java - How to avoid ArrayIndexOutOfBoundsException or IndexOutOfBoundsException?

If your question is I am getting a java.lang.ArrayIndexOutOfBoundsException in my code and I do not understand why it is happening. What does it mean and how can I avoid it?

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Answer

What is java.lang.ArrayIndexOutOfBoundsException / java.lang.IndexOutOfBoundsException?

The JavaDoc curtly states:

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.

What causes it to happen?

This exception means that you have tried to access an index in an
array or array backed list and that index does not exist.

Java uses 0 based indexes. That means all indexes start with 0 as
the index of the first element if it contains any elements.

The IndexOutOfBoundsException message is very explicit, it usually takes the form of:

java.lang.IndexOutOfBoundsException: Index: 1, Size: 1

Where Index is the index that you requested that does not exist and Size is the length of the structure you were indexing into.

As you can see a Size: 1 means the only valid index is 0 and you were asking for what was at index 1.

For example if you have an raw Array of objects or primitive types
the valid indexes are 0 to .length - 1, in the following example the valid indexes would be 0,1,2,3,.

final String days[] { "Sunday", "Monday", "Tuesday" }
System.out.println(days.length); // 3
System.out.println(days[0]); // Sunday
System.out.println(days[1]); // Monday
System.out.println(days[2]); // Tuesday
System.out.println(days[3]); // java.lang.ArrayIndexOutOfBoundsException

This also applies to ArrayList as well as any other Collection classes that may be backed by an Array and allow direct access to the the index.

How to avoid the java.lang.ArrayIndexOutOfBoundsException / java.lang.IndexOutOfBoundsException?

When accessing directly by index:

This uses Guava to convert the raw primitive int[] array to an
ImmutableList. Then it uses the Iterables class to safely
get the value at a particular index and provides a default value when
that index does not exist. Here I chose -1 to indicate an invalid
index value.

final List toTen = ImmutableList.copyOf(Ints.asList(ints));
System.out.println(Iterables.get(toTen, 0, -1));
System.out.println(Iterables.get(toTen, 100, -1));

If you can't use Guava for some reason it is easy to roll your own function to do this same thing.

private static  T get(@Nonnull final Iterable iterable, final int index, @Nonnull final T missing)
{
    if (index < 0) { return missing; }
    if (iterable instanceof List) 
    {
        final List l = List.class.cast(iterable);
        return l.size() <= index ? l.get(index) : missing;
    }
    else
    {
        final Iterator iterator = iterable.iterator();
        for (int i = 0; iterator.hasNext(); i++)
        {
            final T o = iterator.next();
            if (i == index) { return o; }
        }
        return missing;
    }
}

When iterating:

Here is the idiomatic ways to iterate over a raw Array if you need
to know the index and the value:

This is susceptible to one off errors which are the primary causes
of an java.lang.ArrayIndexOutOfBoundsException:

Using a traditional for/next loop:

final int ints[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
for (int i = 0; i < ints.length; i++)
{
    System.out.format("index %d = %d", i, ints[i]);  
}

Using an enhanced for/each loop:

Here is the idiomatic way to iterate over a raw Array with the
enhanced for loop if you do not need to know the actual index:

for (final int i : ints)
{
    System.out.format("%d", i);
    System.out.println();
}

Using a type safe Iterator:

Here is the safe way to iterate over a raw Array with the enhanced
for loop and track the current index and avoids the possibility of
encountering an java.lang.ArrayIndexOutOfBoundsException.

This uses Guava to easily convert the int[] to something Iterable
every project should include it.

final Iterator it = Ints.asList(ints).iterator();
for (int i = 0; it.hasNext(); i++)
{
    System.out.format("index %d = %d", i, it.next());
}

If you can not use Guava or your int[] is huge you can roll your own ImmutableIntArrayIterator as such:

public class ImmutableIntArrayIterator implements Iterator
{
    private final int[] ba;
    private int currentIndex;

    public ImmutableIntArrayIterator(@Nonnull final int[] ba)
    {
        this.ba = ba;
        if (this.ba.length > 0) { this.currentIndex = 0; }
        else { currentIndex = -1; }
    }

    @Override
    public boolean hasNext() { return this.currentIndex >= 0 && this.currentIndex + 1 < this.ba.length; }

    @Override
    public Integer next()
    {
        this.currentIndex++;
        return this.ba[this.currentIndex];
    }

    @Override
    public void remove() { throw new UnsupportedOperationException(); }
}

And use the same code as you would with Guava.

If you absolutely must have the ordinal of the item the following is the safest way to do it.

// assume los is a list of Strings
final Iterator it = los.iterator();
for (int i = 0; it.hasNext(); i++)
{
    System.out.format("index %d = %s", i, it.next());
}

This technique works for all Iterables, it is not an index perse but it does give you the current position in the iteration even for things that do not have a native index.

The safest way:

The best way is to always use ImmutableLists/Set/Maps from Guava as
well:

final List ili = ImmutableList.copyOf(Ints.asList(ints));
final Iterator iit = ili.iterator();
for (int i = 0; iit.hasNext(); i++)
{
    System.out.format("index %d = %d", i, iit.next());
}

Summary:

1. Using raw Array are difficult to work with and should be avoided in most cases. They are susceptible to sometimes subtle one off errors which have plague new programmers even back to the days of BASIC


2. Modern Java idioms use proper type safe Collections and avoid using raw Array structures if at all possible.


3. Immutable types are preferred in almost all cases now.


4. Guava is an indispensable toolkit for modern Java development.