inheritance - 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.