Thread: Why wont my program calculate sales tax?

import javax.swing.*;
public class Purchase
{
 private static double invoiceNumber;
 private static double ammountOfSale;
 private static double salesTax;
 
 public double getInvoiceNumber()
 {
  return invoiceNumber;
 }
 public void setInvoiceNumber(double invoice)
 {
  invoiceNumber = invoice;
 }
 
  public double getAmmountOfSale()
 {
  return ammountOfSale;
 }
 public void setAmmountOfSale(double sale)
 {
  ammountOfSale = sale;
 }
 
 public double setSalesTax()
 {
  salesTax = .05 * ammountOfSale;
	return salesTax;
 }
 public static void displayInfo()
 {
  JOptionPane.showMessageDialog(null, "Invoice#: " + invoiceNumber + 
	                              "\nSale Ammount: $" + ammountOfSale +
																"\nSales Tax: $" + salesTax);
 }
}

Can anyone tell me why the program will not calculate the sales tax? Thanks in advance.