introductionToProgramming/week13/Gearbox.java

package week13;

public class Gearbox {
  private int gear = 1;

  public static class IllegalGearChangeException extends RuntimeException {}

  void changeGear(int newGear) {
    if(newGear == gear) return;
    if(newGear != -1 && (newGear < 1 || newGear > 5)) throw new IllegalArgumentException();
    // newGear is now either -1 or 1..=5
    var gearToCheck = newGear;
    if(gearToCheck == -1) gearToCheck++;
    if(Math.abs(gear - newGear) != 1) throw new IllegalGearChangeException();
    gear = newGear;
  }
}