Apple extends Asset{
  public Apple(Asset yen, float price){
    setPrice(yen, price);
    yen.setPrice(1 / price, this);
  }
  
  public Apple(){
  }
}

Yen extends Asset{
  public Yen(Asset products, float price){
    setPrice(products, price);
    products.setPrice(1 / price, this);
  }
  public Yen(){
  }
}

Bottle{
  public static void main(){
    Yen yen = new Yen();
    Apple apple = new Apple((Asset)yen, 100);
    checkPrice((Asset)apple);
  }
  private checkPrice(Asset asset){
    float price = asset.getPrice();
    Asset unit = asset.getUnitOfPrice();
    checkPrice(unit);
  }
}