如何避免long溢出？Java阶乘作业的BigInteger使用疑问

问题说明 #

作业要求实现程序：让用户输入数字并打印其阶乘；禁止输入负数或大于25的数字，输入不合规则则循环重新输入。原代码使用long存储数据，但输入20-25之间的数时会因long溢出导致结果变为负数。尝试改用BigInteger解决溢出，但所有涉及BigInteger的代码均报错，原代码如下：

boolean correctInputn= false;
    
while(!correctInputn)
{
    long number;// declares variables for storing number
    long factorial = 1;// declare variable for storing factorial
    

    System.out.println("Enter a number between 1 and 25"); // tells user to enter number
    number = scanner.nextLong();
    
    if (number <0)  
    {
        System.out.println("Positive numbers only");// if number entered is negative
        correctInputn = false; 
        continue;
    } // if user enters number less than 0 loops back to code start
   
    else if (number > 25)
    { 
        System.out.println("Number to large to print");
        correctInputn = false; 
        continue;
    } // if user enters number over 25 loops back to code start
    
    else  {
        // if user enter 10, counter starts at 10 and runs to two
        for(long mynumber = number; mynumber >= 1; mynumber--) {
            factorial = factorial*mynumber; // mynumber would contain different values and that is multiplied by value present in factorial value and stored again in factorial variable
        }
    
        System.out.println("The factorial of " + number +" is equal to " + factorial);
        break;
    }
}

解决方案 #

BigInteger是Java的大数处理类，不能像基本类型那样直接用=赋值、*运算，必须使用它提供的方法操作。以下是修改后的正确代码：

import java.math.BigInteger;
import java.util.Scanner;

public class Factorial {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        boolean correctInputn = false;
        
        while (!correctInputn) {
            long number;
            // 用BigInteger初始化阶乘为1
            BigInteger factorial = BigInteger.ONE;

            System.out.println("请输入1到25之间的数字");
            number = scanner.nextLong();

            if (number < 0) {
                System.out.println("只能输入正数");
                continue;
            } else if (number > 25) {
                System.out.println("数字过大，无法计算");
                continue;
            } else {
                for (long mynumber = number; mynumber >= 1; mynumber--) {
                    // 用multiply方法做乘法，将long转成BigInteger参与运算
                    factorial = factorial.multiply(BigInteger.valueOf(mynumber));
                }
                System.out.println(number + "的阶乘是：" + factorial);
                correctInputn = true; // 标记输入正确，退出循环
            }
        }
        scanner.close();
    }
}

关键修改点 #

• 导入包：必须导入java.math.BigInteger才能使用该类
• 初始化：BigInteger不能用1初始化，要使用常量BigInteger.ONE
• 运算方式：不能用*做乘法，必须调用multiply()方法
• 类型转换：将循环中的long类型变量通过BigInteger.valueOf()转成BigInteger才能参与运算
• 循环标记：原代码中correctInputn在合法输入后未设为true，补充后逻辑更严谨

内容的提问来源于stack exchange，提问作者Lau