This adds an extra field to every ArrayList - a tax paid by every carefully constructed ArrayList so that sloppy instances can get away with their sloppiness, which is pretty distasteful. If I was going to "spend" an extra field of ArrayList, it would be to implement a circular array like ArrayDeque and have O(1) deletion of element 0.
An alternative is to make elementData null when there are no elements. We can then reuse the size field for initialCapacity. Yes, it's true that you then have to introduce more null checks into the code, but hotspot will likely do that anyways in the jitted code, and elide them if you provide them, so cost of the null checks may be zero or even negative! Much more difficult would be the alternative of automatically snipping the tail (as with trimToSize) of any inactive ArrayList occasionally, e.g. after a gc cycle.
