运行Java代码时遭遇java.lang.StringIndexOutOfBoundsException，如何修复？

Hey there! Let's break down why you're hitting this exception and fix your code step by step.

1. The Direct Cause of the Exception #

The immediate issue throwing StringIndexOutOfBoundsException is this line:

System.out.println(sb.deleteCharAt(3));

You initialized StringBuilder sb = new StringBuilder(); which creates an empty builder (length = 0). Trying to delete a character at index 3 is impossible—valid indices for an empty builder only range from 0 to -1 (which doesn't exist).

To fix this:

• Remove this line if it's leftover test code, or
• Initialize sb with a string first (e.g., StringBuilder sb = new StringBuilder(str);) and ensure the index you target is between 0 and sb.length() - 1.

2. Other Critical Issues in Your Code #

Your code is incomplete and has a few bugs that will cause unexpected behavior:

• Incorrect loop ranges:
  Your outer loop for (int i = str.length() - 1; i > 0; i--) skips the first character (index 0). The inner loop j > 0 does the same. Change these conditions to i >= 0 and j >= 0 to check all characters.
• Unfinished logic: The line sb.d... cuts off—if you're trying to track duplicates or build a result string, you'll need to complete this code block.
• Static variable misuse: Using a static Occurence variable for counting is risky; if you run this method multiple times, the value won't reset. Use a local variable or Map<Character, Integer> instead for per-string counting.

Example Fixed Code (For Character Occurrence Tracking) #

If your goal is to count how many times each character appears in the input string, here's a cleaned-up version:

import java.util.Scanner;
import java.util.HashMap;
import java.util.Map;

class CharacterCounter {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.next();
        Map<Character, Integer> occurrenceMap = new HashMap<>();

        // Count each character's occurrence
        for (int i = 0; i < str.length(); i++) {
            char c = str.charAt(i);
            occurrenceMap.put(c, occurrenceMap.getOrDefault(c, 0) + 1);
        }

        // Print results
        for (Map.Entry<Character, Integer> entry : occurrenceMap.entrySet()) {
            System.out.printf("Character '%c' occurs %d times%n", entry.getKey(), entry.getValue());
        }

        sc.close();
    }
}

Key Takeaways #

• Always verify that the index you're accessing in a String or StringBuilder falls within 0 <= index < length.
• Avoid static variables for per-operation state unless you explicitly need shared data.
• Ensure loop conditions cover all elements you intend to process.

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