替换switch case方法的方式1--多态

double getSpeed() {
    switch (_type) {
      case EUROPEAN:
        return getBaseSpeed();
      case AFRICAN:
        return getBaseSpeed() - getLoadFactor() * _numberOfCoconuts;
      case NORWEGIAN_BLUE:
        return (_isNailed) ? 0 : getBaseSpeed(_voltage);
    }
    throw new RuntimeException ("Should be unreachable");
  }