Three Dimensional ArrayList?

[Jacob Steingart]

Is it possible to make a three dimensional ArrayList?

[Benjamin Winterberg]

Not directly. But you can box two Lists together for 2-dim list:

List<List><?>> list = new ArrayList<ArrayList<Object>>();

[Rob Spoor]

Which needs another level for a 3d list:

List<List<List<E>>>

Also, don't forget that unlike array initialization, you can't really specify a size. Instead, the outer list is empty by default, and you need to add new lists whenever you need them. What you can do is add null references:

public static <E> List<List<List<E>>> create(int dim1, int dim2, int dim3)
{
    List<List<List<E>>> list1 = new ArrayList<List<List<E>>>(dim1);
    for (int i = 0; i < dim1; i++)
    {
        List<List<E>> list2 = new ArrayList<List<E>>(dim2);
        for (int j = 0; j < dim2; j++)
        {
            List<E> list3 = new ArrayList<E>(dim3);
            for (int k = 0; k < dim3; k++)
            {
                list3.add(null);
            }
            list2.add(list3);
        }
        list1.add(list2);
    }
    return list1;
}

And Ben, you can't declare a List<List<?>> and then assign to it a ArrayList<ArrayList<?>>; ArrayList<List<?>> will work fine though, and can have both ArrayList and LinkedList elements added.

[Benjamin Winterberg]

Arg typo, thanks for clarifying.

[Jacob Steingart]

So you essentially make an ArrayList of Lists, and then assign your values to the list. Now that I think of it, I could probably just make an ArrayList of Arrays- I'm storing data in this, and I have a fixed number of columns, but a variable number of rows. Could you do this by saying something like

import java.util.*;


public class ThreeDimensionalArrayTests {
	
	public static void main(String[] args) {
		ArrayList<Object[]> array = new ArrayList<Object[]>();
		Object[] arr0 = { "1", 2, "3", 4, "5" };
		array.add(arr0);
		System.out.println(array.get(0)[0]);
	}
}

I believe I have usable code!