public void clickOn(int cellX, int cellY) {
		Ball ball = _state[cellY][cellX];
		if (ball != null) {
			_selectedBall = ball;
			_selectedCellX = cellX;
			_selectedCellY = cellY;
		} else if (ball == null && _selectedBall != null) {
			if (canMove(_selectedCellX, _selectedCellY, cellX, cellY)) {
				_state[cellY][cellX] = _selectedBall;
				_selectedBall.setFieldX(cellX);
				_selectedBall.setFieldY(cellY);
				_state[_selectedCellY][_selectedCellX] = null;
				_selectedBall = null;
				_selectedCellX = _selectedCellY = -1;
				checkMove(cellY, cellX, true);
			}
		}
	}