ToDoApp.java

import java.util.*;

// Class representing a single item in the to-do list
class Task {
    int id;
    String title;
    boolean isCompleted;

    public Task(int id, String title) {
        this.id = id;
        this.title = title;
        this.isCompleted = false;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getTitle() {
        return title;
    }

    public boolean isCompleted() {
        return isCompleted;
    }

    public void markCompleted() {
        this.isCompleted = true;
    }

    @Override
    public String toString() {
        return "[" + (isCompleted ? "✓" : " ") + "] " + id + ". " + title;
    }
}

// Class for managing tasks
class ToDoList {
    ArrayList<Task> tasks;

    public ToDoList() {
        this.tasks = new ArrayList<>();
    }

    public void addTask(String title) {
        int id = tasks.size() + 1;
        tasks.add(new Task(id, title));
    }

    public void removeTask(int id) {
        for (int i = 0; i < tasks.size(); i++) {
            if (tasks.get(i).getId() == id) {
                tasks.remove(i);
                break;
            }
        }
       
        for (int i = 0; i < tasks.size(); i++) {
            tasks.get(i).setId(i + 1);
        }
    }

    public void markTaskCompleted(int id) {
        for (Task task : tasks) {
            if (task.getId() == id) {
                task.markCompleted();
                break;
            }
        }
    }

    public void displayTasks() {
        if (tasks.isEmpty()) {
            System.out.println("No tasks available.");
        } else {
            for (Task task : tasks) {
                System.out.println(task);
            }
        }
    }
}

// Main application class
public class ToDoApp {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        ToDoList toDoList = new ToDoList();

        while (true) {
            System.out.println("\nTo-Do List Application");
            System.out.println("1. Add Task");
            System.out.println("2. Remove Task");
            System.out.println("3. Mark Task as Completed");
            System.out.println("4. View Tasks");
            System.out.println("5. Exit");
            System.out.print("Choose an option: ");
            
            int choice = scanner.nextInt();
            scanner.nextLine();

            switch (choice) {
                case 1:
                    System.out.print("Enter task title: ");
                    String title = scanner.nextLine();
                    toDoList.addTask(title);
                    break;
                case 2:
                    System.out.print("Enter task ID to remove: ");
                    int removeId = scanner.nextInt();
                    toDoList.removeTask(removeId);
                    break;
                case 3:
                    System.out.print("Enter task ID to mark as completed: ");
                    int completeId = scanner.nextInt();
                    toDoList.markTaskCompleted(completeId);
                    break;
                case 4:
                    toDoList.displayTasks();
                    break;
                case 5:
                    System.out.println("Exiting application...");
                    scanner.close();
                    return;
                default:
                    System.out.println("Invalid choice. Try again.");
            }
        }
    }
}