java求素数

• 按定义

即除了1和它本身以外不再被其他的除数整数

public static void main(String[] args) {
	for (int i = 2; i < 100; i++) {
		for (int j = 2; j <= i; j++) {
			if (i==j) {
				System.out.println(i);
			}else if (i%j==0) {
				break;
			}
		}
	}
}

• 埃氏筛法

先去掉2的倍数，再去掉3的倍数，再去掉5的倍数，……依此类推，把不大√n的所有素数的倍数剔除，最后剩下的就是素数。代码参考自https://www.cnblogs.com/lsucre/p/6599406.html

public static void main(String[] args) {
	//赋值
	int num = 100;
	boolean[] judge = new boolean[num];
	//judge[0] = false;
	//judge[1] = false;
	for (int i = 2; i < judge.length; i++) {
		judge[i] = true;
	}
	//埃氏筛法
	for (int i = 2; i < Math.sqrt(num); i++) {
		if (judge[i] == true) {
			for (int j = i; j*i < num; j++) {
				judge[j*i] = false;
			}
		}
	}
	//输出
	for (int i = 2; i < num; i++) {
		if (judge[i] == true) {
			System.out.println(i);
		}
	}
}