Prime.java

Below is the syntax highlighted version of Prime.java from §1.3 Conditionals and Loops.

/******************************************************************************
 *  Compilation:  javac Primes.java
 *  Execution:    java Primes n
 *
 *  Determines whether or not n is prime.
 *
 *   % java Prime 81
 *   81 is not prime
 *
 *   % java Prime 17
 *   17 is prime
 *
 *   % java Prime 30864324691489
 *   30864324691489 is not prime
 *
 *
 ******************************************************************************/

public class Prime {

    public static void main(String[] args) {
        long n = Long.parseLong(args[0]);
        boolean isPrime = true;
        if (n < 2) isPrime = false;

        // try all possible factors of n
        // if if n has a factor, then it has one less than or equal to sqrt(n),
        // so for efficiency we only need to check factor <= sqrt(n) or
        // equivalently factor*factor <= n
        for (long factor = 2; factor*factor <= n; factor++) {

            // if factor divides evenly into n, n is not prime, so break out of loop
            if (n % factor == 0) {
                isPrime = false;
                break;
            }
        }

        // print out whether n is prime
        if (isPrime) System.out.println(n + " is prime");
        else         System.out.println(n + " is not prime");
    }
}