Get in if you like generics

[Lukas Stephienn]

Look below and try to answer if this will compile. If it does what output will be generated (don't cheat).
Have fun

class A<A> {
 A A;
 <A> A A(A A) {
  System.out.println(A.getClass().getName());
  return A;
 }
}
class B extends A {
 static public void main(String... String) {
  A<String> A = new A<String>();
  A.A = "A";
  A B = A.A(new B());
 }
}

[Sampada Prabhu]

i think it will compile.....

[S Thiyanesh]

I think it will not compile

[Lukas Stephienn]

We can solve this like men; bring your six-shots and meet in the town middle at noon .

[vivekkumar sharma]

It compiles

[vidya sagar]

Compile or Not compiling?? Question is not interesting??

what the result and How??..Some what Interesting??

Loking for response??

[Firas Zuriekat]

Your's compiles as well as the following: (it looks weird with ;;; but it runs fine)

class A<A extends Number> { A A;;; <A extends Number> A A(A A) {
System.out.println(A.getClass().getName());
return A; }}

class B extends A {

B(){System.out.println(new A().A(128));}
static public void main(String... String) {
A<Integer> A = new A<Integer>();
A.A = 1;
A<Byte> B = new B();

}}

OUTPUT:
C:\myJava>java B
java.lang.Integer
128
[ June 07, 2006: Message edited by: Firas Zureikat ]

[Lukas Stephienn]

You are right guys (except S Thiyanesh - sorry man .
It compiles. Those A's look consfusing, but compiler says it's not problem for him.

Generic A in class declaration shadows class name A.
And generic A in method declaration shadows previously declared generic A.

Main with String...String parameter is ok cause variables can have same name as types.

[Firas Zuriekat]

The following also runs fine:

class A<A extends Number> { A A;;; <C extends Byte> C A(C A) {
System.out.println(A.getClass().getName());
return (C)A; }}

class B extends A {

B(){Byte x= new Byte("2");System.out.println(new A().A(x));}
static public void main(String... String) {
A<Integer> A = new A<Integer>();
A.A = 1;
A<Byte> B = new B();

}}

OUTPUT:
java.lang.Byte
2

REASON method A(C A) works:
Instance variable A extends Number but using "new A()" uses the non generic style (i.e. no need to be strict with generics in terms of what you pass the A constructor)
At the method level, C extends Byte and it is determined by the compiler that it's a byte when you sent the method the varable x. So casting variable A to a Byte is no problem because A is a Byte variable itself.
[ June 07, 2006: Message edited by: Firas Zureikat ]

[Akshay Kiran]

sheesh
dirty bit of coding
If only the makers knew how it would mess up the language....

where'd you pick up this thing from anyway? java puzzlers by bloch?

[warren li]

I don't need to understand this level of generics to get all the generics questions right. :roll:

[Lukas Stephienn]

I made it myself .
It's true this is crapy question but it shows what messy code could be written (whitout errors) with generics.

[warren li]

Originally posted by Lucius Stephienn:
I made it myself .
It's true this is crapy question but it shows what messy code could be written (whitout errors) with generics.

you know that not everyone likes the decision to have generics in java right from the beginning.

[Lukas Stephienn]

you know that not everyone likes the decision to have generics in java right from the beginning.

Yes I know. I was one of them, but If you want to pass SCJP 5.0 you have to learn it. Even if dislike it.

[Swapnil Trivedi]

it was nice code Lucius Stephienn...It was like
for me...
I am preparing for SCJP1.5 but you scared me

[Swapnil Trivedi]

Can anybody tell me that the concept that "a superclass can not access it's members from a subclass with its own object applies only to PROTECTED MEMBERS..."
is true statement...

[Lukas Stephienn]

Dont be scared. These kind of question dont appear on exam (but watch out for shadowing; it's tricky).

Swapnil Trivedi: I dont understand your question, could you be more precise?

[Swapnil Trivedi]

Hi Lucius,
Thanks dear, I have got my doubt

Lucius do you have the link to the mock exams from Sun microsystems.

[ June 09, 2006: Message edited by: Swapnil Trivedi ]
[ June 09, 2006: Message edited by: Swapnil Trivedi ]

[Firas Zuriekat]

static public void main(String... String) {}

Isn't String a keyword? If so, why is is accepted above as a variable name?

I thought keywords are not supposed to be used a variable literals.

[Lukas Stephienn]

From Sun? No. The only links I have are these from javaranch (there was a thread about mocks).

Firas Zureikat: String isn't a keyword, better remember it. Moreover, you can write variable names same as types:

Object Object = new Object();

[Firas Zuriekat]

Ok, It takes time to get used to the fact that a type name could be given to a variable name as in args main() about.

Thanks