得到java.lang.StringIndexOutOfBoundsException错误

问题描述

而我在做一个简单的密码程序。我遇到了这个错误

while i am doing a simple cipher program. i was encounter this error

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -1
at java.lang.String.charAt(Unknown Source)
at Caesar.main(Caesar.java:27)

好吧，我不清楚原因是什么。我需要一些资深人士的帮助，@ @下面是我的代码。

well, i am not quite clear on what is the cause. i require some help from some veteran here @@ below are my code.

import java.util.Scanner;
import java.io.FileReader;
import java.io.IOException;
import java.io.PrintWriter;

public class Caesar {

    public static void main(String[] args){
         String from = "abcdefghijklmnopqrstuvwxyz";
         String to   = "feathrzyxwvusqponmlkjigdcb";
            Scanner console = new Scanner(System.in);
            System.out.print("Input file: ");
            String inputFileName = console.next();
            System.out.print("Output file: ");
        String outputFileName = console.next();

        try{ 
            FileReader reader = new FileReader("C:/"+inputFileName+".txt");
            Scanner in = new Scanner(reader);
            PrintWriter out = new PrintWriter("C:/"+outputFileName+".txt");

                while (in.hasNextLine()){
                    String line = in.nextLine();
                    String outPutText = "";
                    for (int i = 0; i < line.length(); i++){
                        char c = to.charAt(from.indexOf(line.charAt(i)));
                        outPutText += c;
                    }
                    System.out.println("Plaintext: " + line);
                    System.out.println("Ciphertext: " + outPutText);
                    out.println(outPutText);         
                }
                System.out.println("Processing file complete");
                out.close();
        }
        catch (IOException exception){ 
            System.out.println("Error processing file:" + exception);
        }
}
}

推荐答案

这是你在 for循环中的作业： -

char c = to.charAt(from.indexOf(line.charAt(i)));

此处， indexOf 返回 -1 当在中找不到 char 时，从字符串开始，然后它将抛出 StringIndexOutOfBoundsException 。

Here, in indexOf returns -1 when the char is not found in from string, and then it will throw an StringIndexOutOfBoundsException.

您可以在获取字符之前添加一个检查： -

You can add a check before fetching the character: -

int index = from.indexOf(line.charAt(i));

if (index >= 0) {
    char c = to.charAt(index);
    outPutText += c;
}

或： -

char ch = line.charAt(i);

if (from.contains(ch)) {
    char c = to.charAt(from.indexOf(ch));
    outPutText += c;
} 

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