HashMap Menu

package com.company;

import java.util.HashMap;
import java.util.Scanner;

public class Menu {

    Scanner in = new Scanner(System.in);
    public HashMap<String, User> users = new HashMap<>();

    void signUp() {
        String username = "";
        String password = "";
        while (username.length() <= 0) {
            System.out.println("Please enter your Username:");
            username = in.nextLine();
            if (users.containsKey(username)) {
                System.out.println("This username already exist");
                username = "";
            }
        }
        while (password.length() <= 0) {
            System.out.println("Please enter your Password");
            password = in.nextLine();
        }
        User u = new User(username, password);
        users.put(u.getUsername(), u);
    }

    User login() {
        String username = "";
        String password = "";
        int counter = 0;
        int counter2 = 0;
        while (!users.containsKey(username)) {
            System.out.print("Username: ");
            username = in.nextLine();
            if (!users.containsKey(username))
                System.out.println("Username does not exist");
            counter++;
            if (counter == 4) {
                return null;
            }
        }
        while (password.length() <= 0) {
            System.out.print("Password: ");
            password = in.nextLine();
            if (!users.get(username).getPassword().equals(password)) {
                System.out.println("Username and password does not match");
                password = "";
                counter++;
                if (counter2 == 4) {
                    return null;
                }
            }
        }
        System.out.println("Successfully logged in!");
        return users.get(username);
    }

    void changePassword(User user){
        String password = "";
        while(true) {
            System.out.println("Please enter your current Password:");
            password = in.nextLine();
            if (!password.equals(user.getPassword())) {
                System.out.println("Incorrect password");
            } else {
                break;
            }
        }
        password = "";
        while (password.length() <= 0) {
            System.out.println("Please enter your new Password");
            password = in.nextLine();
        }
        user.setPassword(password);
    }
}