ArrayIndexOutOfBoundsException

Why This Happens

Java arrays are zero-indexed, so an array of length 5 has valid indices 0 through 4. Accessing index 5 or any negative index throws this exception. This commonly happens in off-by-one errors in loops.

The Problem

int[] arr = {1, 2, 3, 4, 5};
for (int i = 0; i <= arr.length; i++) {
    System.out.println(arr[i]); // Fails when i == 5
}

The Fix

int[] arr = {1, 2, 3, 4, 5};
for (int i = 0; i < arr.length; i++) {
    System.out.println(arr[i]);
}

Step-by-Step Fix

1. 1

   Identify the out-of-bounds index

   Read the exception message to see which index was used and what the array length is.

2. 2

   Check loop boundaries

   Look for off-by-one errors. Use < arr.length instead of <= arr.length in loop conditions.

3. 3

   Validate index before access

   Add bounds checking before accessing the array, especially if the index comes from user input or a calculation.