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

public class RectangleProgram extends JFrame
{
    private static final int WIDTH = 400;
    private static final int HEIGHT = 200;

    private JLabel lengthL, widthL, areaL;
    private JTextField lengthTF, widthTF, areaTF;
    private JButton calculateB, exitB;

    //Button handlers:
    private CalculateButtonHandler cbHandler;
    private ExitButtonHandler ebHandler;

    public RectangleProgram()
    {
        lengthL = new JLabel("Enter the length: ", SwingConstants.RIGHT);
        widthL = new JLabel("Enter the width: ", SwingConstants.RIGHT);
        areaL = new JLabel("Area: ", SwingConstants.RIGHT);

        lengthTF = new JTextField(10);
        widthTF = new JTextField(10);
        areaTF = new JTextField(10);

        //SPecify handlers for each button and add (register) ActionListeners to each button.
        calculateB = new JButton("Calculate");
        cbHandler = new CalculateButtonHandler();
        calculateB.addActionListener(cbHandler);
        exitB = new JButton("Exit");
        ebHandler = new ExitButtonHandler();
        exitB.addActionListener(ebHandler);

        setTitle("Sample Title: Area of a Rectangle");
        Container pane = getContentPane();
        pane.setLayout(new GridLayout(4, 3));

        //Add things to the pane in the order you want them to appear (left to right, top to bottom)
        pane.add(lengthL);
        pane.add(lengthTF);
        pane.add(widthL);
        pane.add(widthTF);
        pane.add(areaL);
        pane.add(areaTF);
        pane.add(calculateB);
        pane.add(exitB);

        setSize(WIDTH, HEIGHT);
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    private class CalculateButtonHandler implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            double width, length, area;
            length = Double.parseDouble(lengthTF.getText()); //We use the getText & setText methods to manipulate the data entered into those fields.
            width = Double.parseDouble(widthTF.getText());
            area = length * width;
            areaTF.setText("" + area);

        }
    }
    public class ExitButtonHandler implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            System.exit(0);
        }
    }

    public static void main(String[] args)
    {
        RectangleProgram rectObj = new RectangleProgram();
    }
}