public void addUnits(int x, int y, char ch) {
		if (units.isEmpty()) {
			left = right = x;
			top = bottom = y;
		} else {
			if (x < left)
				left = x;
			else if (x > right)
				right = x;
			if (y < top)
				top = y;
			else if (y > bottom)
				bottom = y;
		}
		units.add(new Unit(x, y, ch));
	}