Calin Baenen

Posted on Dec 9, 2020

Can you implement abstract classes, like you can interfaces?

#java #help #watercooler

Like, if I had:

abstract class Test {
    abstract void test();
}

could I do

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

Top comments (14)

Phil Ashby • Dec 9 '20

Almost: you extend abstract classes, and provide the abstract methods, to take your example, you would:

class Test2 extends Test {
    void test() { /* do that lovely stuff */ }
}

this means you can only extend one abstract class of course, as Java is a single-inheritance language.

Calin Baenen • Dec 9 '20

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

Phil Ashby • Dec 9 '20

By definition, if you sub-class a type, your sub-class inherits all the base type methods and fields. You can add method overrides with different parameter types or quantities, but you cannot simply change a return type. I would recommend adding a new method with a well-chosen name rather than trying to fudge something in with the same name as an existing method, again taking your example:

class Test2 extends Test {
    void test() { /* implements abstract method, providing expected (hopefully documented!) behaviour */ }
    void test(int arg) { /* override for test() method that is parametrised in some way, similar behaviour.. */ }
    int getSomeMagicField() { /* implements new behaviour, which returns some useful information */ }
}

Calin Baenen • Dec 10 '20

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 • Dec 10 '20

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 • Dec 10 '20

Thanks!

Comment deleted

Calin Baenen • Dec 19 '20

What is Dart? Another JVM language?

Thierry Poinot • Dec 9 '20 • Edited

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 {/.../}

Hugues Chabot • Dec 10 '20

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

Calin Baenen • Dec 10 '20

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 • Dec 10 '20

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 • Dec 10 '20

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 • Dec 10 '20

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.

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Can you implement abstract classes, like you can interfaces?

Top comments (14)

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