怎样用java实现以上程序

以下是用Java分别实现上述四个问题的简单示例：

1. 寻找1-1000内能被3和7整除的数:

public class Main {
    public static void main(String[] args) {
        for (int i = 1; i <= 1000; i++) {
            if (i % 21 == 0) {
                System.out.println(i);
            }
        }
    }
}

2. 计算阶乘之和 (利用循环):

public int factorialSum(int n) {
    long sum = 0;
    for (int i = 1; i <= n; i++) {
        sum += factorial(i);
    }
    return (int) sum; // 注意长整型溢出，这里转换为int类型
}

private long factorial(int n) {
    if (n == 0 || n == 1) return 1;
    return n * factorial(n - 1);
}

3. 检查奇数个数:

public int countOddNumbers(int[] numbers) {
    int oddCount = 0;
    for (int num : numbers) {
        if (num != 0 && num != -1 && num % 2 != 0) {
            oddCount++;
        } else {
            break;
        }
    }
    return oddCount;
}

4. 判断素数:

public boolean isPrime(int n) {
    if (n <= 1) return false;
    if (n <= 3) return true;
    if (n % 2 == 0 || n % 3 == 0) return false;
    for (int i = 5; i * i <= n; i += 6) {
        if (n % i == 0 || n % (i + 2) == 0) {
            return false;
        }
    }
    return true;
}