Re: Erased data type

On Thursday 30 August 2007 08:10, Mann, Ivan H wrote:
> I have an app with a gui that can develop a lot of data.  There is a
> save button, which writes the data out using serialization.
>
> Then, there is a load button, which reads the data back in, using
> this line:
>
>         destinations = (ArrayList<ScheduleDestination>)
> is.readObject() ;
>
> Where destinations is defined elsewhere as
>
>         ArrayList<ScheduleDestination> destinations ;
>
> On the line that reads the object back in, there is a compiler flag:
>
>         Type Safety: The cast from Object to
> ArrayList<ScheduleDestination> is actually checking against the
> erased type ArrayList.
>
> What does this flag mean?  I have googled fairly creatively and can't
> find anything which tells me what it means or how to make it go away.

Java generics are implemented via "type erasure," which means that the 
types that parameterize a generic type in the source code have no 
representation whatsoever in the compiled bytecodes (they're "erased").

All the type inference and verification that is to be done must be done 
statically by the Java compiler.

So it's telling you that all you're really casting to is ArrayList, not 
ArrayList<anythingInParticular> and thus it cannot be guaranteed that 
somewhere down the line a ClassCastException might be produced when you 
extract an item from that list.


See Angelika Langer's detailed Generics "FAQ" (I use the scare 
quotes 'cause it's really much more than a FAQ—it's a comprehensive 
tutorial):

  <http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html>


> Ivan Mann


Randall Schulz

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