Question

I’m trying to double the length of a 2D array as I add values to it. I know for a 1D an array the code for this is:

int oneD[] = new int[10];
//fill array here

oneD = Arrays.copyOf(oneD, 2 * oneD.length);

so if I have a 2D array and only want to double the amount of rows while keeping say 2 columns I figured I would just do this:

int twoD[][] = new int[10][2];
//fill array here

twoD = Arrays.copyOf(twoD, 2* twoD.length);

This however does not seem to work for the 2D array. How does one go about doubling the length of a 2D array. In this case to make it [20][2] instead.

Answer 1

In your case something like this would do the job:

public static <T> T[][] copyOf(T[][] array, int newLength) {
    // ensure that newLength >= 0
    T[][] copy = new T[newLength][];
    for (int i = 0; i < copy.length && i < array.length; i++) {
        copy[i] = Arrays.copyOf(array[i], array[i].length);
        // this should also work, just not create new array instances:
        // copy[i] = array[i];
    }
    return copy;
}

And you could call this method, just like you called Arrays.copyOf()