That's because you're not using the position. Add something like translate(location.x, location.y); to the top of your display() method and you will actually use the position. Also your check() method doesn't make sense. You probably want to compare it to the width? The forum contains a lot of similar sketches, so you could check those out as well.

With regard to the second question, sideFront[i] is a PVector which has 3 values. See the reference for PVector. So I don't know why you expected 4, except that maybe you got it confused with your frontx array?

One other question, If I modify the code to add velocity to sideFront[i].x why does this not work the same?

It works the same. The velocity gets added. Check it with println.

Your current code seems a bit overly complicated. Perhaps this is because you have other plans with all those arrays. But purely with regard to the code above, it could also be done shorter as shown below.

Adapted Code

1. Face myShape;
2.  
3. void setup() {
4.   size(500,400);
5.   smooth();
6.   myShape = new Face(20,20,25);
7. }
8.  
9. void draw() {
10.   background(200);
11.   myShape.update();
12.   myShape.display();
13. }
14.  
15. class Face {
16.  PVector location, velocity;
17.  float faceScale;
18.  
19.  Face(float _x, float _y, float _faceScale) {
20.    location = new PVector(_x, _y);
21.    velocity = new PVector(2, 0);
22.    faceScale = _faceScale;
23.  }
24.  
25.  void update() {
26.    location.add(velocity);
27.    if (location.x > width-faceScale || location.x < 0) {
28.      velocity.x = -velocity.x;
29.    }
30.  }
31.  
32.  void display() {
33.    pushMatrix();
34.    translate(location.x,location.y);
35.    rect(0, 0, faceScale, faceScale);
36.    popMatrix();
37.  }
38. }

I have the beginShape with the different arrays because I am going to expand into something more unique.

If I change the check() to this:

Yeah, like I said, these types of sketches have been discussed before. Search the forum for safer methods to reverse the velocity using abs.