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);
        }
    }
}