Question

I would like to represent matrix-like data in a suitable data-structure in Java. The dimensions of this matrix depend on user-input. One way would probably be to use a “magic” max-constant, and use a simple multidimensional array. But operations on the data depend heavily on the dimension, and I would like to avoid fixed arrays, since I’d always need to keep track of the used vs. maximal dimension. A more dynamic approach of course is to define something like

private ArrayList<ArrayList<ArrayList<Point>>> arr3d = new ArrayList<ArrayList<ArrayList<Element>>>();

I think this is unacceptably ugly. It might be better to define a custom data-structure, i.e. a one-dimensional ArrayList (or Vector), and then somehow wrap this i.e. map an access specified as [i,j,k] to some element of that single list. Is it possible to overwrite the operator []? How to do this efficiently? Is there some existing library or code I could lean on?

PS: I thought this might be a common problem, but despite best efforts I have not found an existing question. Apologies in advance if I missed some answer.

Answer 1

Why don’t you create your own class to wrap that? It could contain your 3d list internally, but you expose only “user-friendly” methods to access and set elements.

And no, in Java it’s not possible to overload []

A quick helper for that:

public class Cube<T> {
    private final List<List<List<T>>> elements = new ArrayList<List<List<T>>>();

    public T get(final int x, final int y, final int z) {
        if (elements.size() > x) {
            final List<List<T>> rowx = elements.get(x);
            if (rowx.size() > y) {
                final List<T> rowy = rowx.get(y);
                if (rowy.size() > z) {
                    return rowy.get(z);
                }
            }
        }
        return null;
    }
} 

You can even do it in one line if you’re not worried about readability:

public T get(final int x, final int y, final int z) {
    return (elements.size() > x && elements.get(x).size() > y && elements.get(x).get(y).size() > z ? elements.get(x).get(y).get(z) : null);
}

And you need put methods, that creates the rows (as ArrayList) when needed.