firstGUI

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

public class FirstWindow extends JFrame{
    private JLabel lblQuantity;
    private JLabel lblPrice;
    private JTextField txtQuantity;
    private JTextField txtPrice;
    private JTextField txtResult;
    private JButton btnCalculate;
    private JPanel panel;

    public FirstWindow()
    {
        super("First Application!!!");
        setSize(250,300);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);
        buildPanel();
        add(panel);
        //pack();
    }

    public void buildPanel()
    {
        panel = new JPanel();
        lblQuantity = new JLabel("Enter a quantity");
        lblPrice = new JLabel("Enter a price");
        txtQuantity = new JTextField("test", 15);
        txtPrice = new JTextField("test", 15);
        btnCalculate = new JButton("Calculate!");
        txtResult = new JTextField(10);
        panel.add(lblQuantity);
        panel.add(txtQuantity);
        panel.add(lblPrice);
        panel.add(txtPrice);
        panel.add(txtResult);
        panel.add(btnCalculate);
        btnCalculate.addActionListener(new CalcButtonListener());
    }

    private class CalcButtonListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            String strQuantity = txtQuantity.getText();
            String strPrice = txtPrice.getText();
            double result = Double.parseDouble(strQuantity) * Double.parseDouble(strPrice);
            txtResult.setText(result + "");

        }
    }
}