System.out.println("\n❌ " + errorMessage);
            }
        } while (!validator.validate(input));
        return input;
    }

    // Table formatting methods
    private void printTableHeader() {
        System.out.println("+------+----------------------+--------+");
        System.out.println("| ID   | Name                 | Grade  |");
        System.out.println("+------+----------------------+--------+");
    }

    private void printTableFooter() {
        System.out.println("+------+----------------------+--------+");
        System.out.println("Total students: " + students.size());
    }

    // Functional interface for input validation
    @FunctionalInterface
    interface Validator {
        boolean validate(String input);
    }
}

// Main application class
public class StudentManagementSystem {
    public static void main(String[] args) {
        StudentService service = new StudentService();
        Scanner scanner = new Scanner(System.in);
        
        System.out.println("\n🎓 Welcome to Student Management System 🎓");

        while (true) {
            printMainMenu();
            String choice = scanner.nextLine();

            switch (choice) {
                case "1" -> service.addStudent();
                case "2" -> service.viewStudents();
                case "3" -> service.updateStudent();
                case "4" -> service.deleteStudent();
                case "5" -> {
                    System.out.println("\n👋 Exiting system... Goodbye!");
                    scanner.close();
                    System.exit(0);
                }
                default -> System.out.println("\n❌ Invalid choice! Please try again.");
            }
        }
    }

    private static void printMainMenu() {
        System.out.println("\n==================== MAIN MENU ====================");
        System.out.println("1. Add Student");
        System.out.println("2. View Students");
        System.out.println("3. Update Student");
        System.out.println("4. Delete Student");
        System.out.println("5. Exit");
        System.out.print("Enter your choice (1-5): ");
    }
}