Moved to contributed library section.

There is the regular dist() method and the dist() method built into PVector.

See the reference.

Note I don't have a LeapMotion nor used the library, but judging from the code I assume it will return a list of Fingers which contain the positions. When you just draw the points, an enhanced for loop is fine. However for connections you need a double for loop. Additionally a regular for loop is better, because then you can draw each AB-connection once, instead of twice (BA). See the example below.

Code Example

1. ArrayList <PVector> fingerTips = new ArrayList <PVector> ();
2.  
3. void setup() {
4.   size(500, 500);
5.   fill(255);
6.   for (int i=0; i<5; i++) {
7.     fingerTips.add( new PVector(random(width), random(height)) );
8.   }
9. }
10.  
11. void draw() {
12.   background(0);
13.  
14.   for (int i=0; i<fingerTips.size(); i++) {
15.     PVector thisFinger = fingerTips.get(i);
16.     for (int j=i+1; j<fingerTips.size(); j++) {
17.       PVector otherFinger = fingerTips.get(j);
18.       if (thisFinger.dist(otherFinger) < mouseX) stroke(0, 255, 0);
19.       else stroke(0, 0, 255);
20.       line(thisFinger.x, thisFinger.y, otherFinger.x, otherFinger.y);
21.     }
22.   }
23.  
24.   for (PVector thisFinger : fingerTips) {
25.     ellipse(thisFinger.x, thisFinger.y, 10, 10);
26.   }
27. }