One option is to add a "isHitBy(Bullet[] b)" method to your Enemies class, and then each frame pass all the bullets into each enemy, and have their method decide if they've been hit by a bullet, and then do whatever is necessary when that happens.

since a bullet is moving, your Bullet class should store its x and y position (and maybe have a move() method or something like that, it's up to you). Example :
Code:

class Bullet {
  float x;
  float y;
  ...
  void move() {
    y -= speed;
  }
  ...
} 

If you have a method called isHitBy(Bullet b) in your Ennemy class, you can easily retrieve the bullet's position. Example :
Code:

class Ennemy {
  ...
  boolean isHitBy(Bullet b) {
    // make some collision test using b.x and b.y;
  }
  ...
} 

One of the things I really love about Processing is this sort of reaction, and that people share so freely here on the forum.  I remember having the same "it works!" feeling when I was programming an obstacle-course type game for my HP calculator.  (All in RPL on an HP-48S.  Yeah, I was one of *those* geeks in the back of the class.)  Back then, the HP calculator "community" lived in a BBS (which I think was run by HP in Corvallis, OR) that had a forum not unlike this one where people traded tips and hacks for programming the calculator to do many things it wasn't supposed to be able to do.

If I remember correctly, the solution for collision detection was that the vehicle position was compared with each obstacle on the then current "bottom" row of the screen (the road scrolled by from top to bottom).  The bullet detection described above is an extension of the same thing into 2D.

I guess I don't have much of a point here except to say thank you to everyone for sharing -- your expertise, in some cases, and your enthusiasm in all cases!