processing4/java/examples/Books/Processing Handbook/Illustrations/page_476/ArrowParticle.pde

class ArrowParticle extends Particle {  
  float angle = 0.0;  
  float shaftLength = 20.0;
  
  ArrowParticle(int ix, int iy, float ivx, float ivy, float ir) {
    super(ix, iy, ivx, ivy, ir);
  }
  
  void update() {
    super.update();
    angle = atan2(vy, vx);
  }

  void display() {
    stroke(0);
    pushMatrix();
    translate(x, y);
    rotate(angle);
    scale(shaftLength);           
    strokeWeight(1.0/shaftLength);
    line(0, 0, .25, 0);
    line(.25, 0, .174, -.075);
    line(.25, 0, .174, .075);
    popMatrix();
  }
}