我目前正在尝试将两个子类重构成一个超类。子类称为 Taxi 以及 Shuttle 这个超类叫做 Vehicle . 我已经移动了这两个类中的重复代码，并使子类中的构造函数调用方法super，如下所示，但我一直收到错误消息
在调用父类型构造函数之前不能引用“ident”）
为什么？
这里是 Vehicle ```
public class Vehicle
{
// instance variables - replace the example below with your own
protected String ident;
// The Destination Of Taxi and Shuttle
protected String destination;
// The location of this shuttle.
protected String location;

/**
 * Constructor for objects of class Vehicle
 */
public Vehicle(String id, String base)
{
    // initialise instance variables
    this.ident = id;
    location = base;
    destination = null;
}

/**
 * Return the location of the shuttle.
 * @return The location of the shuttle.
 */
public String getLocation()
{
    return location;
}

/**
 * Return the destination of the shuttle.
 * @return The destination of the shuttle.
 */
public String getDestination()
{
    return destination;
}

/**
 * Return the ID of the shuttle.
 * @return The ID of the shuttle.
 */
public String getIdent()
{
    return ident;
}

}

这里是 `Taxi` ```
public class Taxi extends Vehicle
{
    // Whether it is free or not.
    private boolean free;

    /**
     * Constructor for objects of class Taxi.
     * @param base The name of the company's base.
     * @param id This taxi's unique id.
     */
    public Taxi(String id, String base)
    {
        super(ident);
        free = true;
    }

    /**
     * Book this taxi to the given destination.
     * The status of the taxi will no longer be free.
     * @param destination The taxi's destination.
     */
    public void book(String destination)
    {
        setDestination(destination);
        free = false;
    }

    /**
     * Return the status of this taxi.
     * @return The status.
     */
    public String getStatus()
    {
        return vehicle.ident + " at " + location + " headed for " +
        destination;
    }

    /**
     * Indicate that this taxi has arrived at its destination.
     * As a result, it will be free.
     */
    public void signalArrival()
    {
        location = destination;
        destination = null;
        free = true;
    }
}

这里是 Shuttle ```
import java.util.ArrayList;

/**

• A shuttle.

• Shuttles have a unique ID, a location and a list of destinations.

• They operate a circular route.

• @author David J. Barnes

• @version 2016.12.04
  */
  public class Shuttle extends Vehicle
  {

  private ArrayList route;
  // The destination number in route that the shuttle is
  // currently headed for.
  private int destinationNumber;

  /**

  • Constructor for objects of class Shuttle
  • @param id This shuttle's unique id.
  • @param route The route taken by this shuttle.

  •          The first entry is the starting location.

  */
  public Shuttle(ArrayList route)
  {
  super(ident);
  setRoute(route);
  }

  /**

  • Return the status of this shuttle.
  • @return The status.
    */
    public String getStatus()
    {
    return ident + " at " + location + " headed for " +
    destination;
    }

  /**

  • Indicate that this shuttle has arrived at its next destination.
    */
    public void signalArrival()
    {
    location = destination;
    setNextDestination();
    }

  /**

  • Set the next destination of the shuttle.
    */
    private void setNextDestination()
    {
    destinationNumber++;
    if(destinationNumber >= route.size()) {
    // End of the circular route.
    // Start from the beginning again.
    destinationNumber = 0;
    }
    setDestination(route.get(destinationNumber));
    }

  /**

  • Set the route for this shuttle.
  • @param route The circular list of destinations.
    */
    private void setRoute(ArrayList route)
    {
    if(route.size() < 2) {
    throw new IllegalStateException("setRoute must have at least two destinations");
    }
    // Make a copy of the list parameter.
    this.route = new ArrayList();
    this.route.addAll(route);
    destinationNumber = 0;
    location = route.get(destinationNumber);
    setNextDestination();
    }

}