PrimeSieveOfEratosthenes.java

import java.util.Scanner;

public class PrimeSieveOfEratosthenes {

    /**
    *   Program:    PrimeSieveOfEratosthenes.java
    *   Purpose:    Find all primes up to a number using Eratosthenes' Sieve.
    *   Creator:    Chris Clarke, author of books "50 Java Program Source Codes"
                and "50 More Java Source Codes".
                Each contains brand new material.
                Available now in e-book and paperback.
    *   Created:    21.03.2014
    */

    public static void main(String[] args) {

        int number = 0;

        if (args.length>0) {
            try {
                number = Integer.parseInt(args[0]);
                runSieve(number);
            } catch (NumberFormatException e) {
                System.out.println("That\'s not an integer!");
            } // end try/catch
        } else {
            Scanner scan = new Scanner(System.in);
            System.out.print("Enter a number between 1 & 100: ");
            number = scan.nextInt();
            runSieve(number);
        } // end if
    } // end main()

    public static void runSieve(int upperBound) {

        int sqrt = (int) Math.sqrt(upperBound); // square root

        // boolean array of non-primes, to begin with they're all false
        boolean[] isComposite = new boolean[upperBound + 1];

        // find primes up to square root of upper bound
        for (int m = 2; m <= sqrt; m++) {
            if (!isComposite[m]) { // if prime
                // print prime number
                System.out.print(m + " ");

                // mark every multiple of m as non-prime
                for (int k = m * 2; k <= upperBound; k += m) {
                    // mark as not prime
                    isComposite[k] = true;
                }//end for k
            }//end if
        }//end for m

        for (int m = sqrt+1; m <= upperBound; m++) {
            if (!isComposite[m]) { // if prime
                // print prime number
                System.out.print(m + " ");
            }//end if
        }//end for m
    }//end runSieve()

}//end class PrimeSieveOfEratosthenes