I am trying to draw rounded diagonal lines in Java using the ACM library.
A very complicated method would involve drawing a diagonal line, scaling it up to increase its width, drawing an arc on the top of that line with respect to the angles it is rounded.
From the picture you linked to, it looks like you want lines with round end-caps, sometimes called line-joins. I don't know the ACM library too well, but looking at the docs for it, it seems like you could achieve that look by using a GPen with an image that is just a filled-in circle. You could call the setLocation(x,y) and then drawLine(dx, dy) methods on it.