使用子字符串时的Java StringIndexOutOfBounds

Question

我得到了一个StringIndexOutOfBounds错误与这个Java程序行：

String num3 = lottoString.substring(2,2);

告诉我，2超出了范围，但是这个代码应该随机选择一个三位数的彩票号码，从000到999不等。我的错误是什么？

import java.util.Scanner;
public class Lottery
{
    public static void main(String[] args)
    {
        //Declare and initialize variables and objects
        Scanner input = new Scanner(System.in);
        String lottoString = "";

        //Generate a 3-digit "lottery" number composed of random numbers
        //Simulate a lottery by drawing one number at a time and 
        //concatenating it to the string
        //Identify the repeated steps and use a for loop structure
        for(int randomGen=0; randomGen < 3; randomGen++){
            int lotNums = (int)(Math.random()*10);
            lottoString = Integer.toString(lotNums);
        }

        String num1 = lottoString.substring(0,0);
        String num2 = lottoString.substring(1,1);
        String num3 = lottoString.substring(2,2);

        String num12 = num1 + num2;
        String num23 = num2 + num3;
        String num123 = num1 + num2 + num3;

        //Input: Ask user to guess 3 digit number
        System.out.println("Please enter your three numbers (e.g. 123): ");
        String userGuess = input.next();

        //Compare the user's guess to the lottery number and report results
        if(userGuess.equals(num123)){
            System.out.println("Winner: " + num123);
            System.out.println("Congratulations, both pairs matched!");
        }else if(userGuess.substring(0,2).equals(num12)){
            System.out.println("Winner: " + num123);
            System.out.println("Congratulations, the front pair matched!");
        }else if(userGuess.substring(1,3).equals(num23)){
            System.out.println("Winner: " + num123);
            System.out.println("Congratulations, the end pair matched!");
        }else{
            System.out.println("Winner: " + num123);
            System.out.println("Sorry, no matches! You only had one chance out of 100 to win anyway.");
        }
    }
}

Answer 1

正如在另一个答案中提到的，每次迭代循环时，都会将lottoString的值重置为一位数。你需要加在上面，就像这样：

lottoString += Integer.toString(lotNums);

另一个问题是使用substring方法。如果两个索引位置相同，如0,0，则返回一个空字符串。你想要的是：

String num1 = lottoString.substring(0,1);
String num2 = lottoString.substring(1,2);
String num3 = lottoString.substring(2,3);

Answer 2

for(int randomGen=0; randomGen < 3; randomGen++){
    int lotNums = (int)(Math.random()*10);
    lottoString = Integer.toString(lotNums);
}

您将Integer.toString()的结果分配给lottoString。lotNums是介于0到9之间的一个数字。

我想你想

lottoString += Integer.toString(lotNums);