在实例化/声明时指定类的超类型？

Question

Is it possible to specify the parent of a class when I instantiate/declare that class? For example, can I do something similar to this:

MonitoredDevice<DeviceTypeToExtend> device = null;

And then from that statement, the MonitoredDevice class would extend from the type parameter DeviceTypeToExtend. Now, I know that you can't use type parameters to extend from a superclass, but does anyone know of something similar to achieve this goal of "dynamically" extending from a specific parent.

Thanks,

Steve

Answer 1

Inheritance is overrated , use composition instead (it's more powerful and flexible):

MonitoredDevice monitored = new MonitoredDevice();
monitor.setTargetDevice(new BaseDevice());

or a decorator pattern:

new MonitoredDeviceDecorator(new BaseDevice());

Answer 2

No, you can't do that because then the MonitoredDevice class would have to extend directly from its own type parameter, which is not possible. You could instead have something like MonitoredDevice<T extends MonitoredDevice<T>> , as with Enums, and then there would still be raw type information for the supertype.

Answer 3

You have a lot of options. It may help if you post the interfaces for MonitoredDevice & DeviceTypeToExtend, so we can get a better picture of what you're hoping to achieve.

mhaller's feedback makes sense; you probably don't want to do it all with inheritance.

One possibility: use generics. Your MonitoredDevice interface might look like this:

public interface MonitoredDevice<E> {

  E computeFoo();

  void doBar();

}

You could also use composition (probably simplest) or a multi-level class hierarchy (not recommended).