Type Casting Generics

[Yohan Liyanage]

Hi,

Recently, I faced this issue when I was type casting a generic type. The code is as followed (this is a simplified version to show the issue) :

    
List<String> lst = new ArrayList<String>();
Object o = lst;
List<String> m = ( List<String> )   o;

The type casting in the last line gives a compiler warning :

Type safety : Unchecked cast from Object to List<String>

Whats the solution to eliminate this warning ? (Except @SupressWarnings of course)

Thanks.
[ December 29, 2007: Message edited by: Yohan Liyanage ]

[camilo lopes]

change Object by String

  
 
String o = list
  

[Yohan Liyanage]

Sorry, I don't get you.

String o = lst;

I'm afraid that's not possible. lst is a List object. So it cannot be assigned to a String reference variable. It will result in a compiler error. Object is the super-type of all types in Java, so it can be assigned to a Object reference variable.

May be I have mis-understood your reply. Can you please explain ?

P.S. :
As I have stated, the code I have posted is not exactly where I faced this problem. It's a simplified version of it. I faced this issue when I was retrieving an attribute from a Request object in a Servlet. request.getAttribute() always returns the object as an Object.

[Rob Spoor]

AFAIK, there is no way except using the @SupressWarnings annotation. It may suck, but that's the way it is.

[Ilja Preuss]

Rob is right.

[Yohan Liyanage]

Thanks guys.

Will Sun do anything regarding this in a future compiler? It looks really bad when I put @SuppressWarnings in the code.

[Ilja Preuss]

Well, to remove the warning, Sun had to remove the root cause - namely that the generic type of the list is unknown at runtime.

It's not totally unthinkable, but I wouldn't hold my breath...

[Josh Allen]

Yohan Liyanage wrote:Thanks guys.

Will Sun do anything regarding this in a future compiler? It looks really bad when I put @SuppressWarnings in the code.

No, because it's your code that's the problem and not the compiler. You are doing something dangerous and it's being helpful by giving you a warning. Generics provide type safety and you are circumventing that type safety here. This will NEVER change.

Be thankful you have @SuppressWarnings to let the code reader know that you know what you are doing and the warning can be safely ignored.

[Rob Spoor]

Although you are 100% correct, did you really think it was necessary to post in this year old thread?

[Josh Allen]

Rob Prime wrote:Although you are 100% correct, did you really think it was necessary to post in this year old thread?

Point noted, but yes, because I found this on my google search, and others are sure to find it in the same manner. It'll probably still be served up years from now.