Edit:

Well, he edited his post.

If an Object inherits Iterable, you are given the ability to use the for-each loop as such:

for(Object object : objectListVar) {
     //code here
}

So in your case, if you wanted to update your Guns and their Bullets:

for(Gun g : guns) {
     //invoke any methods of each gun
     ArrayList<Bullet> bullets = g.getBullets()
     for(Bullet b : bullets) {
          System.out.println("X: " + b.getX() + ", Y: " + b.getY());
          //update, check for collisions, etc
     }
}

First get your third Gun object:

Gun g = gunList.get(2);

Then iterate over the third gun's bullets:

ArrayList<Bullet> bullets = g.getBullets();

for(Bullet b : bullets) {
     //necessary code here
}

You have several ways to do so:

Basic1:

n = 0;
while (n < arrayList.size()) {
    (arrayList.get(n)).count();
    n++;
}

Basic2:

for (int i = 0; i < arrayList.size(); i++) {
    (arrayList.get(i)).count();
}

Better1:

for(ClassTypeOfArrayList item: arrayList) {
    item.count();
} 

Better2:

Iterator iterator = arrayList.iterator();
while(iterator.hasNext()){
    System.out.println(iterator.next());
}