Question

I have a really simple question, but I just want to know what the correct way of doing it is.

How do you convert a variable of type ArrayList<int[]> to an int[][]?

Answer 1

Use List#toArray(T[]) wherein you pass the array type.

int[][] intMatrix = listOfIntArrays.toArray(new int[0][]);

Demo:

List<int[]> listOfIntArrays = new ArrayList<int[]>();
listOfIntArrays.add(new int[] { 10, 20, 30 });
listOfIntArrays.add(new int[] { 11, 21, 31 });
listOfIntArrays.add(new int[] { 12, 22, 32 });

int[][] intMatrix = listOfIntArrays.toArray(new int[0][]);

System.out.println(Arrays.deepToString(intMatrix));

Result:

[[10, 20, 30], [11, 21, 31], [12, 22, 32]]