Genrics Array creation

[Pradeep bhatt]

I was reading the Generics Tutorial by Gilad Bracha and found that the following code snippet causes compile error,

 <T> T[] makeArray(T t) { return new T[100]; // error
} 

Since type variables don�t exist at run time, there is no way to determine what the
actual array type would be.

I dont understand why it does not compile?

[somkiat puisungnoen]

you can't new <T> in your code.

[Pradeep bhatt]

Originally posted by somkiat puisungnoen:
you can't new <T> in your code.

Thanks ! I dunno why I forgot this.

[Pradeep bhatt]

List<String> [] la = null;  

complies but

 ]List<String> [] la = new ArrayList<String> [10]; 

doesn't. I know why the 2nd one does not compile but why does the first code compile.
[ November 16, 2004: Message edited by: Pradeep Bhat ]

[Nicholas Cheung]

Because your just allocate <T> to null, without *new*ing it?

Nick

[Pradeep bhatt]

Originally posted by Nicholas Cheung:
Because your just allocate <T> to null, without *new*ing it?

Nick

The component type of an array may not be a parameterized type, unless it is an (unbounded) wildcard type , so why should the first code compile?

[Nicholas Cheung]

But in another thread, I saw the author quote this example:

List<String> names = new ArrayList<String>();

I guess the problem might be you cannot new a list, because it is an interface, while you can new concrete class?

Nick

[Pradeep bhatt]

Originally posted by Nicholas Cheung:
But in another thread, I saw the author quote this example:


I guess the problem might be you cannot new a list, because it is an interface, while you can new concrete class?

Nick

Nick,

I m talking about creating an array of List and not just an ArrayList.

[somkiat puisungnoen]

Originally posted by Pradeep Bhat:

List<String> [] la = null;  

complies but

 ]List<String> [] la = new ArrayList<String> [10]; 

doesn't. I know why the 2nd one does not compile but why does the first code compile.

[ November 16, 2004: Message edited by: Pradeep Bhat ]

Why do want to create array of List ???

I think, it's not make sense...

[Nicholas Cheung]

I think, it's not make sense...

Why say so?

Nick

[Pradeep bhatt]

Originally posted by somkiat puisungnoen:

List<String> [] la = null;  

complies but

 ]List<String> [] la = new ArrayList<String> [10]; 

doesn't. I know why the 2nd one does not compile but why does the first code compile.

[ November 16, 2004: Message edited by: Pradeep Bhat ]<hr></blockquote>

Why do want to create array of List ???

I think, it's not make sense...[/QB]

Why not?

[VarunKumar Mallisetty]

Can some one answer this question.
I too came here with the same doubt, but searched the ranch if there is already an existing thread on "Generic Array Creation" error.

I tried this

  
LinkedList<String>[] Slist=new LinkedList<String>[4];

and got the error: Generic Array Creation.

Can some body please explain why i can't create an array of LinkedLists which can operate on strings.

[Ilja Preuss]

Originally posted by VarunKumar Mallisetty:
Can some one answer this question.
I too came here with the same doubt, but searched the ranch if there is already an existing thread on "Generic Array Creation" error.

I tried this

  
LinkedList<String>[] list=new LinkedList<String>[4];

and got the error: Generic Array Creation.

Can some body please explain why i can't create an array of LinkedLists which can operate on strings.

If you were allowed to do it, you could do the following:

LinkedList<String>[] Slist=new LinkedList<String>[4];
Object[] objects = list;
objects[0] = new LinkedList[Number];

This neither leads to a compile time error nor to a runtime error, because only at runtime can be checked whether the last assignment is allowed (you'd get an ArrayStoreException if not), but the type parameter for the LinkedList is erased at runtime.

A more elaborate explanation can be found at http://www.angelikalanger.com/GenericsFAQ/FAQSections/ParameterizedTypes.html#FAQ104

[Tony Morris]

You cannot create an array of a parameterised type unless you use the unbounded wildcard (?) or you generate a compile-time warning. This applies to (for example) java.util.ArrayList - where it is impossible to implement without a compile-time warning.

I pointed this out to a member of JSR-14 way back when (when I was working on the implementation and JSR-14 was an infant) - and in particular that arrays and collections are no longer convertible will cause a headache for many. The response was something along the lines of "oops".

This is (yet another) example of the shifting line/blur that Sun calls "reverse compatibility". There are many out there who proclaim a definitive contract that denotes what can and cannot change as J2SE continues to progress, however, a more detailed analysis proves that this line regularly shifts to meet marketing demands. I think that this one was a screw up on the behalf of the implementers however - since it would seem to violate what the market demands and would inevitably cost. At the time that I humbly pointed it out (it was coincidental that it even came up in conversation), it was probably too late since changing it would have had a propagating effect.

Begin Edit.
@SuppressWarnings didn't work at that time so it was not an option.
End Edit.
[ July 08, 2006: Message edited by: Tony Morris ]