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<code>ArrayIndexOutOfBoundsException</code> in Java

Updated on 2026-04-21 Reviewed by the Java With Us editorial team

ArrayIndexOutOfBoundsException is thrown when you access an array element with an index < 0 or >= array.length. It's an unchecked exception — always a programming bug, never a condition to catch.

What the error looks like

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 5 out of bounds for length 5

The classic off-by-one

int[] arr = new int[5];
for (int i = 0; i <= arr.length; i++) {   // ❌ <= — goes one past the end
    arr[i] = i;
}

for (int i = 0; i < arr.length; i++) {    // ✅ <
    arr[i] = i;
}

for (int x : arr) { ... }                 // ✅ for-each — no index at all

Negative indices

int[] a = {1, 2, 3};
a[-1];                                     // ArrayIndexOutOfBoundsException
a[a.length - 1];                           // ✅ last element

For lists: IndexOutOfBoundsException

List.get(index) throws IndexOutOfBoundsException, a different class. Same underlying bug, different type:

List<String> xs = List.of("a", "b");
xs.get(5);    // IndexOutOfBoundsException: Index 5 out of bounds for length 2

Safe alternatives

// for-each skips the index entirely
for (String s : xs) System.out.println(s);

// Streams
xs.stream().forEach(System.out::println);

// Get-or-default
String s = i < xs.size() ? xs.get(i) : "default";

Common mistakes

  • Loop condition <= instead of < — the most common off-by-one.
  • Using arr.length() — arrays have a length field, not method. Strings use .length(), collections use .size(), arrays use .length.
  • Passing an unchecked user input as an index — validate at the boundary.

Related

Pillar: Java exceptions. See also for loops and ArrayList.

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