You're missing the complete Node and Edge classes.

Perhaps it's easier to just download the example here.

What do you mean by not working? When I copy-paste your code it works.

(minus the... // Part of the relax( ) method in the original Graph.java example source)

You will get that error if you didn't copy the Node (and Edge) classes. Make sure you copy everything.

For example the program itself in the main tab and the two classes each in their own tab.

Or just everything in the main tab, that is also possible.

I can't run the code you uploaded. Because, as stated above, you have to delete the following lines from the Edge tab. If this code is included you can't run the sketch.

1. // Part of the relax( ) method in the original Graph.java example source
2. for (int i = 0 ; i < nedges ; i++) {
3.   Edge e = edges[i];
4.   double vx = nodes[e.to].x - nodes[e.from].x;
5.   double vy = nodes[e.to].y - nodes[e.from].y;
6.   double len = Math.sqrt(vx * vx + vy * vy);
7.   double f = (edges[i].len - len) / (len * 3) ;
8.   double dx = f * vx;
9.   double dy = f * vy;
10.   nodes[e.to].dx += dx;
11.   nodes[e.to].dy += dy;
12.   nodes[e.from].dx += -dx;
13.   nodes[e.from].dy += -dy;
14. }

Once that part has been deleted, as stated above, the sketch runs fine. Don't know what else to say. If you still have problems, perhaps something actually broke (java, processing). But then you should experience this behavior in more sketches and a fresh installation could maybe solve it.