在调用超类型构造函数之前无法引用 "X" 错误消息

Question

我目前正在尝试将 2 个子 class 重构为一个超级 class。 sub classes 称为 Taxi 和 Shuttle，superclass 称为 Vehicle。我已经移动了两者中的重复代码，并使 subclasses 中的构造函数调用 super 方法，如下所示，但我不断收到错误消息

Cannot Reference "ident" before supertype constructor has been called)

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<String> 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<String> 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<String> 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<String>();
        this.route.addAll(route);
        destinationNumber = 0;
        location = route.get(destinationNumber);
        setNextDestination();
    }

}

Answer 1

Vehicle 没有 public Vehicle(String id) 构造函数，只有 public Vehicle(String id, String base) 构造函数 .

Taxi 的构造函数应该调用正确的（现有的）超级构造函数。

注意，我们会将局部变量id传递给超级构造函数，传递给Vehicle它自己的空ident变量是没有意义的。

public Taxi(String id, String base)
{
    super(id, base);
    free = true;
}

Shuttle 的构造函数缺少一个 id 参数（来自其文档），让我们添加它并使用列表的第一个条目作为 base 参数（再次来自它的文档）：

     /**
     * 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(String id, ArrayList<String> route)
    {
        super(id,route.get(0));
        setRoute(route);
    }

在调用超类型构造函数之前无法引用 "X" 错误消息

Cannot Reference "X" before supertype constructor has been called error message

java

inheritance

subclass

superclass