Java - Fill Array With Characters From String

I want to fill an unknown length of string, split it into character and put it into an array, repeating this process until the array is full. This is a multidimensional array I'm using.

    key = input from user as string (length is unknown)
    int[][] fullKey = new int[128][2];
    do {
        for (int ii = 0; ii < key.length(); ii++) {
            fullKey[i][0] = key.charAt(ii);
            i++;
        }
    } while (fullKey[encoding][0] == null);

I know that int is primitive so can't be null but it's just to try and explain what I'm attempting to do

Thanks in advance