ArrayIndexOutOfBound exception even though I check for array length!

Question

I have the following code in some app:

int lowRange=50;
int[] ageRangeIndividual = {6, 10, 18, 25, 45, 65, 90};
int index=0;
for (; index<ageRangeIndividual.length-1 && ageRangeIndividual[index]<=lowRange;index++);

I am getting an "Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 7" in the for line! even though I explicitly specify to break the cycle if index < last indexable item in the array!

This does not happen always, but after some time of running said program (lowRange varies each time the function is called)

What am I not seeing?

Answer 1

The code you've posted doesn't throw an exception, on its own. In fact, your array bounds check is one off - you're being too conservative, unless you really don't intend to check the last value. It would normally just be

index < ageRangeIndividual.length

You say it happens eventually - do you have any other threads changing the value of index or ageRangeIndividual?

I presume you're about to use index in the next line - personally I'd find it clearer if this were wrapped up in a method:

// Rename to something more appropriate, perhaps
public static int findIndexOverMinimum(int[] values, int minimum)
{
    for (int i = 0; i < values.length; i++)
    {
        if (values[i] > minimum)
        {
            return i;
        }
    }
    return -1; // Or throw an exception, whichever is most appropriate
}

I really don't like for loops which have no body, and which use an existing variable for indexing. It's all perfectly valid, but it just feels wrong.

Answer 2

An ArrayIndexOutOfBoundsException appears when you're referring to an array index value that exceeds it's length value minus 1 (you're basically fishing for something that isn't there).

In this case, index would need to reach 7 followed by an an access attempt for this to happen (exceeds 0-6). The expression ageRangeIndividual.length-1 would give 6 (number of elements in an array minus one) and the expression index<ageRangeIndividual.length-1 should evaluate to false. In turn the second expression after the && operator shouldn't be evaluated and so shouldn't trigger the Exception in the first place.

The only thing I can think of is whether the actual code you're testing is different or whether index<ageRangeIndividual.length-1 is being evaluated as intended. According to operator precedence ( http://java.sun.com/docs/books/tutorial/java/nutsandbolts/operators.html ) the substraction should be processed first.