IndexOutOfBoundsException on List

Why This Happens

ArrayList and other List implementations throw IndexOutOfBoundsException when you call get() with an index that is negative or greater than or equal to the list size. Unlike arrays, lists use this parent exception class.

The Problem

List<String> items = List.of("a", "b");
String third = items.get(3); // IndexOutOfBoundsException

The Fix

List<String> items = List.of("a", "b");
if (items.size() > 3) {
    String third = items.get(3);
}

Step-by-Step Fix

1. 1

   Identify the bad index

   Read the exception to see the requested index and the actual list size.

2. 2

   Trace the index source

   Find where the index value comes from and why it exceeds the list size.

3. 3

   Add bounds checking

   Check list.size() before calling get(), or use an iterator or for-each loop instead of index-based access.