Return type of an interface with a wildcard
Hi everyone,
I'm totally new to generics and I had encountered a problem with instantiating
a generic array. I know, that it is impossible to do that and I found a workaround:
<T> List<T>[] newListArray(int size){
return (List<T>[])new List<?>[size];
}
This method returns and array of a generic interfaces and works fine.
The only problem is that I'm not sure how it works ...
Could anybody correct me if I'm wrong:
- the method instantiate an array of interfaces of any type and then
it sees that this any type is cast to a T, which in this case is String ? :
List<String>[] t=b.newListArray(3);
I really got confused here. Could anybody with a good how-it-works-explanation
attitude tell me how it works ?
Thanks,
Adrian
# 3
So what is your issue then? I just told you, it's the same at runtime. And that's why it works. :)
Ok, some more info: there is no cast from "unknown" to String involved. You never created an array on other than on a List of Objects and you never use another array than on a List of Objects. Generics is a compile-time-feature (at least for now).
Message was edited by: stefan.schulz
# 4
OK. Let me try to say it in other words :)
I don't understand how this code works. This
is my issue. I'm not interested in any alternatives,
so maybe if I had asked:
"How does the compiler interpret this code ?"
that would be of more usefulness ;)
Imagine someone who is trying to learn a new language,
and he/she encounters some code which he/she doesn't
understand - can be annoying ...
So basically, I need to know how the compiler interpret
the code, so I will know how to use this syntax (regardless
of its usefulness). And yes - I know that it is ONLY compile-time
type safety. But you still need to understand how it works
in order to use it properly.
Thanks,
Adrian
# 5
<T>
This is a type parameter.
List<T>[]
This tells the compiler that the method returns an array of List<T> objects.
newListArray(int size) {
This is the name of the method and its parameter.
return
This is a return statement.
(List<T>[])
Here you cast something to an array of List<T> objects.
new List<?>[size];
Here you create an array of List<?> objects.
Okay, that's how the compiler interprets the code. I think people are confused by your question, because that's the natural way to break it down even if you aren't a compiler. It would help if you produced a specific question about the code; "Duh" is such a hard question to answer.
# 6
Hi,
Now that is what I needed !
So to be more specific:
new List<?>[size];
I thought you can't use wildcards in constructors.
But I think I know where it is heading for: you can't
use wildcards in constructors but you aren't actually using
any here - am I right ? You are creating an array, not
an object (though array IS and object ...)
Cheers,
Adrian
