为android游戏绘制正方形网格有困难

public class GridView extends View {
    private int rows;
    private int columns;
    private Paint paint;

    public GridView(Context context, int rows, int columns) {
        super(context);
        this.rows = rows;
        this.columns = columns;
        paint = new Paint();
        paint.setColor(Color.BLACK);
        paint.setStyle(Paint.Style.STROKE);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        int width = getWidth();
        int height = getHeight();

        int cellWidth = width / columns;
        int cellHeight = height / rows;

        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < columns; j++) {
                int left = j * cellWidth;
                int top = i * cellHeight;
                int right = left + cellWidth;
                int bottom = top + cellHeight;

                canvas.drawRect(left, top, right, bottom, paint);
            }
        }
    }
}

GridView gridView = new GridView(this, 5, 5);
layout.addView(gridView);