Sieve of Eratosthenes

SieveofEratosthenes.java

import java.util.LinkedList;

public class SieveOfEratosthenes
{
    private static final boolean PRIME = false;
    private static final boolean COMPOSITE = true;
    private boolean[] array;
    private int index;

    /*
     *  PUBLIC
     */

    /* constructor */
    public SieveOfEratosthenes(int upper_bound)
    {
        array = new boolean[upper_bound + 1];
        sieve();
    }

    /* returns ascending list of primes */
    public LinkedList<Integer> ascending()
    {
        LinkedList<Integer> list = new LinkedList<Integer>();

        for(Integer i = 0; i < array.length; i++) {
            if(!array[i])
                list.add(i);
        }
        return list;
    }

    /* returns descending list of primes */
    public LinkedList<Integer> descending()
    {
        LinkedList<Integer> list = new LinkedList<Integer>();

        for(Integer i = array.length - 1; i >= 0; i--) {
            if(!array[i])
                list.add(i);
        }
        return list;
    }

    /* tester function */
    public static void main(String[] args)
    {
    	/* input */
    	int upper_bound = 100;
        long tic, toc;
        if(args.length != 0)
            upper_bound = Integer.parseInt(args[0]);

        /* sieve */
        tic = System.nanoTime();
        SieveOfEratosthenes prime_sieve = new SieveOfEratosthenes(upper_bound);
        int num_primes = prime_sieve.ascending().size();
        toc = System.nanoTime();

        /* elapsed time */
        String elapsed = "";
        double time = 1.0 * (toc - tic) / 1E9;
        if(time > 1E-3) elapsed = Double.toString(time) + " sec.";
        else if(time > 1E-6) elapsed = Double.toString(time * 1E3) + " millisec.";

        /* output */
        System.out.println("Number of primes less than " + upper_bound + ": " + num_primes);
        System.out.println("Approx. elapsed time: " + elapsed);
    }

    /*
     *  PRIVATE
     */

    /* applies sieve */
    private void sieve()
    {
        index = 2;
        crossOutMultiples(index);
        
        while(nextPrime())
            crossOutMultiples(index);
    }

    /* crosses out all multiples of a given prime */
    private void crossOutMultiples(int prime)
    {
        int i = 2 * prime;

        while(i < array.length) {
            array[i] = COMPOSITE;
            i += prime;
        }
    }

    /* returns next prime (ascending) */
    private boolean nextPrime()
    {
        do {
            index++;
            if(index > (array.length - 1))
                return false;
        } while(array[index] == COMPOSITE);
	
        return true;
    }
}