processing4/java/examples/Topics/Simulate/MultipleParticleSystems/ParticleSystem.pde

// An ArrayList is used to manage the list of Particles
// An Iterator is used to remove "dead" particles while iterating over the list
import java.util.Iterator;

class ParticleSystem {

  ArrayList<Particle> particles;    // An arraylist for all the particles
  PVector origin;                   // An origin point for where particles are birthed

  ParticleSystem(int num, PVector v) {
    particles = new ArrayList<Particle>();   // Initialize the arraylist
    origin = v.get();                        // Store the origin point
    for (int i = 0; i < num; i++) {
      particles.add(new Particle(origin));    // Add "num" amount of particles to the arraylist
    }
  }

  void run() {
    // Cycle through the ArrayList using Iterator, because we are deleting while iterating
    Iterator<Particle> it = particles.iterator();
    while (it.hasNext()) {
      Particle p = it.next();
      p.run();
      if (p.isDead()) {
        it.remove();
      }
    }
  }

  void addParticle() {
    Particle p;
    // Add either a Particle or CrazyParticle to the system
    if (int(random(0, 2)) == 0) {
      p = new Particle(origin);
    } else {
      p = new CrazyParticle(origin);
    }
    particles.add(p);
  }

  void addParticle(Particle p) {
    particles.add(p);
  }

  // A method to test if the particle system still has particles
  boolean dead() {
    return particles.isEmpty();
  }

}