Test-Driven Java Development, Second Edition - Second Edition

Implementation

To implement the last test, we should store the location of the placed pieces in an array. Every time a new piece is placed, we should verify that the place is not occupied, or else throw an exception:

private Character[][] board = {  {'\0', '\0', '\0'},
  {'\0', '\0', '\0'},  {'\0', '\0', '\0'}};

public void play(int x, int y) {
  if (x < 1 || x > 3) {
    throw new RuntimeException("X is outside board");
  } else if (y < 1 || y > 3) {
    throw new RuntimeException("Y is outside board");
  }
  if (board[x - 1][y - 1] != '\0') {
    throw new RuntimeException("Box is occupied");
  } else {
    board[x - 1][y - 1] = 'X';
  }
}

We're checking whether a place that was played is occupied and, if it is not, we're changing the array entry value from empty (\0 ...