Generics from ExamLab

[Mark Moge]

Can you explain me why the first method is ok and the second one gives a compile error?

abstract class A<K extends Number>{
abstract <K> A<? super K> useMe(); //ok
abstract <K> A<? extends K> useMe(); //wrong
}

[Tim McGuire]

 

Mark Moge wrote:Can you explain me why the first method is ok and the second one gives a compile error?

abstract class A<K extends Number>{
abstract <K> A<? super K> useMe(); //ok
abstract <K> A<? extends K> useMe(); //wrong
}

yes, Java does not support return-type-based method overloading. Java methods only differ by arguments, not by return type. So, to the compiler the useMe() method and the useMe() method are exactly the same.

That is why the compiler says: useMe() is already defined in A

[Mark Moge]

abstract class A<K extends Number>{
//abstract <K> A<? super K> useMe(); //ok
abstract <K> A<? extends K> useMe(); //still wrong - type parameter ? extends K is not within its bound
}

after commented the first method there is still a compile error. And my question is why A<? super K> is ok and A<? extends K> is wrong.

[Tim McGuire]

 

Mark Moge wrote:

abstract class A<K extends Number>{
//abstract <K> A<? super K> useMe(); //ok
abstract <K> A<? extends K> useMe(); //still wrong - type parameter ? extends K is not within its bound
}

after commented the first method there is still a compile error. And my question is why A<? super K> is ok and A<? extends K> is wrong.

That compiles for me and it should because useMe() is set to return something that extends K which in turn is something that extends Number.

[Mark Moge]

this code

 abstract A<? extends K> useMe(); //compiles without any problem 

but <K> at front hides <K extends Number> from the class definition and gives me a compile error

 <K> abstract A<? extends K> useMe(); // error

Is this code (with <K>) compiles without any problem?

[Tim McGuire]

 

Mark Moge wrote:this code

 abstract A<? extends K> useMe(); //compiles without any problem 

but <K> at front hides <K extends Number> from the class definition and gives me a compile error

 <K> abstract A<? extends K> useMe(); // error

Is this code (with <K>) compiles without any problem?

it compiles for me.

the compiler will not even warn about the hiding, but the Eclipse IDE will warn about this.

what compiler / java version are you using that is not compiling this code?

[Mark Moge]

I have checked again and I can compile it too but my IDE (netbeans) gives me warnings which look like a compile error.
Anyway I dont understand why one method is ok and second one isn't. And if sth is super of K (A<? super K>) then it doesn't have to extends Number (like in a class declaration).

[Rageni Kalra]

Hi,

I am using java 1.7 and i cannot compile this:

abstract class A<K extends Number> {
abstract <K> A<? extends K> useMe();
}

I get this error:

A.java:5: error: type argument ? extends K#1 is not within bounds of type-variab
le K#2
abstract <K> A<? extends K> useMe();
^
where K#1,K#2 are type-variables:
K#1 extends Object declared in method <K#1>useMe()
K#2 extends Number declared in class A
1 error

Can anyone help what is wrong?