计算阶乘

package com.jdk7.chapter1;

public class Factorial {
    /**
     * 计算n!的值，利用公式n×(n-1)×(n-2)×(n-3)×...×3×2×1
     * 注：当n大于17时n!会超出long的取值范围
     */
    public long getFactorial(int n){
        if(n<0 || n>17){
            System.out.println("n的取值区间为[0,17]");
            return -1;
        }else if(n==0){
            return 1;
        }else{
            long result = 1;
            for(;n>0;n--){
                result *=n;
            }
            return result;
        }
        
    }
    
    public static void main(String[] args) {
        Factorial f = new Factorial();
        System.out.println(f.getFactorial(4));
        System.out.println(f.getFactorial(17));
        System.out.println(f.getFactorial(18));
        System.out.println(f.getFactorial(0));
        System.out.println(f.getFactorial(-2));
    }

}

执行结果：
24
355687428096000
n的取值区间为[0,17]
-1
1
n的取值区间为[0,17]
-1