This is the implementation for the second requirement.

We can split this requirement into the following tests:

• When a disc is inserted into an empty column, its position is 0
• When a second disc is inserted into the same column, its position is 1
• When a disc is inserted into the board, the total number of discs increases
• When a disc is put outside the boundaries, a Runtime Exception is thrown
• When a disc is inserted into a column and there's no room available for it, then a Runtime Exception is thrown

Also, these other tests are derived from the first requirement. They are related to the board limits or board behavior.

The Java implementation of the aforementioned tests is as follows:

@Test 
public void whenDiscOutsideBoardThenRuntimeException() {
  int column = -1;
  exception.expect(RuntimeException.class);
  exception.expectMessage("Invalid column " + column);
  tested.putDiscInColumn(column);
}

@Test
public void whenFirstDiscInsertedInColumnThenPositionIsZero() {
  int column = 1;
  assertThat(tested.putDiscInColumn(column),  is(0));
}

@Test
public void whenSecondDiscInsertedInColumnThenPositionIsOne() {
  int column = 1;
  tested.putDiscInColumn(column);
  assertThat(tested.putDiscInColumn(column), is(1));
}
 
@Test
public void whenDiscInsertedThenNumberOfDiscsIncreases() {
  int column = 1;
  tested.putDiscInColumn(column);
  assertThat(tested.getNumberOfDiscs(), is(1));
}
 
@Test 
public void whenNoMoreRoomInColumnThenRuntimeException() {
  int column = 1;
  int maxDiscsInColumn = 6; // the number of rows
  for (int times = 0; times < maxDiscsInColumn; ++times) {
    tested.putDiscInColumn(column);
  }
  exception.expect(RuntimeException.class);
  exception.expectMessage("No more room in column " + column);
  tested.putDiscInColumn(column);
}

This is the necessary code to satisfy the tests:

private static final int ROWS = 6;
 
private static final int COLUMNS = 7;
 
private static final String EMPTY = " ";
 
private String[][] board = new String[ROWS][COLUMNS];
 
public Connect4TDD() {
  for (String[] row : board) Arrays.fill(row, EMPTY);
}

public int getNumberOfDiscs() {
  return IntStream
           .range(0, COLUMNS)
           .map(this::getNumberOfDiscsInColumn)
           .sum(); 
} 

private int getNumberOfDiscsInColumn(int column) {
  return (int) IntStream
                 .range(0, ROWS)
                 .filter(row -> !EMPTY.equals(board[row][column]))
                 .count();
}

public int putDiscInColumn(int column) {
  checkColumn(column);
  int row = getNumberOfDiscsInColumn(column);
  checkPositionToInsert(row, column);
  board[row][column] = "X";
  return row;
}

private void checkColumn(int column) {
  if (column < 0 || column >= COLUMNS)
    throw new RuntimeException("Invalid column " + column);
}

private void checkPositionToInsert(int row, int column) {
  if (row == ROWS)
    throw new RuntimeException("No more room in column " + column); 
}