How to consume methods from service on entity object

Question

I have a entity class:

Product

public String email;
public int age;
public double price;
public double discount;

And I want to create service class for it to create some calculations

Do I need to extend this Product class to consume variables like email, age, price so I can do calculations?

And also do I need to create object from service or I can do it somehow with my Entity object but still use methods on it?

I have this in main for example

Service service = new Service("email", 1, 30)

You see its service, while I want to create object from entity and then on that object consume methods.

So it would be

Product product = new Product ("email", 1, 30)

Main

public class Main {

public static void main(String[] args) {

    Scanner scanner = new Scanner(System.in);
    TaxService taxService = new TaxService("The Little Prince", 12345, 20.25);

    int option = 0;
    do {
        menu();
        option = scanner.nextInt();
        switch (option) {
            case 1:
                taxService.tax();
                break;
            case 2:
                System.out.println(
                        "\nThank you for using the program. Goodbye!\n");
                System.exit(0);

                break;
            default:
                System.out.println("Invalid input");
                break;
        }
    } while (option != 9);
}


public static void menu() {
    System.out.println("Choose action");
    System.out.println("1. Tax calculate\n");
    System.out.println("2: Exit program");


    System.out.print("Enter your selection: ");
}

}

Entity

public class Product {
public String name;
public int UPC;
public double price;
public double tax;


public Product() {
}

public Product(String name, int UPC, double price) {
    this.name = name;
    this.UPC = UPC;
    this.price = price;
    this.tax = 20;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public int getUPC() {
    return UPC;
}

public void setUPC(int UPC) {
    this.UPC = UPC;
}

public double getPrice() {
    return price;
}

public void setPrice(double price) {
    this.price = price;
}

public double getTax() {
    return tax;
}

public void setTax(double tax) {
    this.tax = tax;
}

}

Service

public class TaxService extends Product {

Scanner scanner = new Scanner(System.in);

public TaxService(String name, int UPC, double price) {
    super(name, UPC, price);
}

public void tax() {
    System.out.println("Do you want to change tax percentage? (y / n)");
    String answer = scanner.nextLine();

    if (answer.equalsIgnoreCase("y")) {
        System.out.println("Please enter a new tax percentage: ");
        double newTaxPercentage = scanner.nextInt();
        scanner.nextLine();
        if (newTaxPercentage < 0) {
            System.out.println("Wrong input, try again.");
        } else {
            double calculateTax = (price * (newTaxPercentage / 100));
            double result = calculateTax + price;
            System.out.println("Calculation finished");
            System.out.printf("Product price reported as $%.2f before tax and $%.2f after %.0f%% tax.\n", price, result, newTaxPercentage);
        }
    } else {
        double calculateTax = (price * (tax / 100));
        double result = calculateTax + price;
        System.out.println("Calculation finished");
        System.out.printf("Product price reported as $%.2f before tax and $%.2f after %.0f%% tax.\n", price, result, tax);
    }
}

private boolean isTaxValid() {
    return tax < 0;
}

}

Answer 1

You should separate the responsibilities of the TaxService and Product. The idea is that the TaxService is working with products, it is not a product or a parent class of a product. So inside TaxService class you should not extend Product.

To work with a product you can accept in the methods signatures of TaxService Product as argument:

public void tax(Product product) {
...// some code
// Everywhere when you need properties from the product just call the getters
double result = product.getCalculateTax() + product.getPrice();

Now on creation of the service you don't need to add values in the constructor. So in the main you should have:

Product product = new Product("The Little Prince", 12345, 20.25);
TaxService taxService = new TaxService(scanner); // below I explain why we pass scanner here.
..... // some code
taxService.tax(product);

By the way it is not good practice to open many Scanners for the same resource. You can see here: Java Multiple Scanners

What I will do it to pass the Scanner from the main to the TaxService when I create tax service in the constructor