Hi! I'm having a little bit of a problem with this exercise and I was wondering if anyone could help. here is the problem:

Create a class named Purchase. Each Purchase contains an invoice number, amount of sale, and amount of sales tax. Include set methods for the invoice number and sale amount. Within the set() method for the sale amount, calculate the sales tax as 5% of the sale amount. Also include a display method that displays a purchase’s details. Save as Purchase.class
b. Create an application that declares a Purchase object and prompts the user for purchase details. When you prompt for an invoice number, do not let the user proceed until a number between 1,000 and 8,000 has been entered. When you prompt for a sale amount, do not proceed until the user has entered a nonnegative value. After a valid Purchase object has been created, display the object’s invoice number, sale amount, and sales tax.

Here's my code for my Purchase class
import javax.swing.JOptionPane;
import java.util.Scanner;

public class Purchase
{
//variables
public static int invoice;
public static double saleAmount;
public static double saleTax;

//get&set for Invoice
public void setInvoice(int x)
{
invoice = x;
}
public int getInvoice( )
{
return invoice;
}

//get&set for saleAmount
public void setSaleAmount(double y)
{
saleTax = y * 0.05;
saleAmount = y;
}
public double getSaleAmount( )
{
return saleAmount;
}

//get for saleTax
public double getSaleTax( )
{

return saleTax;
}

//display method
public void display(int invoice, double saleAmount, double saleTax)
{
System.out.println("Invoice number: " + invoice + '\n' + "Sale's Amount: " + saleAmount + '\n' + "Sale's Tax: " + saleTax);
}
}


And the code for the CreatePurchase class


import javax.swing.JOptionPane;
import java.util.Scanner;

public class CreatePurchase

{
public static void main(String[] args)

{
Purchase purchase1 = new Purchase ();

//scanner for sales amount
Scanner inputDevice = new Scanner(System.in);
System.out.println("Please enter the sale amount: ");
Purchase.saleAmount = inputDevice.nextDouble();

//loop for saleAmount
while (Purchase.saleAmount < 1)
{
System.out.print('\n'+ "Error, your sale amount needs to be more than 0. Please enter a valid sale amount: >> ");
Purchase.saleAmount = inputDevice.nextDouble();
}

//scanner for invoice
System.out.println("Please enter an invoice number between 1000 and 8000: ");
Purchase.invoice = inputDevice.nextInt();

//loop for invoice
while (Purchase.invoice < 999 || Purchase.invoice > 8000)
{
System.out.print('\n'+ "Error, please enter a valid invoice number between 1000 and 8000: >> ");
Purchase.invoice = inputDevice.nextInt();
}

//display result
JOptionPane.showMessageDialog(null, "Your invoice number is " + Purchase.invoice + '\n'
+ "Your sale tax is: " + Purchase.saleTax + '\n'
+ "Your grand total is: " + Purchase.saleAmount);

}
}


As you can see, when you run the second class, the saleAmount doesn't include the extra 5% for sale tax and sale tax remains as 0. Is probably something really silly, but I have no idea where to start.