We should also check whether there is a win by filling the vertical line:
@Test public void whenPlayAndWholeVerticalLineThenWinner() { ticTacToe.play(2, 1); // X ticTacToe.play(1, 1); // O ticTacToe.play(3, 1); // X ticTacToe.play(1, 2); // O ticTacToe.play(2, 2); // X String actual = ticTacToe.play(1, 3); // O assertEquals("O is the winner", actual); }
The player wins when the whole vertical line is occupied by his pieces.