You can use Ray2D and then convert to a Line2D. See below on line 31. Press space to create new lines.

Code Example

1. import toxi.geom.*;
2. import toxi.processing.*;
3.  
4. ToxiclibsSupport gfx;
5. ArrayList <Line2D> lines = new ArrayList <Line2D> ();
6.  
7. void setup() {
8.   size(300, 300);
9.   smooth();
10.   gfx = new ToxiclibsSupport(this);
11.   addLine();
12. }
13.  
14. void draw() {
15.   background(255);
16.   for (Line2D line : lines) {
17.     gfx.line(line);
18.   }
19. }
20.  
21. void keyPressed() {
22.   if (key == ' ') {
23.     addLine();
24.   }
25. }
26.  
27. void addLine() {
28.   Vec2D position = new Vec2D(width/2, height/2);
29.   Vec2D angle = Vec2D.randomVector();
30.   float length = random(50, 200);
31.   Line2D line = new Ray2D(position, angle).toLine2DWithPointAtDistance(length);
32.   lines.add(line);
33. }

It would run, but it wouldn't work as you expected. In your code you rotate a vector that is (0,0).

So change line 2 to the line below (or another 'starting direction') and then it will work:

1.   Vec2D direction = new Vec2D(1,0).rotate(theta);

Or you could use Toxiclibs' built in function:

1.   Vec2D direction = Vec2D.fromTheta(theta);

Watch this video for a good explanation of vector math.

For a crappy one, read on.

As you mentioned, vectors are really a direction. Think of Vec2D(1,0) as a line from (0,0) to (1,0) like this _ (sideways). I chose (1,0) to make it as intuitive as possible, however Vec2D(0.0001,0) or Vec2D(10000,0) would give the exact same result in this case. Because for this one, the magnitude doesn't matter, only the direction. What I meant by 'another starting direction' was for example (0,1). Think of that as a line from (0,0) to (0,1) like this | (downwards). In fact any combination of at least one non-zero number would work (but give different results for different starting directions). The only one that does not work is (0,0), because this is the 'origin' of the vector. Going from (0,0) to (0,0) is no direction. So you will not see a line, only a point. I would indeed say that a vector has to be anything other than (0,0) for it to be rotate-able. Hope this helps.