hi, I tried most of this. I see two lines with the same code, but one is marked
ok, the other throws ClassCastException?
n = tokens.get(0); // ok
n = tokens.get(0); // throws ClassCastException
When I ran this (in Java 8), it ran OK.
Any idea why your try behaved differently?
I did see that
n = generalize.get(0); // gives a compile error
which can be overcome with a cast, but that seems like a bad burden to put on
users,
compared to not having the wildcard in the returned result.
-Marshall
On 8/2/2019 3:00 PM, Hai-son X Nguyen wrote:
> Thanks for the reference Marshall,
>
> Try this code:
> public static void main(String[] args) {
>
> List<Number> tokens = new ArrayList<>();
> tokens.add(3);
> tokens.set(0, 4);
>
> ArrayList<? super Number> generalize = (ArrayList<? super Number>) tokens;
> // Upcasts all to number
> Number n = 5;
> generalize.set(0, n); // ok
> var x = generalize.get(0); // ok
> // n = generalize.get(0); // compile error
> System.out.println(tokens.get(0)); // ok prints 5
> x = tokens.get(0); // ok
> n = tokens.get(0); // ok
>
> List unknown = tokens; // warning
> unknown.set(0, "foo"); // warning
>
> // String t = tokens.get(0); // compile error
> // String t = (String) tokens.get(0); // compile error
> System.out.println(tokens.get(0)); // ok prints "foo"
> n = tokens.get(0); // throws ClassCastException
>
> }
>
>> In fact, there's no cast I could figure out to make "generalize.set" work at
>> all, except by changing its definition to use ArrayList (bare with no generic
>> argument):
>>
>> ArrayList generalize = (ArrayList<? extends Number>) tokens; //compiles
>> with
>> warnings,
>> generalize.set(0, 7); // works at runtime
> I thought we can do a upcast and use <? super FeatureStructure> which gets
> around the set problem you found but it looks like there are problems on the
> get side...
> The use of var is possible to get around the immediate errors but that is
> Java 10+
>
> Moved the example to get the runtime error at the end
>
> The run time exception comes in the get() call and setting it to the generic
> type. (
>
> Hai-Son
>
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