What causes a java.lang.ArrayIndexOutOfBoundsException and how do I prevent it?

Question

What does ArrayIndexOutOfBoundsException mean and how do I get rid of it?

Here is a code sample that triggers the exception:

String[] names = { "tom", "bob", "harry" }; for (int i = 0; i <= names.length; i++) {     System.out.println(names[i]); } 

Answer 1

Your first port of call should be the documentation which explains it reasonably clearly:

Thrown to indicate that an array has been accessed with an illegal index. The index is either negative or greater than or equal to the size of the array.

So for example:

int[] array = new int[5]; int boom = array[10]; // Throws the exception 

As for how to avoid it... um, don't do that. Be careful with your array indexes.

One problem people sometimes run into is thinking that arrays are 1-indexed, e.g.

int[] array = new int[5]; // ... populate the array here ... for (int index = 1; index <= array.length; index++) {     System.out.println(array[index]); } 

That will miss out the first element (index 0) and throw an exception when index is 5. The valid indexes here are 0-4 inclusive. The correct, idiomatic for statement here would be:

for (int index = 0; index < array.length; index++) 

(That's assuming you need the index, of course. If you can use the enhanced for loop instead, do so.)

Answer 2

if (index < 0 || index >= array.length) {     // Don't use this index. This is out of bounds (borders, limits, whatever). } else {     // Yes, you can safely use this index. The index is present in the array.     Object element = array[index]; } 

See also:

• The Java Tutorials - Language Basics - Arrays

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Update: as per your code snippet,

for (int i = 0; i<=name.length; i++) { 

The index is inclusive the array's length. This is out of bounds. You need to replace <= by <.

for (int i = 0; i < name.length; i++) {