Java程序字符串越界问题求助：如何实现重复运行？

问题根源 #

使用nextDouble()、nextInt()这类Scanner方法读取数值后，输入末尾的换行符会被留在输入缓冲区里。后续调用nextLine()时，它会直接读取这个换行符，得到空字符串，此时调用charAt(0)必然抛出StringIndexOutOfBoundsException。

修复方案 #

结合程序重复运行的需求，用do-while循环实现逻辑最合理，以下两种修复方式任选其一：

方案1：清除残留换行符 #

在读取完贷款年限后，额外调用一次keyboard.nextLine()吃掉缓冲区里的换行符，再读取用户的Y/N选择：

import java.util.Scanner;
import java.io.*;

public class LoanReport {

    public static void main(String[] args) throws IOException {
        double loanBalance;
        double interestRate;
        int loanYears;
        String answer;
        char again;

        Scanner keyboard = new Scanner(System.in);

        do {
            System.out.print("Enter the loan amount: ");
            loanBalance = keyboard.nextDouble();
            System.out.print("Enter the annual interest rate: ");
            interestRate = keyboard.nextDouble();
            System.out.print("Enter the years of the loan: ");
            loanYears = keyboard.nextInt();

            // 吃掉nextInt()留下的换行符，避免后续nextLine()读空
            keyboard.nextLine();

            Amortization am = new Amortization(loanBalance, interestRate, loanYears);
            am.saveReport("LoanAmortization.txt");
            System.out.println("Report saved to the file LoanAmortization.txt.");

            System.out.print("Would you like to run another report? Enter Y for yes or N for no: ");
            answer = keyboard.nextLine();
            // 处理用户直接回车的情况，默认不继续
            again = answer.isEmpty() ? 'n' : answer.charAt(0);
        } while (again == 'Y' || again == 'y');

        System.exit(0);
    }
}

方案2：用next()读取选择 #

next()方法会自动跳过所有空白字符（包括换行），直接读取有效输入，从根源避免空字符串问题：

import java.util.Scanner;
import java.io.*;

public class LoanReport {

    public static void main(String[] args) throws IOException {
        double loanBalance;
        double interestRate;
        int loanYears;
        String answer;
        char again;

        Scanner keyboard = new Scanner(System.in);

        do {
            System.out.print("Enter the loan amount: ");
            loanBalance = keyboard.nextDouble();
            System.out.print("Enter the annual interest rate: ");
            interestRate = keyboard.nextDouble();
            System.out.print("Enter the years of the loan: ");
            loanYears = keyboard.nextInt();

            Amortization am = new Amortization(loanBalance, interestRate, loanYears);
            am.saveReport("LoanAmortization.txt");
            System.out.println("Report saved to the file LoanAmortization.txt.");

            System.out.print("Would you like to run another report? Enter Y for yes or N for no: ");
            answer = keyboard.next();
            again = answer.charAt(0);
        } while (again == 'Y' || again == 'y');

        System.exit(0);
    }
}

补充说明 #

• 方案1里加了空字符串判断，防止用户直接按回车时再次触发越界异常，更健壮。
• do-while循环天生适合这种“至少执行一次，再根据条件重复”的场景，比你注释里的while循环更贴合需求。

内容的提问来源于stack exchange，提问作者ihateranch808