First GUI that I made that computes figures.

MonlySalesTax.java

import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

/*
 * Name: Geoffrey Zoref
 * Date: 9/30/2018
 * Project: Monthly Sales Tax - GUI
 */



public class MonlySalesTax extends JFrame {

    private JPanel panel;                  // To reference a panel
    private JLabel messageLabel;           // To reference a label
    private JTextField costTextField;      // To reference a text field
    private JTextField markupTextField;
    private JButton calcButton;            // To reference a button

    private final int WINDOW_WIDTH = 340;  // Window width
    private final int WINDOW_HEIGHT = 100; // Window height

    public MonlySalesTax() {
        super("Retail Price Calculator");

        setSize(WINDOW_WIDTH, WINDOW_HEIGHT);

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        buildPanel();

        add(panel);

        setVisible(true);
    }

    private void buildPanel() {

        messageLabel = new JLabel("Enter wholesale cost of item.");
       // messageLabel = new JLabel("Enter markup by percentage");

        costTextField = new JTextField(10);
        markupTextField = new JTextField(10);

        calcButton = new JButton("Calculate");

        calcButton.addActionListener(new CalcButtonListener());

        panel = new JPanel();

        panel.add(messageLabel);
        panel.add(costTextField);
        panel.add(markupTextField);
        panel.add(calcButton);
    }

    private class CalcButtonListener implements ActionListener {

        public void actionPerformed(ActionEvent e) {

            String str1, str2;
            double cost = 0, markup = 0, retailCost = 0;

            str1 = costTextField.getText();
            str2 = markupTextField.getText();

            retailCost = (Double.parseDouble(str1) * Double.parseDouble(str2)/100) + Double.parseDouble(str1)  ;

            JOptionPane.showMessageDialog(null, "wholesale cost is $" + str1 + "\nMarkup is " + str2 + "%\nRetail cost is " + retailCost);

        }
    }

    public static void main(String[] args) {

        new MonlySalesTax();
    }
}


/**
 * Create a GUI application where the user enters the wholesale cost of an item and its markup percentage into text
 * fields. (For example, if an item’s wholesale cost is $5 and its markup percentage is 100%, then its retail price is
 * $10.) The application should have a button that displays the item’s retail price when clicked.
 */