Java: Creating a subclass object from a parent object

Question

Newbie Java question. Say I have:

public class Car{
  ...
}

public class Truck extends Car{
  ...
}

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 */

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

Answer 1

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);
    }
    ...
}

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

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;
    } 
    ...
}

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.

Answer 2

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

Essentially, yes - you can't just convert an object in Java.

Fortunately you don't have to write all the code yourself - look into commons-beanutils, specifically methods like cloneBean. This has the added advantage that you don't have to update it every time it gets a new field!

Answer 3

You will need a copy constructor, but your copy constructor can use reflection to find the common fields between the two objects, get their values from the "prototype" object, and set them on the child object.

Answer 4

Yes, you have to do this manually. You'll also need to decide how "deeply" to copy things. For instance, suppose the Car has a collection of tyres - you could do a shallow copy of the collection (such that if the original object changes the contents of its collection, the new object would see the change too) or you could do a deep copy which created a new collection.

(This is where immutable types like String often come in handy - there's no need to clone them; you can just copy the reference and know that the contents of the object won't change.)

Answer 5

You could use the reflection API to loop through each of the Car fields and assign the value to the equivalent Truck fields. This can be done within truck. Further it is the only way to access the private fields of Car - at least in an automatic sense, providing that a security manager is not in place and restricting access to private field.