Java: Best way to store to an arbitrary index of an ArrayList

Question

I know that I cannot store a value at an index of an ArrayList that hasn\'t been used yet, i.e. is less than the size. In other words, if myArrayList.size() is 5, then if I

Answer 1

If you are looking for a sparse array (where most of the indices will be empty), a Map of some sort (probably a HashMap) will be your best bet. Any array-esque solution will be forced to reserve space for all the empty indices, which is not very space-efficient, and a HashMap is fast enough for most normal purposes.

If you will eventually fill up the array up to some n, you will need to add nulls in a loop to get to the index you want. You can make this somewhat more efficient by giving it an initial capacity of the number of elements you will eventually want to store (this prevents the ArrayList from needing to resize itself). new ArrayList(n) will work fine. Unfortunately, there is no simple way of making it a certain size to begin with except adding things in a loop when you make it.