Java中的回调接口

Question

So I'm pretty new to Java and I'm having trouble wrapping my head around this concept.

I got it working by doing this:

What I don't understand is that in my WorkerClass I have a constructor that takes as a parameter a type of CallBackInterface.

In my Main Activity class I make a instance of my WorkerClass, by passing the current instance of Main Activity..why does it work? Shouldn't it give me an error because the constructor from WorkerClass requires a different parameter?

Sorry if this sounds dumb, but I wanna understand why it works, and not just remember that it works like this so I will just use it in the future.

Answer 1

Your activity implements CallBackInterface, so you are not passing a main activity instance into the constructor, but rather instance of CallbackInterface implementation.

If you remove this implementation you will get an error.

Answer 2

why does it work?

it works because instead of thinking:

WorkerClass needs an interface in the constructor

you can(must) think as

WorkerClass needs "something" that can do a callback in the constructor .

MainActivity implements that CallbackInterface so this (the actually instance of the main Activity) is holding that contract and can be pass as argument in the constructor.