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What it looks like

java.lang.ArrayIndexOutOfBoundsException: Index 5 out of bounds for length 5

Extends IndexOutOfBoundsException in java.lang.

What it means

Java arrays have a fixed length and valid indexes run from 0 to length minus 1. Reading or writing any other index throws this exception, with a message that gives both the index and the length. It is almost always an off-by-one in a loop or an assumption about the size of data that arrived from outside the program.

Common causes

• A loop condition of i <= array.length instead of i < array.length.
• Using the last index plus one, or a negative index from arithmetic such as i - 1 at i = 0.
• Splitting a string and assuming a number of parts: line.split(",")[2] on a short line.
• Reading command-line arguments or a CSV row without checking its length.

How to fix it

1. Compare the index with array.length before using it, or iterate with an enhanced for loop.
2. Check the result of split before indexing, and use the limit argument when the trailing parts matter.
3. Use a List and its methods when the size changes, instead of resizing arrays by hand.
4. Log the index and length (the message already contains them) to see which assumption failed.

Broken and fixed

for (int i = 0; i <= values.length; i++) {
    sum += values[i]; // fails when i == values.length
}

for (int i = 0; i < values.length; i++) {
    sum += values[i];
}

🔎 Unravel the failing code 🔎 Unravel the fix

In Spring and enterprise code

Input parsing code is the usual source in services: a file with a missing column or a message with fewer fields than expected. Validate the shape of external data before indexing into it.

Debug it faster

Paste the whole stack trace into the stack trace beautifier to hide framework frames, jump to the root cause and find the first line of your own code. It runs in your browser, so the trace never leaves your machine.

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