Index Out of Bounds / IndexError — Java, Python & C# Fix Guide

What Does This Error Mean?

You tried to access an array/list element at a position that doesn't exist. Arrays are zero-indexed, so an array of length 5 has valid indices 0–4. Accessing index 5 (or -1) throws this error.

Error Names by Language

• Java: ArrayIndexOutOfBoundsException / IndexOutOfBoundsException
• Python: IndexError: list index out of range
• C#: IndexOutOfRangeException
• JavaScript: Returns undefined (no error!) — which causes TypeErrors later

Common Causes

• Off-by-one error in a loop: for (i = 0; i <= arr.length; i++) should be <
• Empty array/collection: accessing [0] when list is empty
• Assuming data exists: API returned fewer items than expected

How to Fix It

1. Check length before access

// Java
if (index >= 0 && index < list.size()) {
    return list.get(index);
}

# Python
if 0 <= index < len(my_list):
    return my_list[index]

2. Fix loop boundaries

// Wrong: i <= length (accesses one past end)
for (int i = 0; i < arr.length; i++) { ... }

3. Use safe access patterns

# Python: use .get() or slicing
value = my_list[index] if index < len(my_list) else default

// Java: Optional pattern
Optional.ofNullable(list.size() > i ? list.get(i) : null)