如何为特殊的“多重继承”情况在Java中定义类和接口

Question

Given a standard class which I cannot change (greenfoot Actor; caveat: I am not familiar with either java or greenfoot), I want to create two subclasses which represent transportable goods and transporting means.

The problem appears when a transporting means, such as a bike, is small enough to be a transportable good.

class Transportation extends Actor {...}
class Truck extends Transportation {...}
interface Transportable {...}
class Container extends Actor implements Transportable {...}
class Bike extends Transportation implements Transportable {...}

Now if I want to define methods to load/unload goods, in class Transportation , what shall be the class of the parameter?

void load (Transportable goods) {
    :
    /* not exactly sure about call of getworld(); it returns the "world" */
    World.getWorld().removeObject(goods) <- this is where an Actor is needed
    :
} 

For bookkeeping in the greenfoot world (ie, which objects are "inside the world", meaning outside of any Transportation ), I need to call a method on goods inside load which requires an Actor - and the compiler rightfully tells me that Transportable objects are not Actor s.

But interfaces ( Transportable ) cannot inherit from classes ( Actor ).

The solutions I have seen on SO say to make the top class ( Actor , in this case) have an associated interface, but what do I do when I do not control the top class?

EDIT: added more code in load() as requested; added class Truck

Answer 1

You will have to shuffle around some of your classes and create new interfaces.

class Actor {
 // Use Greenfoot Actor class instead of this one
}

interface ITransportation {

    public void load(Transportable goods);
}

class Transportation extends Actor implements ITransportation{

    @Override
    public void load(Transportable goods) {

    }
}

class Truck extends Transportation{

}

interface ITransportable {

}

class Transportable extends Actor implements ITransportable {

}
class Container extends Transportable {

}

class HybridGood implements ITransportable, ITransportation {

    private Transportation transportation;
    private Transportable transportable;

    @Override
    public void load(Transportable goods) {
        transportation.load(goods);
    }
}
class Bike extends HybridGood {

}

With this approach, you will be able to

• Share the common functionality of Transportation and Transportable
  type in HybridGoods class.
• You can access Actor type in load/unload method.