小白求助

public class Circle
{
private float x;
private float y;
private float rad;
private Paint paint=new Paint();
private int alpha=255;
public Circle(float x,float y)
{
this.x=x;
this.y=y;
}
public void draw(Canvas canvas)
{
paint.setAlpha(alpha);
paint.setStyle(Paint.Style.STROKE);
paint.setStrokeWidth(10);
paint.setColor(Color.RED);
canvas.drawCircle(x,y,rad,paint);
change();
}
private void change()
{
rad+=3;
if(alpha>0)
{
alpha-=5;
}
}
public int getAlpha()
{
return alpha;
}
}
为什么画圆的时候，alpha的效果没有出现