https://qiita.com/lotoka/items/2472e4b5dec92e8d6ed1#comment-bf8e252c1446c4ee0030

Othello.java

enum Stone {
    NONE("."),
    BLACK("X"),
    WHITE("O"),
    INVALID("?");

    private final String text;

    private Stone(String text) {
        this.text = text;
    }

    public String toString() {
        return text;
    }

    public Stone turn() {
        return this == BLACK ? WHITE :
               this == WHITE ? BLACK :
               this;
    }
}

class Board {
    private final Stone[][] cells;
    public static final String X = "abcdefgh";
    public static final String Y = "12345678";

    public Board() {
        this(8, 8);
    }

    public Board(int width, int height) {
        cells = new Stone[height][width];
        for (int y = 0; y < height; y++) {
            for (int x = 0; x < width; x++) {
                cells[y][x] = Stone.NONE;
            }
        }
    }

    public void print() {
        System.out.println(" " + X);
        for (int y = 0; y < cells.length; y++) {
            System.out.print(Y.charAt(y));
            for (int x = 0; x < cells[y].length; x++) {
                System.out.print(cells[y][x]);
            }
            System.out.println();
        }
    }

    public boolean isValid(int x, int y) {
        return y >= 0 && y < cells.length && x >= 0 && x < cells[y].length;
    }

    public boolean put(int x, int y, Stone stone) {
        if (!isValid(x, y)) {
            return false;
        }
        cells[y][x] = stone;
        return true;
    }

    public Stone at(int x, int y) {
        return isValid(x, y) ? cells[y][x] : Stone.INVALID;
    }

    public int count(Stone stone) {
        int counter = 0;
        for (var row: cells) {
            for (var cell: row) {
                if (cell == stone) {
                    counter += 1;
                }
            }
        }
        return counter;
    }
}

public class Othello {
    private final Board board;

    public Othello() {
        board = new Board();
        board.put(3, 3, Stone.WHITE);
        board.put(4, 3, Stone.BLACK);
        board.put(3, 4, Stone.BLACK);
        board.put(4, 4, Stone.WHITE);
    }

    public void print() {
        System.out.println("BLACK(" + Stone.BLACK + "):" + board.count(Stone.BLACK) +
                           " vs " +
                           "WHITE(" + Stone.WHITE + "):" + board.count(Stone.WHITE));
        board.print();
    }

    public boolean canPut(int x, int y, Stone stone) {
        if (board.at(x, y) != Stone.NONE) {
            return false;
        }
        for (int dy = -1; dy <= 1; dy++) {
            for (int dx = -1; dx <= 1; dx++) {
                if (dy == 0 && dx == 0) {
                    continue;
                }
                if (canReverse(x, y, stone, dx, dy)) {
                    return true;
                }
            }
        }
        return false;
    }

    private boolean canReverse(int x, int y, Stone stone, int dx, int dy) {
        if (board.at(x + dx, y + dy) != stone.turn()) {
            return false;
        }
        do {
            x += dx;
            y += dy;
        } while (board.at(x, y) == stone.turn());
        return board.at(x, y) == stone;
    }

    public boolean put(char x, char y, Stone stone) {
        return put(Board.X.indexOf(x), Board.Y.indexOf(y), stone);
    }

    boolean put(int x, int y, Stone stone) {
        if (!canPut(x, y, stone)) {
            return false;
        }
        board.put(x, y, stone);
        for (int dy = -1; dy <= 1; dy++) {
            for (int dx = -1; dx <= 1; dx++) {
                if (dy == 0 && dx == 0) {
                    continue;
                }
                reverse(x, y, stone, dx, dy);
            }
        }
        return true;
    }

    private void reverse(int x, int y, Stone stone, int dx, int dy) {
        if (!canReverse(x, y, stone, dx, dy)) {
            return;
        }
        while (board.at(x + dx, y + dy) == stone.turn()) {
            x += dx;
            y += dy;
            board.put(x, y, stone);
        }
    }

    public static void main(String[] args) {
        System.out.println(Stone.WHITE.turn().name());
        System.out.println(Stone.BLACK.turn().name());
        Othello othello = new Othello();
        othello.print();
        var player = Stone.BLACK;
        System.out.println(othello.put('a', '1', player));
        System.out.println(othello.put('d', '3', player));
        othello.print();
        player = player.turn();
        othello.put('c', '3', player);
        othello.print();
    }
}