DSA Java Project – To Do List

Program 1

// To-Do List Using Singly Linked List

import java.util.Scanner;

class Task 
{
    int id;
    String description;
    boolean completed;
    Task next;

    public Task(int id, String description) 
    {
        this.id = id;
        this.description = description;
        this.completed = false;
        this.next = null;
    }
}

class ToDoList 
{
    private Task head;
    private int taskCount;

    public ToDoList() 
    {
        head = null;
        taskCount = 0;
    }

    public void addTask(Scanner scanner) 
    {
        scanner.nextLine();  // clear buffer
        System.out.print("Enter task description: ");
        String desc = scanner.nextLine();

        Task newTask = new Task(++taskCount, desc);

        if (head == null) 
        {
            head = newTask;
        } else {
            Task temp = head;
            while (temp.next != null) {
                temp = temp.next;
            }
            temp.next = newTask;
        }

        System.out.println("Task added successfully.");
    }

    public void displayTasks() 
    {
        if (head == null)
        {
            System.out.println("To-Do List is empty.");
            return;
        }

        System.out.println("\nTo-Do List:");
        Task temp = head;
        while (temp != null)
         {
            String status = temp.completed ? "[X]" : "[ ]";
            System.out.println("ID: " + temp.id + " | " + status + " " + temp.description);
            temp = temp.next;
        }
    }

    public void markCompleted(Scanner scanner) {
        System.out.print("Enter task ID to mark as completed: ");
        int id = scanner.nextInt();

        Task temp = head;
        while (temp != null) 
        {
            if (temp.id == id) 
            {
                temp.completed = true;
                System.out.println("Task marked as completed.");
                return;
            }
            temp = temp.next;
        }

        System.out.println("Task with ID " + id + " not found.");
    }

    public void deleteTask(Scanner scanner) 
    {
        System.out.print("Enter task ID to delete: ");
        int id = scanner.nextInt();

        Task temp = head;
        Task prev = null;

        while (temp != null && temp.id != id) {
            prev = temp;
            temp = temp.next;
        }

        if (temp == null) {
            System.out.println("Task with ID " + id + " not found.");
            return;
        }

        if (prev == null) {
            head = temp.next;
        } else {
            prev.next = temp.next;
        }

        System.out.println("Task deleted successfully.");
    }
}

public class ToDoListApp
{
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        ToDoList list = new ToDoList();
        int choice;

        do {
            System.out.println("\n----- To-Do List Menu -----");
            System.out.println("1. Add Task");
            System.out.println("2. Display Tasks");
            System.out.println("3. Mark Task as Completed");
            System.out.println("4. Delete Task");
            System.out.println("5. Exit");
            System.out.println("---------------------------");
            System.out.print("Enter your choice: ");

            while (!scanner.hasNextInt()) {
                System.out.println("Please enter a valid number.");
                scanner.next(); // clear invalid input
            }

            choice = scanner.nextInt();

            switch (choice) {
                case 1:
                    list.addTask(scanner);
                    break;
                case 2:
                    list.displayTasks();
                    break;
                case 3:
                    list.markCompleted(scanner);
                    break;
                case 4:
                    list.deleteTask(scanner);
                    break;
                case 5:
                    System.out.println("Exiting program.");
                    break;
                default:
                    System.out.println("Invalid choice. Try again.");
            }

        } while (choice != 5);

        scanner.close();
    }
}