Re: Method chaining with generics


"Hemal Pandya" <hemalpandya@xxxxxxxxx> wrote in message
news:1125487477.912674.260290@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>
> Chris Uppal wrote:
>> Raymond DeCampo wrote:
>>
>> > Is there any case
>> > when casting a List to List<Object> would cause a problem?
>>
>> I haven't been following the thread, so this may be irrelevant. But if
>> you
>> start with a List that is intended to hold Strings (whether it's declared
>> as a
>> List or a List<String> doesn't matter provided it reaches your code as a
>> List)
>> and cast that to a List<Object>, then you'll be able to add a Rectangle
>> to it,
>> which would not be correct.
>
> Very true. There are two different cases depending on how the List
> started. If it was intended to hold Strings but created as a List, not
> a List<String> then the error is at the point of creation. This issue
> exists in all java until 5.0. If it was created as a List<String> then
> the error is where it was converted to List.
>
> But given a List, which by definition is a list of Objects, is there a
> case where casting it to List<Object> is dangerous?

This is exactly the issue that the compiler is warning about: Due to
type erasure, there's no way for the runtime to ensure that something which
is a "List" is actually a "List<Object>" and not, for example, a
"List<String>".

public static test(List someList) {
List<Object> listOfObjects = (List<Object>)someList;
}

public static void main(String args[]) {
test(new List<String>());
}

In the above sample code, the runtime will NOT throw a class cast
exception, because it will not be able to detect that the list passed was
actually a list of Strings and not a list of Objects.

Hence the warning in the compiler.

- Oliver


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