I am writing a simulation in Java whereby objects act under Newtonian physics. An object may have a force applied to it and the resulting velocity causes it to move across the screen. The nature of the simulation means that objects move in discrete steps depending on the time ellapsed between the current and previous iteration of the animation loop; e.g

public void animationLoop() {
  long prev = System.currentTimeMillis();
  long now;

  while(true) {
    long now = System.currentTimeMillis();
    long deltaMillis = now - prev;
    prev = now;

    if (deltaMillis > 0) { // Some time has passed
      for (Mass m : masses) {
        m.updatePosition(deltaMillis);
      }

      // Do all repaints.
    }
  }
}

A problem arises if the animation thread is delayed in some way causing a large amount of time to ellapse (the classic case being under Windows whereby clicking and holding on minimise / maximise prevents a repaint), which causes objects to move at an alarming rate. My question: Is there a way to determine the time spent in the animation thread rather than the wallclock time, or can anyone suggest a workaround to avoid this problem?

My only thought so far is to contstrain deltaMillis by some upper bound.