[java]埃拉托斯特尼筛法检定素数

package test1.number;

public class Eratosthenes {
	public static void main(String[] args) {
		int max = 20; 
		try {
			max = Integer.parseInt(args[0]);
		} 
		catch (Exception e) {
		} 

		boolean[] isprime = new boolean[max + 1];

		for (int i = 0; i <= max; i++)
			isprime[i] = true;

		isprime[0] = isprime[1] = false;

		int n = (int) Math.ceil(Math.sqrt(max)); 

		for (int i = 0; i <= n; i++) {
			if (isprime[i]) 
				for (int j = 2 * i; j <= max; j = j + i)
					isprime[j] = false; 
		}

		int largest;
		for (largest = max; !isprime[largest]; largest--); 

		System.out.println("The largest prime less than or equal to " + max
				+ " is " + largest);
	}

}