方法签名泛型中的java对此进行了扩展

Question

This does not work because of this.class it must be Tower , but I want that all classes that extends from Tower are assignable from the current Tower like ArrowTower or something else.

public Tower upgrade(Class<? extends this.class> c) {
    try {
        return c.getConstructor().newInstance();
    } catch (Exception e) {
        e.printStackTrace();
    }
    return null;
}

I can check it so, but I want that the complier says you thats the wrong extends hierarchy or something else

if (!this.getClass().isAssignableFrom(c))
{
    return null;
}

I want it so because there are an ArrowTower and a FireArrowTower that are extends from ArrowTower and there are a CannonTower and a FireCannonTower that are extends from CannonTower . But if want to upgrade an ArrowTower its possible to upgrade it to an CannonTower , but this should not be allowed.

See:

    Tower t1 = new ArrowTower();
    t1 = t1.upgrade(FireArrowTower.class);

    Tower t2 = new CannonTower();
    t2 = t2.upgrade(Tower.class); // Here it musst be an Error

Sorry for my bad english I'm from Germany :D

Answer 1

This is impossible due to the principle that, loosely speaking, a subclass should support everything the superclass supports (see also the Liskov substitution principle).

Let me illustrate. With the structure you describe, you want the compiler to accept:

Tower myTower = new Tower();
myTower.upgrade(ArrowTower.class)

.. but not:

GunTower myTower = new GunTower();
myTower.upgrade(ArrowTower.class);

right? That would cause trouble, though. Look:

Tower myTower = new GunTower();
myTower.upgrade(ArrowTower.class);

Here we only made the type of 'myTower' a bit less specific, and now you'd like the compiler to suddenly disallow this call. That's surprising, and indeed Java doesn't allow it.