Test-Driven Java Development, Second Edition - Second Edition

Test – winning condition III

Now that horizontal and vertical lines are covered, we should move our attention to diagonal combinations:

@Test 
public void whenPlayAndTopBottomDiagonalLineThenWinner() {
  ticTacToe.play(1, 1); // X
  ticTacToe.play(1, 2); // O
  ticTacToe.play(2, 2); // X
  ticTacToe.play(1, 3); // O
  String actual = ticTacToe.play(3, 3); // X
  assertEquals("X is the winner", actual);
}

The player wins when the whole diagonal line from the top-left to bottom-right is occupied by his pieces.