Inferring that what exceptions are thrown from a lambda

[Stuart Marks]

Not sure what you mean by "quickly end up at Exception." Depends on 
what's in the lambda. Certain things, like Future.get() or 
ObjectInputStream.writeObject() are declared to throw two checked 
exceptions whose common supertype is Exception. But there also seem to 
be many cases where code throws a single checked exception, or none at 
all, and for these cases callers benefit by having to handle only the 
right checked exception or none at all.

s'marks

On 9/5/13 7:47 PM, Howard Lovatt wrote:
> Sure, but as your example shows you quickly end up at Exception and
> therefore I start there! The enclosing method can catch the more
> specific exceptions, if you know what they are, and rethrow the specific
> exceptions so that the enclosing method doesn't have to throw Exception
> but can throw the specific exceptions.
>
>   -- Howard.
>
>
> On 6 September 2013 12:22, Stuart Marks <stuart.marks at oracle.com
> <mailto:stuart.marks at oracle.com>> wrote:
>
>     Applying this to the OP's example, this would propagate "throws
>     Exception" out to the callers, forcing them to catch or to throw
>     Exception themselves. That's not what the OP wanted. Instead, the
>     example using a lambda that throws IOException should only be forced
>     to handle or declare IOException, and the example using a lambda
>     that doesn't throw any checked exceptions shouldn't have to deal
>     with exceptions at all.
>
>     With the new inference work in the compiler, the additional "X
>     extends Throwable" type argument seems to do the trick for some
>     common cases. (I tend to prefer type variable X -- "exception" --
>     over E, since E is already used for enums and for collection element
>     types.) It doesn't work if the lambda throws multiple different
>     checked exception types. For example, the following doesn't work:
>
>          public void multiple() throws ExecutionException,
>     InterruptedException {
>              String result = tryRepeatedly(10, () -> {
>                  if (/*condition*/)
>                      throw new ExecutionException(null);
>                  else
>                      throw new InterruptedException();
>              });
>          }
>
>     Here, the compiler infers the least upper bound for the lambda's
>     exception type, which in this case is Exception, so that's what has
>     to be listed in the throws clause instead of listing multiple
>     exception types. Oh well.
>
>     s'marks
>
>
>
>     On 9/4/13 7:22 PM, Howard Lovatt wrote:
>>     It is generally easier to do this:
>>
>>          public interface Action<T> {
>>               T run() throws Exception;
>>           }
>>
>>     Which is what Callable does, infact the above is
>>     Callable apart from the name changes (Callable -> Action, call ->
>>     run).
>>
>>
>>     On 5 September 2013 06:11, Stuart Marks <stuart.marks at oracle.com
>>     <mailto:stuart.marks at oracle.com>> wrote:
>>
>>         On 8/31/13 11:04 AM, Esko Luontola wrote:
>>         > But if the lambda doesn't thrown anything, the compiler
>>         thinks that the
>>         > method may throw the most generic exception. The following
>>         code fails to
>>         > compile with "error: unreported exception Throwable; must be
>>         caught or
>>         > declared to be thrown"
>>         >
>>         >       public void doesNotCompile() {
>>         >           String result = Resilient.tryRepeatedly(10, () ->
>>         "result");
>>         >       }
>>
>>         A change to support this went in fairly recently. This now
>>         compiles for
>>         me using JDK 8 b105. It fails with the error you mention when
>>         using
>>         older builds, e.g., JDK 8 b88, which is one I happened to have
>>         lying
>>         around. (Note, I am referring to JDK 8 builds, not Lambda builds.)
>>
>>         I believe that if a lambda throws no checked exceptions, and its
>>         functional interface method is declared "throws E", then E is now
>>         inferred to be RuntimeException.
>>
>>         s'marks
>>
>>
>>
>>
>>     --
>>       -- Howard.
>
>
>
>
> --
>    -- Howard.