Why does ArrayList#rangeCheck not check if the index is negative?

Question

ArrayList#get, set and remove call the rangeCheck method at first. This method does not check if the index is negative. It only checks if the index is greater than or equal to the length of the array. Javadoc explains the reason; an array access throws an ArrayIndexOutOfBoundsException if index is negative.

private void rangeCheck(int index) {
    if (index >= size)
        throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
public E get(int index) {
    rangeCheck(index);

    return elementData(index);
}

According to Java Langauage Specification, an array access throws an ArrayIndexOutOfBoundsException if index is not only negative but also gte.

All array accesses are checked at run time; an attempt to use an index that is less than zero or greater than or equal to the length of the array causes an ArrayIndexOutOfBoundsException to be thrown.

I think that rangeCheck should check both negative and gte or, for performance, should check neither. Why does not rangeCheck check if the index is negative?

Answer 1

Very simply because in an ArrayList the backing array may be larger than the current size.

In the current implementation, an ArrayList's backing array capacity increases by a factor of 1.5 each time the current maximum capacity is exceeded. The default initial capacity is 10, and when you try to add the 11th element to the list the array gets reallocated with a capacity of 15. When you exceed 15 it goes to 22, etc.

At any given time, the capacity is likely larger than the current number of elements in the ArrayList.

Checking for negative indexes is left to the JVM (on the backing array) and ArrayList itself needs to check only at the upper end of the current list.