more understanding of java gui. #from analysis to design

More understanding of Java gui

package gui;
import javax.swing.*; //i am to use JOptionPane
public class Rectangle {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
         double width, length, area, perimeter;
         String lengthstr, widthstr, outputstr;
         
         lengthstr = JOptionPane.showInputDialog("Enter the Length:");
         length = Double.parseDouble(lengthstr);
         
         widthstr = JOptionPane.showInputDialog("Enter the Width");
         width = Double.parseDouble(widthstr);
         
         area = length * width;
         perimeter = 2 * (length + width);
         
         outputstr = "Length: "+ length + "\n" + 
                     "Width: " + width + "\n" +
        		     "Area: " + area +"square units\n" +
                     "Perimeter"+ perimeter +"units\n";
         
         JOptionPane.showMessageDialog(null, outputstr,
        		       "Rectangle",JOptionPane.INFORMATION_MESSAGE);
         
         System.exit(0);
	}

}