Complex Number GUI

import javax.swing.JOptionPane;

public class ComplexGUI {

    private final static String TITLE = "Complex Number Program";
    private final static String CONTINUE_PROMPT = "Do this again?";

    // **********************************************

    private static void testComplex(Complex c1, Complex c2) {
        Complex plus = c1.add(c2);
        Complex minus = c1.subtract(c2);
        Complex times = c1.multiply(c2);
        Complex quot = c1.divide(c2);

        String output = "";
        output += String.format("c1      = " + c1 + "\n");
        output += String.format("c2      = " + c2 + "\n");
        output += String.format("c1 + c2 = " + plus + "\n");
        output += String.format("c1 - c2 = " + minus + "\n");
        output += String.format("c1 * c2 = " + times + "\n");
        output += String.format("c1 / c2 = " + quot + "\n");
        output += String.format("-c1     = " + c1.negate() + "\n");
        output += String.format("-c2     = " + c2.negate() + "\n");
        output += String.format("Conj c1 = " + c1.conjugate() + "\n");
        output += String.format("Conj c2 = " + c2.conjugate() + "\n");
        output += String.format("|c1|    = " + c1.abs() + "\n");
        output += String.format("|c2|    = " + c2.abs() + "\n");
        output += String.format("Dist    = " + c1.distance(c2) + "\n");
        output += String.format("==?     = " + c1.equals(c2) + "\n");
        output += String.format(">?      = " + c1.greaterThan(c2) + "\n");
        output += String.format("<?      = " + c1.lessThan(c2) + "\n");

        JOptionPane.showMessageDialog(null, output, TITLE, JOptionPane.INFORMATION_MESSAGE, null);
    }


    // **********************************************

    private static Complex buildComplex(String prompt) {
        Complex comp = null;

        ComplexDriverPanel cip = new ComplexDriverPanel();
        int result = JOptionPane.showConfirmDialog(null, cip, "Please Enter Values For " + prompt,
                JOptionPane.OK_CANCEL_OPTION);
        if (result == JOptionPane.OK_OPTION) {
            double real = cip.getReal();
            double imag = cip.getImag();
            comp = new Complex(real, imag);
        } else {
            System.exit(1);
        }
        return comp;
    }

    // **********************************************

    private static void process(String args[]) {
        Complex c1 = buildComplex("Complex Number 1");
        Complex c2 = buildComplex("Complex Number 2");
        testComplex(c1, c2);
    }


    // **********************************************

    private static boolean doThisAgain(String prompt) {
        int option = JOptionPane.showConfirmDialog(null, prompt, TITLE, JOptionPane.YES_NO_OPTION,
                JOptionPane.QUESTION_MESSAGE);
        return option == JOptionPane.YES_OPTION;
    }

    // **********************************************

    public static void main(String args[]) {
        do {
            process(args);
        } while (doThisAgain(CONTINUE_PROMPT));
    }

}