2.1： 判断一个正整数是否为质数的算法。

import java.util.Scanner;

public class EXP2_1 {
	public int isPrime(long a) {
		if (a == 1)
			return 0;
		for (int i = 2; i <= Math.sqrt(a); i++) {
			if (a % i == 0)
				return 0;
		}
		return 1;
	}

	public static void main(String[] args) {
		EXP2_1 exp = new EXP2_1();
		System.out.println("输入一个数判断其是否是质数：");
		Scanner s = new Scanner(System.in);
		long a = s.nextLong();
		System.out.println(exp.isPrime(a));
	}
}
import java.util.Scanner;

public class EXP