I downloaded some code from ejohn.org page and was trying to make some changes. Trying to understand arrays I added some to make the code more flexible, but something is wrong. Note that I commented the original code, which actually works but its not preciselly what I want.

/*Eye e1, e2, e3, e4, e5;*/
int eyeMax = 100;
Eye[] e = new Eye[eyeMax];

void setup()
{
 size(1024, 768);
 smooth();
 noStroke();
 
 for (int i=1; i<=e.length; i++){
   int xRand = round(random(width));
   int yRand = round(random(height));
   int sRand = round(random(height/7));
   e[i] = new Eye(xRand, yRand, sRand);
 }

/*  e1 = new Eye( 50,  16,  int(random(500)));
 e2 = new Eye( 64,  85,  40);  
 e3 = new Eye( 90, 200, 120);
 e4 = new Eye(150,  44,  40);
 e5 = new Eye(175, 120,  80);*/
}

void draw()
{
 background(102);
 
 for (int i=1; i<=eyeMax; i++){
  e[i].update(mouseX, mouseY);
  e[i].display();
 }
   
/*  e1.update(mouseX, mouseY);
 e2.update(mouseX, mouseY);
 e3.update(mouseX, mouseY);
 e4.update(mouseX, mouseY);
 e5.update(mouseX, mouseY);

 e1.display();
 e2.display();
 e3.display();
 e4.display();
 e5.display();*/
}

class Eye
{
 int ex, ey;
 int size;
 float angle = 0.0;
 
 Eye(int x, int y, int s) {
   ex = x;
   ey = y;
   size = s;
}

 void update(int mx, int my) {
   angle = atan2(my-ey, mx-ex);
 }
 
 void display() {
   pushMatrix();
   translate(ex, ey);
   fill(255);
   ellipse(0, 0, size, size);
   rotate(angle);
   fill(153);
   ellipse(size/4, 0, size/2, size/2);
   popMatrix();
 }
}

There is an error in the non commented code: loop must go from 0 to e.length - 1: for (i = 0; i < e.length; i++)
Common idiom in Java (and C and lot of other programming languages).