get columns from two dimensional array in java

Question

In a two dimensional array I can easily get the arrays for the rows, how can I get the columns as arrays too? I need a solution that works for objects too, not just primitives. Thanks

        int counter = 1;
    int[][] matrix = new int[9][9];

    for (int x = 0; x < matrix.length; x++) {
        for (int y = 0; y < matrix[0].length; y++) {
            matrix[x][y] = counter;
            System.out.print(counter + " ");
            counter++;
        }
        System.out.println();
    }

    for (int x = 0; x < matrix.length; x++) {
        int[] row = matrix[x];  
    }

Answer 1

There's no "out-of-the-box" way, but you can create a static method for this:

public static Object[] getColumn(Object[][] array, int index){
    Object[] column = new Object[array[0].length]; // Here I assume a rectangular 2D array! 
    for(int i=0; i<column.length; i++){
       column[i] = array[i][index];
    }
    return column;
}

Answer 2

Here is a method using Java 8 streams:

int[] getColumn(int[][] matrix, int column) {
    return IntStream.range(0, matrix.length)
        .map(i -> matrix[i][column]).toArray();
}

And if you want to cope with rows that are too short:

int[] getColumn(int[][] matrix, int column, int defaultVal) {
    return IntStream.range(0, matrix.length)
        .map(i -> matrix[i].length < column ? defaultVal : matrix[i][column])
        .toArray();
}

Answer 3

You can do it this way:

private <T> T[] getColumn(int address, T[][] from) {
    return (T[]) Arrays.stream(from).map(x -> x[address]).toArray(Object[]::new);
}