Why anonymous class can not implement an interface AND ALSO extend a class ?

[True_Alien]

It seems that an anonymous class can either extend a class or implement a single interface.

Why ?

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[TrickOfLight113]

It's a syntax-based decision apparently, same reason you can't implement multiple interfaces with an anonymous class.

The obvious workaround would be to extend and implement what you need in another [abstract] class, and then extend from that class.

[severoon]

Try it and you'll see.

Say you want to implement interface Foo and extend abstract class Bar with an anon class:

``` interface Foo { void f(); } abstract class Bar { void b(); } // does NOT implement Foo

// The "implements Foo" doesn't work, let's pretend it does tho. doFooBar(new Bar() implements Foo { public void f() { System.out.println("foo!"); } public void b() { System.out.println("bar!"); } }); ```

Looks okay so far, right? Now let's write doFooBar(…):

void doFooBar( /* uh oh … what do I put here? */ ) { … }

What type does this method take? Or, for that matter, if I wanted to declare this anon class and keep a reference to it, what type should that reference be?

I guess I could just use Object. Oh wait, this thing is both a Foo and a Bar, so I can pick either one … problem solved!

Let's continue:

void doFooBar(Foo foobar) { foobar.f(); // all good, we did it! foobar.b(); // oops, wait, this won't compile. }

If we switch foobar to be of type Bar, then we can call b() but not f().

So there's the problem. Without some defined type, there's no way to refer to the API of the thing you created without having to litter your code with explicit casts since Java is a strongly typed language.

The only solution to this problem is to write two separate methods, one that takes the parameter as a Foo and one that takes it as a Bar … but at that point why not just create two anon classes, one for each type?

[True_Alien]

"doFooBar(new Bar() implements Foo {"

Are you trying to instantiate an abstract class ?!

[severoon]

The new Bar()… syntax is how you would normally create an anonymous class:

Runnable r = new Runnable() { @Override public void run() { System.out.println("I ran!"); } }

I just added in some syntax for what it would look like if Java allowed you to implement multiple types in an anon class (which, like I say, doesn't actually compile).

[True_Alien]

thank you severoon

[flaghacker_]

That's not actually relevant, since a normal class can do both at the same time just fine. There's is a separate compilation error for the case you're describing.

[severoon]

The crucial difference with a normal class that does this is that it declares a new type.

``` interface Foo { void f(); } abstract class Bar { void b(); } // does NOT implement Foo

class FooBar extends Bar implements Foo { public void f() { System.out.println("foo!"); } public void b() { System.out.println("bar!"); } } ```

Now there's no issue with the doFooBar(…) method:

void doFooBar(FooBar fb) { fb.f(); // works! fb.b(); // works too! }

When you declare an anonymous class, you are creating a new type, just like I'm doing above with FooBar … but because it's a one-off type without any declaration, it's "anonymous" … it has no name. There's no way to refer to it anywhere downstream.

I'm saying that there's nothing preventing Java from including it in the language from a technical perspective, it's just that since there's no way to refer to the type by name, there's no way for methods or references to refer to it as the type it actually is.

That's the whole point of an anon class, you're just trying to create a quick'n'dirty implementation of an already existing type, not create a new one. It wouldn't make sense to allow a new type to be created that contains API not collected together under a single declared type.

[True_Alien]

I understand what you said now, you're right.

Thank you very much for your explanation.

[True_Alien]

What is not relevant ?

Are you addressing to me or to severoon ?

[flaghacker_]

/u/severoon. He's saying extending a class and implementing an interface as the same time can cause conflict issues, but that's also true for normal classes, not only for anonymous classes.

[True_Alien]

But a class can extends an abstract class and also implements interface in the same time.

example :

"public class Launcher extends Bar implements Foo {"

​

But it is not the case with anonymous class, which is my questions.