Question

I draw an oval on a canvas:

RectF f = new RectF(-r, -r, r, r);
canvas.drawOval(f , mPaint);  

Why I see warning on RectF?

Avoid object allocations during draw/layout operations (preallocate
and reuse instead)

Answer 1

It’s just what it says: doing a new RectF inside onDraw creates an object every time anything is drawn. This can be a lot of objects. Just use a single RectF:

RectF mRect = new RectF();

protected void onDraw(Canvas canvas) {
    mRect.set(-r, -r, r, r);
    canvas.drawOval(mRect, mPaint);
}

Just to be clear: your original code is logically correct. This is just a performance improvement (albeit an important one).