Re: [eclipse-dev] Java: Overloading & Lambdas = TROUBLE

["Daniel Megert" <daniel_megert@xxxxxxxxxx>]

Thanks Stephan for
that detailed heads-up. I would also like to thank you for improving the
compiler in that area and that as of 4.12 M3 more cases will be detected
and reported by the compiler.

Dani



From:
       Stephan
Herrmann <stephan.herrmann@xxxxxxxxx>
To:
       "General
development mailing list of the Eclipse project." <eclipse-dev@xxxxxxxxxxx>
Date:
       21.05.2019
16:03
Subject:
       [EXTERNAL]
[eclipse-dev] Java: Overloading & Lambdas = TROUBLE
Sent
by:        eclipse-dev-bounces@xxxxxxxxxxx

---

Hi folks,

As your compiler engineer, and alarmed by a recent event, I have to warn
you 
that Java 8 brought one particularly nasty pitfall: If your API defines
same-named methods which differ only in the type of a parameter, and if
that 
type is a functional interface you are effectively requiring that all users
of 
this API need to understand details of Java which I wish they would never
need 
to think about:

First you need to teach all users of the API, that type inference in Java
8 
happens in several phases, and that overload resolution happens before
the final 
phase. Then you need to teach all users of that API, that some functional
expressions (lambdas, method references, under certain conditions) are
not 
pertinent to applicability. If they confirm that they understand JLS sections
§15.12, §15.13, §15.27 and all of §18, then they may get the license to
invoke 
your API.

I'm not kidding. Overloading in Java was problematic well before Java 8,
in 
terms of: it's very easy to write programs which seasoned Java programmers
will 
not understand. Java 8 "improved" this by the above problem.

The general problem can surface in two ways: perhaps the compiler says
that a 
given method invocation is ambiguous. In that case be happy about this
alert and 
write your code in a way that not for a fraction of a second anybody will
have 
to guess which of the methods will be invoked (explicit type arguments
etc pp).

If the compiler accepts the method invocation, you may be in trouble, because
your intuition about which method will be chosen by the compiler may very
well 
be wrong. Not knowing which method a given call will invoke doesn't sound
very 
nice, right?

In the particular, recent event, it is likely too late to change the affected
API, but please refrain from adding more like that - and be extra careful
when 
using API that is already affected.

Thanks,
Stephan
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