如题：编写一道JAVA阶成程序

Question

编写应用程序求1!+2!+…+20!的和

Answer 1

public static void main(String[] args) {
int sum = 0;
int subSum = 1;
for (int i = 1; i <= 20; i++) {
subSum *= i;
System.out.println(i + "的阶乘: " + subSum);

sum += subSum;
}

System.out.println("总和为: " + sum);
}

运行结果:
1的阶乘: 1
2的阶乘: 2
3的阶乘: 6
4的阶乘: 24
5的阶乘: 120
6的阶乘: 720
7的阶乘: 5040
8的阶乘: 40320
9的阶乘: 362880
10的阶乘: 3628800
11的阶乘: 39916800
12的阶乘: 479001600
13的阶乘: 1932053504
14的阶乘: 1278945280
15的阶乘: 2004310016
16的阶乘: 2004189184
17的阶乘: -288522240
18的阶乘: -898433024
19的阶乘: 109641728
20的阶乘: -2102132736
总和为: 268040729

请注意,从 "17的阶乘: -288522240" 开始,后面阶乘已经超出整数最值了,所以需要改成使用long类型来保存.

正确程序如下:
public static void main(String[] args) {
long sum = 0;
long subSum = 1;
for (long i = 1; i <= 20; i++) {
subSum *= i;
System.out.println(i + "的阶乘: " + subSum);

sum += subSum;
}

System.out.println("总和为: " + sum);
}

正确程序结果:
1的阶乘: 1
2的阶乘: 2
3的阶乘: 6
4的阶乘: 24
5的阶乘: 120
6的阶乘: 720
7的阶乘: 5040
8的阶乘: 40320
9的阶乘: 362880
10的阶乘: 3628800
11的阶乘: 39916800
12的阶乘: 479001600
13的阶乘: 6227020800
14的阶乘: 87178291200
15的阶乘: 1307674368000
16的阶乘: 20922789888000
17的阶乘: 355687428096000
18的阶乘: 6402373705728000
19的阶乘: 121645100408832000
20的阶乘: 2432902008176640000
总和为: 2561327494111820313

Answer 2

看下谁的算法比我的简单高效，呵呵

public class FacCalculator {

public static void main(String args[]) {

long sum = 0L;

long previous = 1L;

for (int i = 1; i <= 20; i++) {
previous = previous * i;
sum += previous;
}

System.out.println("1!+2!+3!+...+18!+19!+20! = " + sum);
}
}

-------------
1!+2!+3!+...+18!+19!+20! = 2561327494111820313

Answer 3

//Main.java 测试类
public class Main {

public static void main(String[] args) {
Dog dog = new Dog();
Cat cat = new Cat();
dog.cry();
cat.cry();
}
}

//抽象类
abstract class Animal {
public abstract void cry();
//如果不马上实现cry()方法，请标记abstract
}

class Cat extends Animal{

@Override
public void cry() {
System.out.println("小猫喵喵叫");
}
}

class Dog extends Animal{

@Override
public void cry() {
System.out.println("小狗汪汪叫");
}
}