Products exercise

import java.util.Scanner;

public class Products {

    double usersMoney = 0; // builds up as user buy more stuff.
    Scanner input = new Scanner(System.in);

       public static void main(String[] args) {
        Products product = new Products();
        product.menu();
       }

    public void menu() {
        printMenu();
        String userInput = input.nextLine().toUpperCase();
        while (!userInput.contains("Q")) {
            switch (userInput) {
                case "1":
                    printProductMenu();
                    break;
                case "2":
                    calculateChange();
                    break;
                case "Q":
                    System.exit(0);


            }
        }
    }

    public static void printMenu(){
        System.out.println("Welcome to the Local Mart. Please select an Option form below: ");
        System.out.println("1 - Display Products");
        System.out.println("2 - Proceed to Purchase");
    }

    public void printProductMenu() {
        System.out.println("1 - Soda Bottle, Price: £2.00");
        System.out.println("2 - Packet of Crisps, Price: £1.00");
        System.out.println("3 - Pack of Gum, Price: £0.50p");
        System.out.println("4 - Go back");
        int userInput = input.nextInt();

        switch (userInput) {
            case 1:
                this.usersMoney += 2;
                System.out.println("Your total is:" + usersMoney);
                break;

            case 2:
                this.usersMoney += 1;
                System.out.println("Your total is:" + usersMoney);
                break;

            case 3:
                this.usersMoney += 0.50;
                System.out.println("Your total is:" + usersMoney);
                break;

            case 4:
                menu();
                break;
        }
    }

    public void calculateChange() {
        System.out.println("You have to pay" + usersMoney);
        double change = 0;
        int userInput = input.nextInt();
        while (usersMoney != 0) {

            if (userInput < usersMoney) {
                change = usersMoney - usersMoney;
                System.out.println("You still have to pay" + change);
            }
            if (userInput > usersMoney) {
                change = userInput - usersMoney;
                System.out.println("Your remaining change is" + change);
            }
        }
        System.out.println("Congratulations you've paid the amount, Enjoy :)");
    }



}