实现字符串反转时触发String index out of range: -1错误，求助排查

Hey there! Let's break down why you're seeing that frustrating String index out of range: -1 error when trying to reverse your string. That error pops up when your code tries to access a string index that doesn't exist—and since string indices start at 0, -1 is always invalid.

What's Causing the Issue? #

From the code snippet you shared, the most likely culprit is not handling empty input and potentially letting your loop run past the valid index range. Here's the scenario:

• If a user enters an empty string (or IO.readString() returns an empty string), str.length() becomes 0. When you calculate str.length() -1, that gives you -1. Trying to call str.charAt(-1) will immediately throw that index out of range error.
• Even with non-empty strings, if your loop condition doesn't stop when the index hits 0, you might end up trying to access index -1 in the final iteration.

Corrected Full Code #

Here's a complete, fixed version of your code with explanations:

public class april{ 
    public static void main(String [] args){
        System.out.println("What do you want backwards?");
        System.out.println("Your new word is " + reverse(IO.readString())); 
    } 

    public static String reverse(String str) {
        // First, handle empty or null input to avoid index errors
        if (str == null || str.isEmpty()) {
            return str;
        }

        // Create a char array to hold reversed characters
        char[] reversedChars = new char[str.length()];
        int originalIndex = str.length() - 1; // Start at the last valid character index
        int reversedIndex = 0; // Start at the beginning of the array

        // Loop only while we have valid indices to access
        while (originalIndex >= 0) {
            reversedChars[reversedIndex] = str.charAt(originalIndex);
            originalIndex--;
            reversedIndex++;
        }

        // Convert the char array back to a String
        return new String(reversedChars);
    }
}

Key Fixes & Explanations #

• Defensive Input Check: The if (str == null || str.isEmpty()) check ensures we don't try to process a string with no characters, which would lead to that problematic -1 index calculation.
• Valid Index Boundaries: We start originalIndex at str.length() -1 (the last valid character in the string) and stop the loop when originalIndex hits 0—so we never try to access index -1.
• Clear Character Placement: We populate the char array from the start using reversedIndex, which lets us build the reversed string correctly without gaps or overflows.

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