Java Generics

**help_desk** · August 7th, 2014, 11:05 AM

Hi guys,

Please, this question is from the OCP Java SE 6 Programmer Practice Exams textbook, Assessment Test 2 question 11. Below is the code for the question:

class A{}
class B extends A {}
class C extends B {}

 
  public class Carpet<V extends B>{   //line 1
 
           public <X extends V> Carpet<? extends V> method(Carpet<? super X> e){ //line 2
              //insert code here
          }
}

I don't quite understand the syntax of the generic declaration at the class level(line 1) and at the method level(line 2). Anyone care to explain this to me?

Thanks

**KevinWorkman** · August 7th, 2014, 11:07 AM

What don't you understand about it?

What happened when you wrote a little test program that used this code?

**help_desk** · August 7th, 2014, 11:25 AM

The syntax. I don't know if you ever wrote with the C language. There used to be weird syntax like this with the C language.. having to do with pointers. pointers of a pointer, pointer arrays , and so on. I am just trying to understand the syntax. I can't write a code IF I am lost on the syntax. The best option I have concluded is to go find a simple tutorial on Java generics but perhaps, an answer on this will also help me as I meet more complex syntaxes on generics.

Thanks

**Cornix** · August 7th, 2014, 11:44 AM

I hope you do know what the "public class Carpet" stands for.
The rest is the syntax for generics in java.

You have a generic type parameter called X and you tell the compiler that, whatever X will later be, it must be a class or interface that extends/implements the class/interface B.

In your method you use a wildcard type "?" that means you dont care what class is being used here. But it must still extend/implement the class/interface V.

You will never see any code looking like this. This is constructed purely to mess with people, in reality, only the most twisted of minds would wright anything like it.

**KevinWorkman** · August 7th, 2014, 11:48 AM

Well I can tell you that this isn't C and has nothing to do with pointers or arrays.

I recommend going through the basic tutorials and writing an example program that plays with this code. You keep posting questions from exams, which you won't be able to pass if you can't do this kind of research.

**help_desk** · August 7th, 2014, 11:51 AM

Ok. Thanks for the flippant response.

**KevinWorkman** · August 7th, 2014, 12:27 PM

I'm not trying to be flippant; I'm trying to be helpful. The best way to figure out the type of a questions you've been asking is to visit the basic tutorials, do some googling, and write a little test program. That should be your first step, and telling you to try that out is the best thing I can do to help you.

If you do the googling, read the tutorials, and write a test program and are still confused, then by all means, post here what you've tried and what you're confused about. If you can't do the bare minimum, then maybe programming isn't for you. But it's up to you.

**help_desk** · August 7th, 2014, 01:20 PM

Originally Posted by KevinWorkman

I'm not trying to be flippant; I'm trying to be helpful. The best way to figure out the type of a questions you've been asking is to visit the basic tutorials, do some googling, and write a little test program. That should be your first step, and telling you to try that out is the best thing I can do to help you.

If you do the googling, read the tutorials, and write a test program and are still confused, then by all means, post here what you've tried and what you're confused about. If you can't do the bare minimum, then maybe programming isn't for you. But it's up to you.

And you think I have not thought through that OR I don't know that I have to test something before asking? I was interested in understanding the complex syntax. IF I had not donw some googling, I won't even know it is generic. I just hope one could be given the benefit of doubt when one posts his question.

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