TodosApp.java

import java.util.ArrayList;
import java.util.Scanner;
import java.util.UUID;

public class TodosApp {
    private static Scanner scanner = new Scanner(System.in); // this is used by userInput function keep that in mind.
                                                             // It's probably not a good thing
    private static ArrayList<Todo> todos = new ArrayList<>();

    public static void main(String[] args) {
        boolean running = true;

        while (running) {
            displayTodos();
            showMenu();
            String userChoice = getUserInput();

            switch (userChoice) {
                case "1":
                    addTodo();
                    break;
                case "2":
                    completeTodo();
                    break;
                case "3":
                    deleteTodo();
                    break;
                case "4":
                    running = false;
                    break;
            }

        }

        System.out.println("Not running anymore ;-;");
    }

    private static void showMenu() {

        String template = """
                \n--- TODOS App Menu ---
                1. Add Todo
                2. Mark Todo as completed %s
                3. Delete Todo %s
                4. Quit!
                    """;

        String message = String.format(template,
                todos.isEmpty() ? "(disabled)" : "",
                todos.isEmpty() ? "(disabled)" : "");

        System.out.println(message);
    }

    private static void addTodo() {
        System.out.println("Enter todo:");
        String title = getUserInput();
        todos.add(new Todo(title));
        System.out.println("Todo named: " + title + " was added bitch. ");
    }

    private static void displayTodos() {
        if (!todos.isEmpty()) {
            System.out.println("Your todos: ");
            for (int i = 0; i < todos.size(); i++) {
                System.out.println(i + 1 + ". " + todos.get(i).returnTodoString());
            }
        } else {
            System.out.println("You have nothing to do. Time to relax 8-)");
        }

    }

    private static void completeTodo() {

        if (todos.isEmpty()) {
            System.out.println("You don't have todos to manage! \n create some by chosing option one in the menu ");
            return;
        }

        displayTodos();
        System.out.println("What have you done?: ");
        String userChoice = getUserInput();

        boolean validation = validateUserInputInt(userChoice);
        if (validation == false) {
            return;
        }

        int parsedUserChoice = Integer.parseInt(userChoice) - 1;

        Todo todo = todos.get(parsedUserChoice);

        todo.toggleComplete();
    }

    public static void deleteTodo() {
        if (todos.isEmpty()) {
            System.out.println("You don't have todos to manage! \n create some by chosing option one in the menu ");
            return;
        }

        displayTodos();
        System.out.println("What todo you want to delete?: ");
        String userChoice = getUserInput();

        boolean validation = validateUserInputInt(userChoice);

        if (validation == false) {
            return;
        }

        int parsedUserChoice = Integer.parseInt(userChoice) - 1;

        System.out.println("Removed todo: " + todos.get(parsedUserChoice).returnTodoString());

        todos.remove(parsedUserChoice);
    }

    private static String getUserInput() {
        return scanner.nextLine().trim();
    }

    // wanned to make external function to validate user input. It isn't so good as
    // I thought It would be xD and it's redundant xD
    private static boolean validateUserInputInt(String userInput) {
        String regex = "^[1-9]\\d{0,8}$"; // you can make it so it dynamically adds the size of the todos to it, but I
                                          // was too lazy for that there are other todos apps to be coded in other
                                          // languages / framewroks etc.

        if (!userInput.matches(regex)) {
            System.out.println("You can only use positive numbers,\n and you can't go higher than: 999999999. ");
            return false;
        }

        int validatedUserInput = Integer.parseInt(userInput) - 1;

        if (validatedUserInput > (todos.size() - 1)) {
            System.out.println("There is no todo with this number. ");
            return false;
        }

        return true;
    }
}

class Todo {
    public String title;
    public boolean checked = false;
    public String id = UUID.randomUUID().toString();

    public Todo(String title) {
        this.title = title;
    }

    public void toggleComplete() {
        this.checked = !this.checked;
        System.out.println("Marked as completed: " + this.title);
    }

    public String returnTodoString() {
        return this.title + (this.checked ? " [x] (completed) " : " [ ] (not completed)");
    }
}