https://qiita.com/Xenon_DX/items/bac85f084188c2b8e4dc#comment-f72f4209b8932f248444

Maze.java

enum Cell { WALL, ROAD };

class Map {
    public final int width;
    public final int height;
    private final Cell[][] cells;

    public Map(int width, int height) {
        if (width < 5 || height < 5 || width % 2 == 0 || height % 2 == 0) {
            new IllegalArgumentException("縦・横共に5以上の奇数で作成してください。");
        }
        this.width = width;
        this.height = height;
        cells = new Cell[height][width];
    }

    public void fillWall() {
        for (var row: cells) {
            for (int x = 0; x < row.length; x++) {
                row[x] = Cell.WALL;
            }
        }
    }

    public void dig(int x, int y) {
        cells[y][x] = Cell.ROAD;
    }

    public boolean isInside(int x, int y) {
        return 1 <= x && x < width - 1 && 1 <= y && y < height - 1;
    }

    public boolean isDigable(int x, int y) {
        return isInside(x, y) && cells[y][x] == Cell.WALL;
    }

    public boolean isDigableForward(int x, int y) {
        return isDigable(x, y - 2) || isDigable(x, y + 2) ||
               isDigable(x - 2, y) || isDigable(x + 2, y);
    }

    public boolean isDigable() {
        for (int y = 1; y < height; y += 2) {
            for (int x = 1; x < width; x += 2) {
                if (cells[y][x] == Cell.WALL) {
                    return true;
                }
            }
        }
        return false;
    }

    public void show() {
        for (var row: cells) {
            for (var cell: row) {
                System.out.print(cell == Cell.WALL ? "##" : "  ");
            }
            System.out.println();
        }
    }
}

class Digger {
    private Map map;
    private static final int DIR_X[] = { -1, 1, 0, 0 };
    private static final int DIR_Y[] = { 0, 0, -1, 1 };

    public Digger(Map map) {
        this.map = map;
    }

    private int random(int max) {
        return 1 + 2 * (int) ((Math.random() * (max - 1)) / 2);
    }

    public void dig() {
        map.fillWall();
        while (map.isDigable()) {
            int x, y;
            do {
                x = random(map.width);
                y = random(map.height);
            } while (!map.isDigable(x, y));
            dig(x, y);
        }
    }

    private void dig(int x, int y) {
        map.dig(x, y);
        while (map.isDigableForward(x, y)) {
            int dx, dy;
            do {
                int direction = (int) Math.floor(Math.random() * DIR_X.length);
                dx = DIR_X[direction];
                dy = DIR_Y[direction];
            } while (!map.isDigable(x + dx * 2, y + dy * 2));
            map.dig(x + dx, y + dy);
            dig(x + dx * 2, y + dy * 2);
        }
    }
}

public class Maze {
    private final Map map;

    public Maze(int width, int height) {
        map = new Map(width, height);
    }

    public void make() {
        new Digger(map).dig();
    }

    public void show() {
        map.show();
    }

    public static void main(String[] args) {
        Maze maze = new Maze(21, 21);
        maze.make();
        maze.show();
    }
}