Java: Creating a subclass object from a parent object

Question

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.

If you are using Spring in your project you may use ReflectionUtils.

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.)

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

Not at all!

Try this way:

public class Car{
    ...
}

public class Truck extends Car{
    ...

    public Truck(Car car){
        copyFields(car, this);
    }
}


public static void copyFields(Object source, Object target) {
        Field[] fieldsSource = source.getClass().getFields();
        Field[] fieldsTarget = target.getClass().getFields();

        for (Field fieldTarget : fieldsTarget)
        {
            for (Field fieldSource : fieldsSource)
            {
                if (fieldTarget.getName().equals(fieldSource.getName()))
                {
                    try
                    {
                        fieldTarget.set(target, fieldSource.get(source));
                    }
                    catch (SecurityException e)
                    {
                    }
                    catch (IllegalArgumentException e)
                    {
                    }
                    catch (IllegalAccessException e)
                    {
                    }
                    break;
                }
            }
        }
    }

you can use reflection i do it and work fine for me:

public Child(Parent parent){
    for (Method getMethod : parent.getClass().getMethods()) {
        if (getMethod.getName().startsWith("get")) {
            try {
                Method setMethod = this.getClass().getMethod(getMethod.getName().replace("get", "set"), getMethod.getReturnType());
                setMethod.invoke(this, getMethod.invoke(parent, (Object[]) null));

            } catch (NoSuchMethodException | SecurityException | IllegalAccessException | IllegalArgumentException | InvocationTargetException ex) {
                //not found set
            }
        }
    }
 }