Java：从父对象创建子类对象 [英] Java: Creating a subclass object from a parent object

问题描述

Newbie Java问题。说我有：

Newbie Java question. Say I have:

public class Car{
  ...
}

public class Truck extends Car{
  ...
}

我已经有一个Car对象，如何从这个Car对象创建一个新的Truck对象，以便所有的Car对象的值复制到我的新的Truck对象？
理想情况下，我可以这样做：

Suppose I already have a Car object, how do I create a new Truck object from this Car object, so that all the values of the Car object is copied into my new Truck object? Ideally I could do something like this:

Car c = new Car();
/* ... c gets populated */

Truck t = new Truck(c);
/* would like t to have all of c's values */

写我自己的复制构造函数？这将不得不更新每次汽车获得一个新的领域...

Would I have to write my own copy constructor? This would have to be updated everytime Car gets a new field...

推荐答案

是的，只是添加一个构造函数卡车。您可能想要向Car添加一个构造函数，但不一定是公共的：

Yes, just add a constructor to Truck. You will probably want to add a constructor to Car also, though not necessarily public:

public class Car {
    protected Car(Car orig) {
    ...
}

public class Truck extends Car {
    public Truck(Car orig) {
        super(orig);
    }
    ...
}

一般来说最好是让叶子（你可能想标记那些最终的）或抽象。

As a rule it's generally best to make classes either leaf (and you might want to mark those final) or abstract.

看起来像你想要一个 / code>对象，然后将相同的实例变成 Truck 。一个更好的方法是将行为委托给 Car （ Vehicle ）中的另一个对象。所以：

It looks as if you want a Car object, and then have the same instance turn into a Truck. A better way of doing this is to delegate behaviour to another object within Car (Vehicle). So:

public final class Vehicle {
    private VehicleBehaviour behaviour = VehicleBehaviour.CAR;

    public void becomeTruck() {
        this.behaviour =  VehicleBehaviour.TRUCK;
    } 
    ...
}

code> Cloneable ，那么您可以自动将对象复制到同一类的实例。但是还有一些问题，包括必须复制易变对象的每个字段，这些对象容易出错，并且禁止使用final。

If you implement Cloneable then you can "automatically" copy an object to a instance of the same class. However there are a number of problems with that, including having to copy each field of mutable objects which is error-prone and prohibits the use of final.

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