Simple Particle System by Daniel Shiffman.

Particles are generated each cycle through draw(), fall with gravity and fade out over time A ParticleSystem object manages a variable size (ArrayList) list of particles.

 
ParticleSystem ps;

void setup() {
  size(640, 360);
  colorMode(RGB, 255, 255, 255, 100);
  ps = new ParticleSystem(1, new PVector(width/2,height/2,0));
  smooth();
}

void draw() {
  background(0);
  ps.run();
  ps.addParticle(mouseX,mouseY);
}






// A class to describe a group of Particles
// An ArrayList is used to manage the list of Particles 

class ParticleSystem {

  ArrayList particles;    // An arraylist for all the particles
  PVector origin;        // An origin point for where particles are born

  ParticleSystem(int num, PVector v) {
    particles = new ArrayList();              // 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 backwards b/c we are deleting
    for (int i = particles.size()-1; i >= 0; i--) {
      Particle p = (Particle) particles.get(i);
      p.run();
      if (p.dead()) {
        particles.remove(i);
      }
    }
  }

  void addParticle() {
    particles.add(new Particle(origin));
  }
  
    void addParticle(float x, float y) {
    particles.add(new Particle(new PVector(x,y)));
  }

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

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

}




// A simple Particle class

class Particle {
  PVector loc;
  PVector vel;
  PVector acc;
  float r;
  float timer;
  
  // Another constructor (the one we are using here)
  Particle(PVector l) {
    acc = new PVector(0,0.05,0);
    vel = new PVector(random(-1,1),random(-2,0),0);
    loc = l.get();
    r = 10.0;
    timer = 100.0;
  }

  void run() {
    update();
    render();
  }

  // Method to update location
  void update() {
    vel.add(acc);
    loc.add(vel);
    timer -= 1.0;
  }

  // Method to display
  void render() {
    ellipseMode(CENTER);
    stroke(255,timer);
    fill(100,timer);
    ellipse(loc.x,loc.y,r,r);
    displayVector(vel,loc.x,loc.y,10);
  }
  
  // Is the particle still useful?
  boolean dead() {
    if (timer <= 0.0) {
      return true;
    } else {
      return false;
    }
  }
  
   void displayVector(PVector v, float x, float y, float scayl) {
    pushMatrix();
    float arrowsize = 4;
    // Translate to location to render vector
    translate(x,y);
    stroke(255);
    // Call vector heading function to get direction (note that pointing up is a heading of 0) and rotate
    rotate(v.heading2D());
    // Calculate length of vector & scale it to be bigger or smaller if necessary
    float len = v.mag()*scayl;
    // Draw three lines to make an arrow (draw pointing up since we've rotate to the proper direction)
    line(0,0,len,0);
    line(len,0,len-arrowsize,+arrowsize/2);
    line(len,0,len-arrowsize,-arrowsize/2);
    popMatrix();
  } 

}