矩形之间的空格

Question

我将开发一个应用程序，我用这种方法绘制矩形：

private int columnWidth = 1;
private int rowHeight = 1;
private int nbColumns = 1;
private int nbRows = 1;
public static final int DEFAULT_SIZE = 150;

private void initWorld() {
    WindowManager wm = (WindowManager) getContext().getSystemService(Context.WINDOW_SERVICE);
    Display display = wm.getDefaultDisplay();
    Point point = new Point();
    display.getSize(point);
    // we calculate the number of columns and rows for our World
    nbColumns = point.x / DEFAULT_SIZE;
    nbRows = point.y / DEFAULT_SIZE;
    // we calculate the column width and row height
    columnWidth = point.x / nbColumns;
    rowHeight = point.y / nbRows;
    world = new World(nbColumns, nbRows);
}
// Method to draw each cell of the world on the canvas
private void drawCells(Canvas canvas) {
    for (int i = 0; i < nbColumns; i++) {
        for (int j = 0; j < nbRows; j++) {
            Cell cell = world.get(i, j);
            r.set((cell.x * columnWidth) - 1, (cell.y * rowHeight) - 1,
                    (cell.x * columnWidth + columnWidth) - 1,
                    (cell.y * rowHeight + rowHeight) - 1);
            // we change the color according the alive status of the cell
            p.setColor(cell.alive ? DEFAULT_ALIVE_COLOR : DEFAULT_DEAD_COLOR);
            canvas.drawRect(r, p);
        }
    }
}

谁能告诉我，我怎样才能在矩形之间得到一些空格或线条？