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Java Forum / General / January 2006Override an inner class?
Inner classes are real useful, especially if you do something like SomeOtherClass soc = <something>; SomeOtherClass.Inner o = soc.new Inner() { public void overrideStuff() {} }; I like being able to override functionality of inner classes this way when the inner class does something that really belongs to a particular enclosing object. For example, SpriteWorld.Sprite. But there seems to be no way to make a non-anonymous class that extends another class's inner class. I've tried using the static keyword here and there but it doesn't work. It seems like this should be possible without messing up the integrity of the object architecture. TIA Dan > But there seems to be no way to make a non-anonymous class that extends > another class's inner class. Sure there is: class A{ class B{ } } class C extends A{ class E extends A.B{ } } (Of course the declaration of E makes no sense without the "extends A".) I know about that one...but it seems silly to override a SpriteWorld and do nothing, just to subclass Sprite...and then I'll have to put all my Sprite override code in one place. I guess I'll have to make an interface that the Sprite calls to do its work. I think Java should have allowed static class BetterSprite extends SpriteWorld.Sprite { Since it wouldn't break mono-morphism or packages or anything. Or at least let it be a first-level class. Thanks Dan > But there seems to be no way to make a non-anonymous class that extends > another class's inner class. That's correct (Torkel's example notwithstanding... in Torkel's cose, he is overriding an inner class that's inherited into the class where he extends it, so it's not really subclassing another class's inner class.) > I've tried using the static keyword here > and there but it doesn't work. It should work if you make the inner class static... but then it's not an inner class any more, so it's still not possible to subclass another class's inner class except with an anonymous class. > It seems like this should be possible without messing up the integrity > of the object architecture. Sure, it is theoretically possible. However, it's not possible within the Java language.  Signaturewww.designacourse.com The Easiest Way To Train Anyone... Anywhere. Chris Smith - Lead Software Developer/Technical Trainer MindIQ Corporation >> But there seems to be no way to make a non-anonymous class that extends >> another class's inner class. [quoted text clipped - 15 lines] > Sure, it is theoretically possible. However, it's not possible within > the Java language. Part of being an inner class is having access to the containing class's private parts. Allowing a subclass to have that same access would be a security hole. I don't (offhand) see any security issues so long as the subclass doesn't have that special access. > Part of being an inner class is having access to the containing class's > private parts. Allowing a subclass to have that same access would be a > security hole. I don't (offhand) see any security issues so long as the > subclass doesn't have that special access. Sure. Just as is the case with the anonymous inner class, the subclass would have access to private members of its own enclosing lexical scopes, but not the enclosing lexical scopes of the superclass. Subclasses in Java never have access to private members that are accessible to their superclasses. Signaturewww.designacourse.com The Easiest Way To Train Anyone... Anywhere. Chris Smith - Lead Software Developer/Technical Trainer MindIQ Corporation > That's correct (Torkel's example notwithstanding... in Torkel's cose, he > is overriding an inner class that's inherited into the class where he > extends it, so it's not really subclassing another class's inner > class.) True (reading "hiding" for "overriding"). But what would it mean to subclass another class's inner class? You have to get a surrounding instance from somewhere. > > That's correct (Torkel's example notwithstanding... in Torkel's cose, he > > is overriding an inner class that's inherited into the class where he > > extends it, so it's not really subclassing another class's inner > > class.) > > True (reading "hiding" for "overriding"). Actually, read "subclassing" for "overriding". Sorry about that; my mistake. > But what would it mean to > subclass another class's inner class? You have to get a surrounding > instance from somewhere. Yes. The syntax would have to provide a means to specify separate enclosing instances for the superclass and the subclass. With anonymous inner classes, there is such a mechanism: the enclosing instance for the anonymous subclass is 'this', while the enclosing instance for the superclass is specified as a qualifier for the "new" expression. There is no such obvious syntax for named inner classes, so I understand why the language doesn't provide the ability. Signaturewww.designacourse.com The Easiest Way To Train Anyone... Anywhere. Chris Smith - Lead Software Developer/Technical Trainer MindIQ Corporation > Actually, read "subclassing" for "overriding". Yes, of course, the inherited inner class wasn't hidden in my example. > Yes. The syntax would have to provide a means to specify separate > enclosing instances for the superclass and the subclass. Would it? Perhaps I misunderstand the suggestion, but I thought it was to be able to modify the behavior of an existing inner class, giving both the same enclosing instance, much like class Outer { class InnerSuper { } class InnerSub extends InnerSuper { } } but with InnerSub defined in a different file. > Would it? Perhaps I misunderstand the suggestion, but I thought it was to > be able to modify the behavior of an existing inner class, giving both the [quoted text clipped - 12 lines] > > but with InnerSub defined in a different file. Right, but there are implications to InnerSub being in a different file. It is, therefore, in a different top-level class... and assuming it's an inner class (which was part of the original description), it must have an enclosing instance of that top-level class. Signaturewww.designacourse.com The Easiest Way To Train Anyone... Anywhere. Chris Smith - Lead Software Developer/Technical Trainer MindIQ Corporation >> Would it? Perhaps I misunderstand the suggestion, but I thought it was >> to [quoted text clipped - 16 lines] > > Right, but there are implications to InnerSub being in a different file. Again, I thought the suggestion was to get around those implications. > There > is no such obvious syntax for named inner classes, so I understand why > the language doesn't provide the ability. Java language does. class A { class Inner { } } A a = new A(); A.Inner ai = a.new Inner() {}; // anonymous subclass of inner class class AInnerSubclass extends A.Inner { // non-anonymous one AInnerSubclass(A a) { a.super(); } } ai = new AInnerSubclass(a); Regards, piotr > class AInnerSubclass extends A.Inner { // non-anonymous one > AInnerSubclass(A a) { > a.super(); > } > } True, you can supply an enclosing instance in the constructor. This simply didn't occur to me. It's rather remarkable that the compiler can figure this out. class A { class Inner { } } A a = new A(); A.Inner ai = a.new Inner() {}; // anonymous subclass of inner class class AInnerSubclass extends A.Inner { // non-anonymous one AInnerSubclass(A a) { a.super(); } } This simply doesn't work. The compiler generates an error for the reason that A.Inner is not accessible here; there is no enclosing instance. And there isn't. (Which A is AInnerSubclass going to refer to when it asks for A.this?) > This simply doesn't work. This compiles just fine: class A{ class B{ } } class C extends A.B{ C(A a){ a.super(); } } The interesting thing is that the compiler actually ponders the body of the constructor in C to make sure that an enclosing instance is provided. > class A { > class Inner { [quoted text clipped - 12 lines] > > This simply doesn't work. It does, simply... copy and paste my example into main() method, then show compilation errors here (if any). > The compiler generates an error for the > reason that A.Inner is not accessible here; there is no enclosing > instance. And there isn't. My example is very simple and it will not solve all your problems... sorry. You have to follow other Java "rules", even in this case... > (Which A is AInnerSubclass going to refer to > when it asks for A.this?) None. In my example A is not the enclosing class of AInnerSubclass, than A.this is simply not allowed. You can access the enclosing instance of type A referring it as final variable or storing (e.g. as field) the instance of A passed to AInnerSubclass constructor. A.this would be allowed in the context of AInnerSubclass only if there were class A enclosing it. BTW, this is quite interesting feature of Java allowing single instance of inner class to have many instances of enclosing classes. Of course, you can simply forget about it, if you don't need this feature... '-) Regards, piotr > class AInnerSubclass extends A.Inner { // non-anonymous one > AInnerSubclass(A a) { > a.super(); > } > } Learn something every day. Hmm. Signaturewww.designacourse.com The Easiest Way To Train Anyone... Anywhere. Chris Smith - Lead Software Developer/Technical Trainer MindIQ Corporation >But there seems to be no way to make a non-anonymous class that extends >another class's inner class. I've tried using the static keyword here >and there but it doesn't work. what happens when you redefine inner classes of inner classes? SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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