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What it looks like

java.lang.IndexOutOfBoundsException: Index 3 out of bounds for length 3

Extends RuntimeException in java.lang.

What it means

List implementations such as ArrayList throw IndexOutOfBoundsException from get, set, add(index, e) and remove(index) when the index is negative or not smaller than the size. It is the parent of ArrayIndexOutOfBoundsException and StringIndexOutOfBoundsException. An empty list is the most common trigger, because index 0 is already out of range.

Common causes

• Calling list.get(0) on a list that is empty, for example a query that returned no rows.
• Removing elements inside a loop that counts up, which shifts the remaining indexes.
• Using the size as an index, or an index computed from another collection with a different length.
• Calling subList(from, to) with a range larger than the list.

How to fix it

1. Check isEmpty() or size() before indexing.
2. Use streams or findFirst() to get an Optional instead of get(0).
3. Iterate with an Iterator and remove(), or use removeIf, when deleting while looping.
4. Use List.getFirst() and getLast() (Java 21) when you really want the ends; they throw a clearer NoSuchElementException.

Broken and fixed

List<Order> orders = repo.findByCustomer(id);
Order first = orders.get(0); // empty list -> exception

Order first = orders.stream().findFirst()
        .orElseThrow(() -> new OrderNotFoundException(id));

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In Spring and enterprise code

Repository and query methods return empty lists, not null, so get(0) without a size check is a very common production error in service code.

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