Doubt in generic method

[Siva Masilamani]

Hi
Please consider the below code.

 public class TestGeneric {

    public static void main(String[] args) {
        A<Integer> a = new A<Integer>();
        new GenClass<B>().getNow(a);
    }
}

class A<E> {
}

class B extends A {
}

class C extends B {
}

class D {
}

class GenClass<E extends A> {

    public <T extends E> A<? super T> getNow(A<? super T> a) {
        return new A<A>();
    }
}   

In the program i am creating an instance for GenClass with generic type as B,since generic type in GenClass can take anything that extends A ,it is ok to create an instance with generic type with B.Infact i canpass the generic type as A or B or C.

But my doubt is when invoking the method i pass the reference to a type A<Integer> so i thought the generic type T inside the method would be considered as Integer,so that the method will throw error.I thought since T is considered as Integer then T extends E would fail ,but the following method compiles successfully.

But if I am returning anything that is not A or super of A then it is showing the compilation error.

I do not understand the relation between these fragment T extends E and the return type ? super T and the argument ? super E.Are they not relate to each other?

please help me.

[Siva Masilamani]

please somebody help.

[Henry Wong]

Siva Masilamani wrote:But my doubt is when invoking the method i pass the reference to a type A<Integer> so i thought the generic type T inside the method would be considered as Integer,so that the method will throw error.I thought since T is considered as Integer then T extends E would fail ,but the following method compiles successfully.

I agree. It should not compile for the reason given. Wait a sec... Just tried it. And it did not compile.

Henry

[Siva Masilamani]

It compiles and runs fine for me.

I am using Netbeans IDE with jdk 1.6

still confused???

[Leandro Coutinho]

The code compile and runs fine.
I also have the same doubts. It's a strange code.

E -> B
? -> Integer
T -> ???

What T would be?

[Siva Masilamani]

Henry,Leandro

Are you able to find the reason why it compiles ?

[Henry Wong]

Siva Masilamani wrote:Henry,Leandro

Are you able to find the reason why it compiles ?

It did *not* compile for me. I tested it with the Eclipse Java 5 compiler.

Henry

[Siva Masilamani]

Thanks Henry

I checked the code in Eclipse and it shows error.

But why i am able to return only A or Super of A i.e Object not B or C.

I have passed the parameter to the function as A<C> a=new A<C>.(Replaced Integer as C)

[zhong chen]

It compiles on my machine. I use dos and jdk1.6.3.

[Siva Masilamani]

Did you use Eclipse or Netbeans?

I tried with WSID 6.2 with default JRE and it did not compile which is correct.If it compiles then it is wrong.

I think this is a bug with in Some JDK version.

I use JDK 1.6.16 in Netbeans

[rutuparna andhare]

it compiles fine (notepad & dos) jdk 1.6
there are two views to solve this type of problem
1 view>

paramether passing must be same as argument
A<Integer>a=new A<Integer>();
parameter a i.e A<Integer>
argument A<? super T> //anythig whos super is T

2 view>

so what is T ?
see left <T extends E>
so what is E ?
A<E>
again what is E?

in main
we wrote that
A<Integer>a=new A<Integer>();
-------------------------------------------
so Integer Integer matches

[Siva Masilamani]

[quote=rutuparna andhare]it compiles fine (notepad & dos) jdk 1.6
there are two views to solve this type of problem
1 view>

paramether passing must be same as argument
A<Integer>a=new A<Integer>();
parameter a i.e A<Integer>
argument A<? super T> //anythig whos super is T

2 view>

so what is T ?
see left <T extends E>
so what is E ?
A<E>
again what is E?

in main
we wrote that
A<Integer>a=new A<Integer>();
-------------------------------------------
so Integer Integer matches
[/quote]

E - Generic type is not from class A instead from class GenClass and has no relation to A.
This code should not be compiled as it is.

But some IDE especially Netbeans compules fine but Eclipse shows error.

[rutuparna andhare]

i respect everyones thought
i think this will help

i think E is confusing ! right

public <T extends E> A<? super T> getNow(A<? super T> a)

in this sentance whatever is in argument A<----T---> relates to leftside <T extends E> but only and only for class "A"

to check this you can
compile this code
--------------------------------------------------------------

public class TestGeneric
{

public static void main(String [] args)
{
A<Integer> a = new A<Integer>();
new GenClass<B>().getNow(a);
}
}

class A<E>
{
static void call()
{
System.out.println("in class A");
}
}

class B extends A
{
static void call()
{
System.out.println("in class B");
}
}

class C extends B
{
static void call()
{
System.out.println("in class C");
}
}

class D { }

class GenClass<E extends A>
{
public <T extends E> A<? super T> getNow(A<? super T> a)
{
E.call(); // shows what type of class E is
return new A<A>();
}

static void call()
{
System.out.println("in class GenClass");
}
}
------------------------------------------------------------------------------------

[Siva Masilamani]

in this sentance whatever is in argument A<----T---> relates to leftside <T extends E> but only and only for class "A"

Could you please explain this more elaborately?

i am soo confused now.

Then why different IDE gives different result?

Which is one is corrct?

I am being struck in this for past 3 days and i am afraid that my SCJP exam is on Nov 14th

Many thanks in advance