I agree with Alexander, and indeed when I compiled your code I saw the error I
expected (AspectJ 1.8.5):
ajc -1.5 *.java *.aj
/Users/aclement/provaBug/ClassImplementingInterf.java:3 [error] The type
ClassImplementingInterf must implement the inherited abstract method
MyInterface.myMethod()
public class ClassImplementingInterf {
AspectJ wants to ensure the type system is consistent when you are applying
ITDs. Here you have said a class implements an interface but you haven’t
provided an implementation for the method, it is an error. In your more recent
reply you said " If i had to implement directly the interface by hard coding it
in the classes i can also specify an implements clause in the class
declaration, isn't it?” - yes, you could, but you don’t have to directly
implement the interface by hard coding it in the class. You can use an ITD for
it:
public aspect ConcreteAspect extends MyAspect {
declare parents: ClassImplementingInterf implements MyInterface;
// implementors of this interface not providing an implementation of
this method will get this one
public void MyInterface.myMethod(){
System.out.println("myMethod");
}
pointcut mypointcut(): call(public void OtherClass.method(int));
pointcut interfacemethod(): call(void *.*.myMethod());
}
So no changes required in ClassImplementingInterf. This pattern above is very
common.
I’m surprised you aren’t seeing errors like the one above, I wonder what
compilation sequence you are using to build the pieces. Why haven’t I gotten
around to implementing traits :)
cheers,
Andy
> On Mar 5, 2015, at 2:33 AM, Savoja Luca <[email protected]> wrote:
>
> Hi and thanks for the reply,
>
> Well I think that adding an interface with declare parents and not specifying
> an implementation is not a problem in AspectJ,
> if it were not so, what should serve declare parents? If i had to implement
> directly the interface by hard coding it in the classes i can also specify an
> implements clause in the class declaration, isn't it?
>
> I know it's a strange behaviour to add an interface with aspectj and pretend
> that classes works as the interface was present, but i'm trying to create an
> aspect that implements the GoF command pattern (like in the work of Jan
> Hannemann and Gregor Kiczales
> https://www.cs.ubc.ca/labs/spl/projects/aodps.html
> <https://www.cs.ubc.ca/labs/spl/projects/aodps.html> )
>
> And I want to underline that everything work fine except the implicit cast
> when extracting the ClassImplementingInferf instance from the collection.
>
> I've made some other tests and i found out that
> MyInterface var2 = (MyInterface)(ClassImplementingInterf)x.get(0);
> Works fine, but
> MyInterface fromLst = x.get(0);
> throw ClassCastException
>
> I think it's a strange behaviour!
>
> I hope i was clearer.
> Regards,
> Luca
>
>
> 2015-03-04 20:46 GMT+01:00 Alexander Kriegisch <[email protected]
> <mailto:[email protected]>>:
> Hi Luca.
>
> I am nowhere near a PC now, so I cannot run/test your code, but from what I
> see I am really surprised you do not get more errors earlier in the process
> because it just does not make sense to ITD-declare a class to implement an
> interface without also specifiying an actual implementation. I wonder why it
> even compiles. Don't you see any compilation errors? Maybe the class file
> just (partly) works by chance because Ajc has produces halfway usable
> bytecode before terminating.
>
> Probably you will get more qualified feedback from Andy Clement or even from
> me when I have time to actually switch on my home office PC (which can be
> days from now). My hint is just quick and first-aidish.
>
> Regards
> --
> Alexander Kriegisch
> http://scrum-master.de <http://scrum-master.de/>
>
>
> > Am 04.03.2015 <tel:04.03.2015> um 17:05 schrieb Savoja Luca
> > <[email protected] <mailto:[email protected]>>:
> >
> > Hello everyone,
> >
> > I found a situation where i get a ClassCastException that might not be
> > thrown.
> > With declare parents I force a class to implement an interface, in an
> > aspect i put an object of this class in a collection of the interface's
> > type. And everythig works fine thus far.
> > Then when i extract the object from the collection i get a
> > ClassCastException.
> >
> >
> > Here's an example that illustrates what i'm describing you.
> >
> > I've a simple interface:
> >
> > public interface MyInterface {
> > void myMethod();
> > }
> >
> > and a class that i force to implement it by declare parents.
> >
> > public class ClassImplementingInterf {
> > //if the the interface methods are present in the implementing class
> > there's no cast error
> > /*
> > public void myMethod(){ //NB: commented, not present.
> > System.out.println("myMethod");
> > }
> > */
> > }
> >
> > then I have an abstract aspect:
> >
> > public abstract aspect MyAspect {
> > private ArrayList<MyInterface> x = new ArrayList<MyInterface>();
> >
> > abstract pointcut interfacemethod();
> > abstract pointcut mypointcut();
> >
> > void around(): mypointcut(){
> > ClassImplementingInterf obj = new ClassImplementingInterf();
> >
> > x.add(obj); //working
> > MyInterface var = (MyInterface) obj; //working
> > ClassImplementingInterf fromListWithCast =
> > (ClassImplementingInterf) x.get(0); //working
> >
> > System.out.println("Just before MyInterface fromLst =
> > x.get(0)");
> > MyInterface fromLst = x.get(0); //ClassCastException
> >
> > System.out.println("**** intercepted OtherClass.method *****
> > "); //not reached!
> > }
> >
> > void around(): interfacemethod(){
> > System.out.println("**** intercepted interface method **** ");
> > }
> > }
> >
> > and it's concrete aspect:
> >
> > public aspect ConcreteAspect extends MyAspect {
> > declare parents: ClassImplementingInterf implements MyInterface;
> >
> > pointcut mypointcut(): call(public void OtherClass.method(int));
> > pointcut interfacemethod(): call(void *.*.myMethod());
> > }
> >
> > and an other class (useless, i use it only for intercept the method)
> > public class OtherClass {
> > public void method(int n){
> > System.out.println(n*2);
> > }
> > }
> >
> > The main class is:
> > public class ClassMain {
> > public static void main(String[] args) {
> > OtherClass o = new OtherClass();
> >
> > MyInterface ci = new ClassImplementingInterf();
> > ci.myMethod(); //this works!! the class can act like it has
> > the interface!
> >
> > o.method(15);
> > }
> > }
> >
> >
> > As you can see i can put an ClassImplementingInterf instance in a
> > MyInterface variable or collection, and I can also cast it to MyInterface.
> > But when extracting it from a collection it doesn't work.
> >
> > I think there's some kind of reflective controls when extracting an element
> > from a collection that fails when i do it.
> > Sure enough if i add to ClassImplementingInterf a method that has the same
> > signature of the interface the ClassCastException disappears and everything
> > works fine.
> >
> > I've attached to this message a .rar package that contains my code.
> >
> > Best Regards,
> > --
> > Luca Savoja.
> > <provaBug.rar>
> > _______________________________________________
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>
> --
> Luca.
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