How to combine array with arraylist in Java?

Question

I need a 5-dimensional data structure in Java with "double" as type for all the cells. For 3 of dimensions I know the size, so it fits to array category. But for 2 of dimensions I don't know the size beforehand; looks like ArrayList. I have been able to manage the combination for 3 dimensions in the past:

ArrayList(Double)[][] prXifY = (ArrayList(Double)[][]) new ArrayList[m][n];

But despite long hours working on it (and through search in the net), I wasn't able to scale it. I need something like this:

ArrayList(ArrayList(Double))[][][] prXiXjY = (ArrayList(ArrayList(Double))[][][]) new ArrayList(ArrayList<Double))[m][m][n];

When I tried the above code, it says: "Cannot create a generic array of ArrayList(ArrayList(Double))"

I will appreciate quick and complete answers.

By the way, this is my very first post ever. I tried my best to do a good job on searching beforehand and explaining the problem clearly. Comments on these matters are appreciated as well. :)

Answer 1

An ArrayList is an object, and instantiated differently than an array is. In general, to say that you want an ArrayList that holds doubles, you might use something like:

ArrayList<Double> list = new ArrayList<Double>();

to specify that you have an ArrayList which holsts ArrayLists which hold doubles...

ArrayList<ArrayList<Double>> list = new ArrayList<ArrayList<Double>>();

You see where this is going, I hope.

This just creates the top-level list - it doesn't create any of the cells themselves. For that, you'll need additional new statements. This is going to get messy fast, and you may want to stop and consider if there is a better way to store the data than in a 5-dimension array.

Answer 2

I think what you want would look something like this

List<List<List<List<List<Double>>>>> myList= new ArrayList<List<List<List<List<Double>>>>> ();

as you can tell this looks insane and will be very hard to maintain. You should probably look at alternative methods of doing this.

Answer 3

Multi dimensional arrays can be created like so:

    ArrayList<Double> oneDimensionalArrayList = new ArrayList<>();
    ArrayList<ArrayList<Double>> twoDimensionalArrayList = new ArrayList<>();
    ArrayList<ArrayList<ArrayList<Double>>> threeDimensionalArrayList = new ArrayList<>();
    ArrayList<ArrayList<ArrayList<ArrayList<Double>>>> fourDimensionalArrayList = new ArrayList<>();
    ArrayList<ArrayList<ArrayList<ArrayList<ArrayList<Double>>>>> fiveDimensionalArrayList = new ArrayList<>();

But I would definitely recommend considering whether a 5 dimensional array is what you require for the problem at hand; it smells like something is wrong