java.lang.ArrayIndexOutOfBoundsException

java.lang.ArrayIndexOutOfBoundsException in Java means your Java code attempted to access an array with an illegal index that is either negative or greater than or equal to the array length.

JavaPattern: Off-by-one errors

How do I fix java.lang.ArrayIndexOutOfBoundsException?

To fix java.lang.ArrayIndexOutOfBoundsException in Java, locate the line causing the error and check the values right before it runs. Most occurrences happen because of using <= arr.length in a standard for-loop condition. Apply a defensive check or safe fallback, make sure collections or objects exist before access, and verify return values from upstream functions.

1. Inspect bounds & typesPrint or inspect collection sizes and verify variables are not empty or None/undefined.

2. Guard with safe accessUse optional chaining (?.), dictionary get(), or guard conditions before dereferencing.

3. Trace upstream originsFind where the value was instantiated or returned rather than only patching the symptom.

What does java.lang.ArrayIndexOutOfBoundsException mean?

The Java Virtual Machine (JVM) checks array boundaries on every indexing operation. When code accesses arr[i] where i < 0 or i >= arr.length, the JVM throws ArrayIndexOutOfBoundsException at runtime.

Why does java.lang.ArrayIndexOutOfBoundsException happen?

1. Using <= arr.length in a standard for-loop condition

An array of size N has indices 0 through N-1. When i equals arr.length, the final iteration crashes.

Breaks

for (int i = 0; i <= arr.length; i++) {
    System.out.println(arr[i]); // crashes on last pass!
}

Works

for (int i = 0; i < arr.length; i++) {
    System.out.println(arr[i]);
}

2. Accessing index 0 on an empty array

If the array has length 0, even index 0 is out of bounds.

Breaks

int first = numbers[0]; // crashes if numbers is empty!

Works

if (numbers.length > 0) {
    int first = numbers[0];
}

How to find it in your own code

Ensure loop conditions use strict inequality i < arr.length. Use the enhanced for-loop (for (int x : arr)) when indices are not strictly required. Verify array length > 0 before accessing fixed indices.

Try it: fix this bug

easy

java.lang.StringIndexOutOfBoundsException: String index out of range: 5

public class Main {
    public static String extractPrefixSafely(String text) {
        // substring throws StringIndexOutOfBoundsException if endIndex > length
        return text.substring(0, 5);
    }

    public static void main(String[] args) {
        System.out.println(extractPrefixSafely("abc"));
    }
}

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