Next, we should let the player know the status of the game.

As we are throwing exceptions when an error occurs, this is already covered, so we only need to implement these two tests. Furthermore, for the sake of testability, we need to introduce a parameter within the constructor. By introducing this parameter, the output becomes easier to test:

private OutputStream output;

@Before
public void beforeEachTest() {
  output = new ByteArrayOutputStream(); 
  tested = new Connect4TDD(new PrintStream(output)); 
}

@Test
public void whenAskedForCurrentPlayerTheOutputNotice() {
  tested.getCurrentPlayer();
  assertThat(output.toString(), containsString("Player R turn")); 
}

@Test
public void whenADiscIsIntroducedTheBoardIsPrinted() {
  int column = 1;
  tested.putDiscInColumn(column);
  assertThat(output.toString(), containsString("| |R| | | | | |"));
}

One possible implementation is to pass the preceding tests. As you can see, the class constructor now has one parameter. This parameter is used in several methods to print the event or action description:

private static final String DELIMITER = "|";

public Connect4TDD(PrintStream out) {
  outputChannel = out;
  for (String[] row : board) Arrays.fill(row, EMPTY); 
}

public String getCurrentPlayer() {
  outputChannel.printf("Player %s turn%n", currentPlayer);
  return currentPlayer;
}

private void printBoard() {
  for (int row = ROWS - 1; row >= 0; row--) {
    StringJoiner stringJoiner = new StringJoiner(DELIMITER, DELIMITER, DELIMITER); 
    Stream.of(board[row]).forEachOrdered(stringJoiner::add); 
    outputChannel.println(stringJoiner.toString()); 
  }
}

public int putDiscInColumn(int column) {
  ... 
  printBoard();
  switchPlayer();
  return row;
}