Spring s0, s1;

void setup(){
 size(400, 400);
 s0 = new Spring(new PVector(0, 0), new PVector(100, 100));
 s1 = new Spring(s0.b, new PVector(200, 200));
}

void draw(){
 background(255);
 if(mousePressed){
   s1.b.x = mouseX;
   s1.b.y = mouseY;
 }
 s0.updateSpring();
 s1.updateSpring();
 line(s0.a.x, s0.a.y, s0.b.x, s0.b.y);
 line(s1.a.x, s1.a.y, s1.b.x, s1.b.y);
 ellipse(s0.a.x, s0.a.y, 20, 20);
 ellipse(s0.b.x, s0.b.y, 20, 20);
 ellipse(s1.a.x, s1.a.y, 20, 20);
 ellipse(s1.b.x, s1.b.y, 20, 20);
}

class Spring{
 float restLength;
 float length;
 float stiffness;
 float vx, vy;
 PVector moveBy;
 PVector a, b;
 Spring(PVector a, PVector b){
   this.a = a;
   this.b = b;
   stiffness = 0.5f;
   restLength = length = a.dist(b);
   moveBy = new PVector();
 }
 void updateSpring(){
   vx = b.x - a.x;
   vy = b.y - a.y;
   length = a.dist(b);
   float diff = 0;
   if(length > 0f){
     diff = (length - restLength) / length;
   }
   float mul = diff * stiffness;
   moveBy.x = -vx*mul;
   moveBy.y = -vy*mul;
   a.x -= moveBy.x;
   a.y -= moveBy.y;
   b.x += moveBy.x;
   b.y += moveBy.y;
 }
}