GST Calculator ( Intrastate / Interstate ), depends on GST.java

Shop.java

import javax.swing.*;
public class Shop {
    JFrame f;

    void ShopSrc() {
        GST call = new GST();
        f = new JFrame();


        String n1 = JOptionPane.showInputDialog(f, "Enter price of product.");
        double pr = Double.parseDouble(n1);

        String n2 = JOptionPane.showInputDialog(f, "Enter quantity of product.");
        double qt = Double.parseDouble(n2);

        double tc = pr * qt;

        String n3 = JOptionPane.showInputDialog(f, "Input a for intra - state and b for inter state transaction.");
        char op = n3.charAt(0);

        if (n3.charAt(0) == 'a') {
            String n4 = JOptionPane.showInputDialog(f, "Please input SGST rate in numbers.");
            double a = Double.parseDouble(n4);
            JOptionPane.showMessageDialog(f, "Price of Product = Rs." + pr + "\nQuantity of Product = " + qt + "\nCost before GST addition = Rs." + tc);
            call.main(tc, op, a);
        } else if (n3.charAt(0) == 'b') {
            String n5 = JOptionPane.showInputDialog(f, "Please input IGST rate in numbers.");
            double b = Double.parseDouble(n5);
            JOptionPane.showMessageDialog(f, "Price of Product = Rs." + pr + "\nQuantity of Product = " + qt + "\nCost before GST addition = Rs." + tc);
            call.main(tc, op, b);
        } else {
            JOptionPane.showMessageDialog(f, "Input proper GST rate in numbers only!");
        }
    }

    public static void main(String[] args) {
        Shop obj = new Shop();
        obj.ShopSrc();
    }
}