JDK 8 - Lambda - Reflection issues

[Florian Weimer]

On 03/13/2014 09:56 AM, Pavel Bucek wrote:

> And then there is a Session object, which has method "void
> addMessageHandler(MessageHandler handler);". Obvious common use of this
> method is:
>
> session.addMessageHandler(new MessageHandler.Whole<String>() {
>      @Override
>          public void onMessage(String message) {
>              // ...
>          }
> });
>
> I can see my IDE automatically offers me to transform this to lambda
> expression (this is actually what worries me a little, because all users
> will see that and do it - because why not - it seems to be equivalent
> with anonymous class). When this suggestion is accepted, previous
> statement is transformed into:
>
> session.addMessageHandler((MessageHandler.Whole<String>) message -> {
>      // ...
> });
>
> which looks prettier, but just does not work and cannot work :/

Why doesn't it work?  Does the implementation try to extract the String 
actual type argument?  Then it's unsound because

      session.addMessageHandler(new MessageHandler.Whole<T>() {
           @Override
           public void onMessage(T message) {
                // ...
           }
      });

type-checks (assuming that T is a type parameter), but will not work at 
run time, either.

-- 
Florian Weimer / Red Hat Product Security Team