Basic stupidity Java generics - comp.lang.java.programmer

Hi all, Probably I am doing something very twisted, but I keep getting an "incompatible types" compile error and I don't know why. I brought it back to the very simple script below. Could somebody please explain me, why is this incorrect, it seems perfectly logical to me... :( This is a big problem for me, because I want to hide all implementation behind interfaces, and I need the return type IContainer<Ithing> to be compatible with Container<Thing>. Apparently it isn't. Anyone? Many things for any help! Cheers, Paul public class Test { public interface IThing { } public interface IContainer<TYPE extends IThing> { } public class Thing implements IThing { } public class Container<TYPE extends Thing> implements IContainer<TYPE> { } public Test() { IContainer<IThing> var = new Container<Thing>(); } public static void main(String[] args) { new Test(); } } $ javac Test.java Test.java:23: incompatible types found : Test.Container<Test.Thing> required: Test.IContainer<Test.IThing> IContainer<IThing> var = new Container<Thing>(); ^ 1 error

Damn! sorry, I found the problem. I spend two hours before posting the question and I found the answer just minutes after the posting: This is not correct: IContainer<IThing> var =3D new Container<Thing>(); It needs to be: IContainer<? extends IThing> var =3D new Container<Thing>(); Again, sorry, and thanks for reading. Cheers, Paul On 4 sep, 21:58, "pjvleeu...@gmail.com" <pjvleeu...@gmail.com> wrote: > Hi all, > Probably I am doing something very twisted, but I keep getting an > "incompatible types" compile error and I don't know why. I brought it > back to the very simple script below. Could somebody please explain > me, why is this incorrect, it seems perfectly logical to me... :( > This is a big problem for me, because I want to hide all > implementation behind interfaces, and I need the return type > IContainer<Ithing> to be compatible with Container<Thing>. Apparently > it isn't. > Anyone? Many things for any help! > Cheers, > Paul > > public class Test { > =A0 =A0 =A0 =A0 public interface IThing { > =A0 =A0 =A0 =A0 } > =A0 =A0 =A0 =A0 public interface IContainer<TYPE extends IThing> { > =A0 =A0 =A0 =A0 } > =A0 =A0 =A0 =A0 public class Thing implements IThing { > =A0 =A0 =A0 =A0 } > =A0 =A0 =A0 =A0 public class Container<TYPE extends Thing> implements > IContainer<TYPE> { > =A0 =A0 =A0 =A0 } > =A0 =A0 =A0 =A0 public Test() { > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 IContainer<IThing> var =3D new Container<= Thing>(); > =A0 =A0 =A0 =A0 } > =A0 =A0 =A0 =A0 public static void main(String[] args) { > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 new Test(); > =A0 =A0 } > > } > > $ javac Test.java > Test.java:23: incompatible types > found =A0 : Test.Container<Test.Thing> > required: Test.IContainer<Test.IThing> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 IContainer<IThing> var =3D new Container<= Thing>(); > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 = =A0 =A0 =A0^ > 1 error

pjvleeuwen@gmail.com wrote: > Damn! sorry, I found the problem. I spend two hours before posting the > question and I found the answer just minutes after the posting: > This is not correct: IContainer<IThing> var = new Container<Thing>(); > It needs to be: IContainer<? extends IThing> var = new > Container<Thing>(); > > Again, sorry, and thanks for reading. > No problem, but thanks for posting back with the solution. In the future, please do not top-post. > Cheers, > Paul > > On 4 sep, 21:58, "pjvleeu...@gmail.com" <pjvleeu...@gmail.com> wrote: > > Hi all, > > Probably I am doing something very twisted, but I keep getting an > > "incompatible types" compile error and I don't know why. I brought it > > back to the very simple script below. Could somebody please explain > > me, why is this incorrect, it seems perfectly logical to me... :( > > This is a big problem for me, because I want to hide all > > implementation behind interfaces, and I need the return type > > IContainer<Ithing> to be compatible with Container<Thing>. Apparently > > it isn't. > > Anyone? Many things for any help! > > Cheers, > > Paul > > > > public class Test { > > public interface IThing { > > } > > public interface IContainer<TYPE extends IThing> { > > } > > public class Thing implements IThing { > > } > > public class Container<TYPE extends Thing> implements > > IContainer<TYPE> { > > } > > public Test() { > > IContainer<IThing> var = new Container<Thing>(); > > } > > public static void main(String[] args) { > > new Test(); > > } > > > > } > > > > $ javac Test.java > > Test.java:23: incompatible types > > found : Test.Container<Test.Thing> > > required: Test.IContainer<Test.IThing> > > IContainer<IThing> var = new Container<Thing>(); > > ^ > > 1 error > -- Sabine Dinis Blochberger Op3racional www.op3racional.eu

On Thu, 4 Sep 2008 12:58:52 -0700 (PDT), "pjvleeuwen@gmail.com" <pjvleeuwen@gmail.com> wrote, quoted or indirectly quoted someone who said : >public interface IContainer<TYPE extends IThing> { public class Container<TYPE extends Thing> implements It is confusing and probably incorrect to redefine the meaning of TYPE -- Roedy Green Canadian Mind Products The Java Glossary http://mindprod.com

Roedy Green <see_website@mindprod.com.invalid> writes: > On Thu, 4 Sep 2008 12:58:52 -0700 (PDT), "pjvleeuwen@gmail.com" > <pjvleeuwen@gmail.com> wrote, quoted or indirectly quoted someone who > said : > >>public interface IContainer<TYPE extends IThing> { > public class Container<TYPE extends Thing> implements > > It is confusing and probably incorrect to redefine the meaning of TYPE Might be confusing (although I personally don't think so, and would just have called them both "T"), but it's definitly not incorrect. It's just two unrelated type variables that happen to have the same name. It's not more incorrect than having two different methods both containing a variable called "i", or two methods both having an argument called "x". /L -- Lasse Reichstein Nielsen DHTML Death Colors: <URL:http://www.infimum.dk/HTML/rasterTriangleDOM.html> 'Faith without judgement merely degrades the spirit divine.'