I’ve written simple ToDo Console Application with option to create account and log into. Actutally, this option is unnecessary, because you have to create account, but I don’t know DataBase yet. I wanted to be as much OOP as it can be. Thanks for all suggestions.

Main class

public class Main { public static void main(String[] args) {     ToDoEngine engine = new ToDoEngine();     engine.displayMainMenu();     engine.displayUserMenu();     } } 

AccountMaker

import java.util.HashMap; import java.util.Scanner;  public class AccountMaker {  private HashMap<String, User> loginDetails = new HashMap<>(); private String login, password; private Scanner input = new Scanner(System.in);   void inputLoginAndPassword() {     System.out.println("Input your login");     login = input.next();      System.out.println("Input your password");     password = input.next(); }  void createAccount() throws InterruptedException {     User newUser = new User(login, password);     loginDetails.put(login, newUser);     System.out.println("Account has been created successfully");     Thread.sleep(2000); }  HashMap<String, User> getLoginDetails() {     return loginDetails; }  } 

AccountLogger

import java.util.Scanner;  public class AccountLogger {  private AccountMaker accountMaker; private Scanner input = new Scanner(System.in); private String login, password;   AccountLogger(AccountMaker accountMaker) {     this.accountMaker = accountMaker; }   void inputLoginAndPassword() {     System.out.println("Input your login");     login = input.next();      System.out.println("Input your password");     password = input.next(); }  boolean isLoginDataIncorrect() {     User user = accountMaker.getLoginDetails().get(login);     try {         if (user.getPassword().equals(password)) {             System.out.println("You've logged in.");             return false;         } else {             System.out.println("Bad login or password");         }     } catch (NullPointerException e) {         System.out.println("Bad login or password");     }     return true; } } 

User

public class User { private Tasker tasker; private String login; private String password;  User(String login, String password) {     this.login = login;     this.password = password; }  User(Tasker tasks) {     this.tasker = tasks; }  void addNewTask(String task) {     tasker.createTask(task); }  void deleteTask(String task) {     tasker.deleteTask(task); }  void showAllTasks() {     tasker.showAllTasks(); }  String getPassword() {     return password; }  } 

Tasker

import java.util.ArrayList; import java.util.List;  public class Tasker {  private List<String> listOfTasks = new ArrayList<>();   void createTask(String task) {     listOfTasks.add(task); }  void deleteTask(String task) {     listOfTasks.remove(task); }  void showAllTasks() {     for (String listOfTask : listOfTasks) {         System.out.println(listOfTask);     } } } 

ToDoEngine

import java.util.InputMismatchException; import java.util.Scanner;  public class ToDoEngine {  private Scanner input = new Scanner(System.in); private AccountMaker accountMaker = new AccountMaker(); private AccountLogger accountLogger = new AccountLogger(accountMaker); private boolean loopIsTrue = true; private Tasker tasks = new Tasker(); private User user = new User(tasks);  void displayMainMenu() {     System.out.println("What do you wanna to do?");     System.out.println("1. Add Account 2. Log into my account");      while (loopIsTrue) {         try {             getOptionsOfMainMenu(input.nextInt());         } catch (InputMismatchException e) {             System.out.println("You've inputed something wrong!");             System.out.println("What do you wanna to do?");             System.out.println("1. Add Account 2. Log into my account");             input.next();         } catch (InterruptedException e) {             e.printStackTrace();         }     } }  private void getOptionsOfMainMenu(int option) throws InterruptedException {     if (option >= 1 && option <= 2) {         switch (option) {             case 1:                 accountMaker.inputLoginAndPassword();                 accountMaker.createAccount();                 System.out.println("Now let's log into you account.");             case 2:                 while (loopIsTrue) {                     accountLogger.inputLoginAndPassword();                     if (!accountLogger.isLoginDataIncorrect()) {                         loopIsTrue = false;                     }                 }                 break;         }     } }  public void displayUserMenu() {     loopIsTrue = true;     while (loopIsTrue) {         System.out.println("What do you wanna to do?");         System.out.println("1. Add task 2. Show my tasks 3. Delete task 4. Exit");         getOptionsOfUserMenu(input.nextInt());     } }  private void getOptionsOfUserMenu(int option) {     if (option >= 1 && option <= 4) {         switch (option) {             case 1:                 System.out.println("Write down your task.");                 input.nextLine();                 user.addNewTask(input.nextLine());                 System.out.println("Task was added.");                 break;             case 2:                 System.out.println("----------------");                 System.out.println("YOUR TASK LIST:");                 user.showAllTasks();                 System.out.println("----------------");                 break;             case 3:                 System.out.println("Write down your task that you want to delete from task list.");                 input.nextLine();                 user.deleteTask(input.nextLine());                 System.out.println("Task was deleted");                 loopIsTrue = false;                 break;             case 4:                 loopIsTrue = false;                 break;         }     } } } 

Here is github link to repository that contains these classes: https://github.com/must1/ToDoApp