New JEP: Concise Method Bodies

Brian Goetz brian.goetz at
Fri Oct 5 22:18:54 UTC 2018

> Can the expression before the :: refer to any method parameters? Of 
> course, it would still expect to pass those parameters into the 
> method, so it's /weird/ to have the same parameter used in both ways, 
> but does it make sense to forbid it?

Yes, it makes sense to forbid.  Here's why.

The framing of

     method declaration = behavior literal

is meant to suggest that the thing on the RHS can be computed ONCE, and 
that's the "body" of the method.  Therefore, it should only be 
computable based on what is known in the context of the method (static 
state for static methods, instance state for instance method.)  It is 
not merely a shorthand for "take the method ref, turn it into a lambda, 
and then turn the body of that lambda into a method body."

So the thing on the left of the :: must be something that could be, say, 
the RHS of a field initializer of the appropriate static-ness.

> Now we want to introduce a second way to use method references, where 
> that creation/evaluation distinction doesn't exist. 

Same distinction.  You can think of the creation as happening in <init> 
/ <clinit>.

Now, a fair question is: when should we evaluate that LHS?  And, is it 
an error or warning if that LHS is, for example, a nonfinal field?  Good 
question!  Thanks for bringing this up.

> Finally, to my earlier screed about the trade-offs of "multiple ways 
> to do the same thing" I want to add a nice (very paraphrased) point 
> that was made on reddit by a now-deleted account. (Thanks, mystery 
> person.) Offering users a choice between two ways to do the same thing 
> isn't /just/ offering them a choice. It's also burdening them with 
> having to read and maintain code that made each of those choices, for 
> whatever reasons that may be unknowable now. 

Yes, this is a valid concern.  It will spur the usual "when should I use 
an anonymous class vs named class" FAQs, style guide wars, etc.   FWIW, 
the C# folks went through this, there was the usual noise at the 
beginning, but after the dust settled, the concise method bodies (the 
arrow form, they don't have the stronger form) has become a fairly 
popular feature.

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