Question

How to change rows to columns in ArrayList<Integer[]>? For instance:

ArrayList<Integer[]> arr = ArrayList<Integer[]>();
arr.add(new Integer[]{1,2,3});
arr.add(new Integer[]{4,5,6});

It should be:

[1]: 1 4
[2]: 2 5
[3]: 3 6

If it´s impossible with ArrayList, what are the other options to store 2D data and change rows to columns?

Answer 1

Anything wrong with int[][]? That would be the standard approach:

public static void main(String[] args) {
    int[][] table = new int[][] { { 1, 2, 3 }, { 4, 5, 6 } };

    // This code assumes all rows have same number of columns
    int[][] pivot = new int[table[0].length][];
    for (int row = 0; row < table[0].length; row++)
        pivot[row] = new int[table.length];

    for (int row = 0; row < table.length; row++)
        for (int col = 0; col < table[row].length; col++)
            pivot[col][row] = table[row][col];

    for (int row = 0; row < pivot.length; row++)
        System.out.println(Arrays.toString(pivot[row]));
}

Output:

[1, 4]
[2, 5]
[3, 6]

If you must use Collections, use as a start:

List<List<Integer>> table = new ArrayList<List<Integer>>();
table.add(Arrays.asList(1, 4));
table.add(Arrays.asList(2, 5));
table.add(Arrays.asList(3, 6));