Java in a Nutshell, 5th Edition

An Example Program

Example 1-1 shows a Java program to compute factorials.^[3] Note that the numbers at the beginning of each line are not part of the program; they are there for ease of reference when we dissect the program line-by-line.

Example 1-1. Factorial.java: a program to compute factorials

 1 /**
 2  * This program computes the factorial of a number
 3  */
 4 public class Factorial {                   // Define a class
 5   public static void main(String[] args) { // The program starts here
 6     int input = Integer.parseInt(args[0]); // Get the user's input
 7     double result = factorial(input);      // Compute the factorial
 8     System.out.println(result);            // Print out the result
 9   }                                        // The main() method ends here
10 
11   public static double factorial(int x) {  // This method computes x!
12     if (x < 0)                             // Check for bad input
13       return 0.0;                          // If bad, return 0
14     double fact = 1.0;                     // Begin with an initial value
15     while(x > 1) {                         // Loop until x equals 1
16       fact = fact * x;                     // Multiply by x each time
17       x = x - 1;                           // And then decrement x
18     }                                      // Jump back to start of loop
19     return fact;                           // Return the result
20   }                                        // factorial() ends here
21 }                                          // The class ends here

Compiling and Running the Program

Before we look at how the program works, we must first discuss how to run it. In order to compile and run the program, you need a Java development kit (JDK) of some sort. Sun Microsystems created the Java language and ships a free JDK for its Solaris operating system and also for Linux and Microsoft Windows platforms. ...