eulerpc.java

//save previous particle forces and velocities
ArrayList prevForces = new ArrayList();
ArrayList prevVelocities = new ArrayList();
for(int i=0;i<particles.size();++i) {
  Particle p = (Particle) particles.get(i);
  prevVelocities.add(p.velocity);
  prevForces.add(p.force);
}
//compute estimate
for(int i=0;i<particles.size();++i) {
  Particle p = (Particle) particles.get(i);
  p.position += deltaT*p.velocity;
  p.velocity += deltaT*p.force/p.mass;
}
computeForces();
for(int i=0;i<particles.size();++i) {
  Particle p = (Particle) particles.get(i);
  PVector prevV = (PVector) prevVelocities.get(i);
  PVector prevF = (PVector) prevForces.get(i);
  p.position += deltaT*(p.velocity+prevVelocity)/2.0;
  p.velocity += deltaT*(p.force+prevForce)/p.mass/2.0;
}