INTRODUCTION TO OBJECTS AND INPUT/OUTPUT - Java Programming: From Problem Analysis to Program Design - page 144

Note that this statement is needed only for programs that have GUI components such as
input/output dialog boxes.
Example 3-12 shows a program that calculates the area and circumference of a circle and
uses input/output dialog boxes.

EXAMPLE 3-12

The following program prompts the user to enter the radius of a circle. The program
then outputs the circle's radius, area, and circumference. The class Math defines the
named constant PI (p), which is PI ¼ 3.141592653589793 . We will use this value to
find the area and circumference. (Note that to use this value, we use the expression
Math.PI .)

//Program to determine the area and circumference of a circle

import javax.swing.JOptionPane;

public class AreaAndCircumferenceProgram
{

public static void main(String[] args)
{

double radius; //Line 1

double area; //Line 2

double circumference; //Line 3

String radiusString; //Line 4

String outputStr; //Line 5

radiusString =
JOptionPane.showInputDialog
("Enter the radius: "); //Line 6

radius = Double.parseDouble(radiusString); //Line 7

area = Math.PI * radius * radius; //Line 8

circumference = 2 * Math.PI * radius; //Line 9

outputStr = "Radius: " + radius + "\n" +
"Area: " + area + " square units\n" +
"Circumference: " + circumference +
" units"; //Line 10

JOptionPane.showMessageDialog( null , outputStr,
"Circle",
JOptionPane.INFORMATION_MESSAGE); //Line 11

System.exit(0); //Line 12

}

}