public static long fact(long n) {
    if (n < 0) {
        throw new IllegalArgumentException("参数不能小于0");
    }
    if (n > 20) {
        throw new ArithmeticException("参数不能大于20");
    }
    long r = 1;
    for (long i = 1; i <= n; i++) {
        r = r * i;
    }
    return r;
}

@Test
public void testFact() {
    assertThrows(ArithmeticException.class, () -> fact(21L)); 
}