Can you implement abstract classes, like you can interfaces?

Like, if I had:

abstract class Test {
    abstract void test();
}

could I do

class Test2 implements Test {/*...*/}

?

Comments

[Calin Baenen]

What is Dart? Another JVM language?

[Calin Baenen]

Okay. Thanks.

I really wish there was a way to do so (for the "daring").
I have something I wanted to be considered a java.lang.Runnable (because it has similar functionality), the only difference is that run returns an Object, and run is a default method (in my future library, this is called a CodeShell).

[Alain Van Hout]

For a 'Runnable that returns an object' have a look at the Supplier class. For a runnable that takes an object as part of its method call, there's Consumer, and if you want both combined there's Function.

[Calin Baenen]

Thanks!

[Hugues Chabot]

In a subclass, you can override the return type of a method: stackoverflow.com/a/14694885

[Calin Baenen]

Yes, but it must be a subclass of the original return type, and I seen this.
But, I don't think void has any subclasses, and I don't think you can subclass void (even though void IS a class (void.class proves)).

[Hugues Chabot]

Exactly, nothing extends void. I am not sure of what you want to do but could java.util.function.Supplier be a better suited interface than Runnable?

[Calin Baenen]

No, because Runnable implies it's a piece of code that's run (one that's not seeking a result), much to the likes of Thread (which implements Runnable).
If Runnable was useless, it wouldn't have been made.

I want my class to be considered part of Runnable because it matches the format with similar (if not (essentially) the same) methods, and because a lot of things use Runnable in place of functions in Java.

[Alain Van Hout]

Note that a runnable doesn't 'return' void. What the 'void' return type means is that it doesn't return something.

Things like Supplier, Consumer and Function are used as much as Runnable.

[Thierry Poinot]

An abstract class is a class that you can't instantiate, but another class can "extends" it.

You must extends class and abstract class and you must implements interface.

So you must write :

class Test2 extends Test {/.../}

[Calin Baenen]

Can I subclass (extend) without inheriting any of the methods?
Or can I override the return type of the only given method?