code.java

```java
import java.util.ArrayList;
import java.util.Scanner;

public class ToDoList {

    private static ArrayList<String> tasks = new ArrayList<>();
    private static Scanner scanner = new Scanner(System.in);

    public static void main(String[] args) {

        int choice;

        do {
            System.out.println("\n===== TO-DO LIST APPLICATION =====");
            System.out.println("1. Add Task");
            System.out.println("2. Delete Task");
            System.out.println("3. Display Tasks");
            System.out.println("4. Mark Task Complete");
            System.out.println("5. Exit");

            System.out.print("Enter your choice: ");
            choice = scanner.nextInt();
            scanner.nextLine(); // consume newline

            switch (choice) {

                case 1:
                    addTask();
                    break;

                case 2:
                    deleteTask();
                    break;

                case 3:
                    displayTasks();
                    break;

                case 4:
                    markTaskComplete();
                    break;

                case 5:
                    System.out.println("Exiting application...");
                    break;

                default:
                    System.out.println("Invalid choice!");
            }

        } while (choice != 5);

        scanner.close();
    }

    // Add Task
    private static void addTask() {
        System.out.print("Enter task description: ");
        String task = scanner.nextLine();

        tasks.add(task);

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

    // Delete Task
    private static void deleteTask() {

        if (tasks.isEmpty()) {
            System.out.println("No tasks to delete!");
            return;
        }

        displayTasks();

        System.out.print("Enter task number to delete: ");
        int taskNumber = scanner.nextInt();
        scanner.nextLine();

        if (taskNumber > 0 && taskNumber <= tasks.size()) {
            tasks.remove(taskNumber - 1);
            System.out.println("Task deleted successfully!");
        } else {
            System.out.println("Invalid task number!");
        }
    }

    // Display Tasks
    private static void displayTasks() {

        if (tasks.isEmpty()) {
            System.out.println("No tasks available!");
            return;
        }

        System.out.println("\n===== TASK LIST =====");

        for (int i = 0; i < tasks.size(); i++) {
            System.out.println((i + 1) + ". " + tasks.get(i));
        }
    }

    // Mark Task Complete
    private static void markTaskComplete() {

        if (tasks.isEmpty()) {
            System.out.println("No tasks available!");
            return;
        }

        displayTasks();

        System.out.print("Enter task number to mark complete: ");
        int taskNumber = scanner.nextInt();
        scanner.nextLine();

        if (taskNumber > 0 && taskNumber <= tasks.size()) {

            String completedTask = tasks.get(taskNumber - 1);

            tasks.set(taskNumber - 1, "[COMPLETED] " + completedTask);

            System.out.println("Task marked as completed!");
        } else {
            System.out.println("Invalid task number!");
        }
    }
}
```