Why was the double colon operator chosen for Java 8?

Question

Prior to Java 8, my observation was that the de facto standard for referring to member methods was by using the # symbol (such as Object#toString()). Then along came Java 8 which instead chose the :: operator for method references for seemingly no reason.

Has there been an official explanation or justification for why :: in particular was chosen?

Java has no such thing as "member methods"; functions are not a first class citizen in Java. The `::` was simply borrowed to have a convenient way of defining a method reference, with the left part being a class or instance of it, and the right part being the method name. — fge, Mar 06 '15 at 09:41
# was never part of Java API standard syntax for refering to methods. You might want to review http://stackoverflow.com/a/22245383/579580 to better understand :: — aviad, Mar 06 '15 at 09:42
Why asking _"Has there been an official explanation or justification for why :: in particular was chosen?"_ is considered as **primarily opinion-based** ? — gontard, Mar 06 '15 at 09:52
@gontard Beats me, but I find that somewhat amusing for some reason. I would ask "how does that qualify as opinion-based?", but then they would probably close this comment too for being primarily opinion-based. :) — BambooleanLogic, Mar 06 '15 at 09:53
Nevermind... [there is an official explanation or justification](http://openjdk.5641.n7.nabble.com/Method-reference-double-colon-syntax-td58953.html) — gontard, Mar 06 '15 at 09:57
@gontard: Thanks, that's exactly what I was looking for. If this question gets reopened, feel free to post that as an answer (with an excerpt or something to follow the no-link-only guidelines) and I'll happily accept it. — BambooleanLogic, Mar 06 '15 at 10:03

gontard · Accepted Answer · 2022-03-19T06:56:08.300

You could find an "official explanation or justification" from Brian Goetz. You should read the complete discussion but this is an extract :

The :: infix syntax:
   ClassName::methodName 
   ClassName<T>::methodName 
   ClassName::<U>genericMethodName 
works acceptably well. Some people like it, and some people hate it -- just like #. There's never going to be a perfect syntax for anything that makes everyone jump up in unison and say "yeah, that's it!" But :: is OK, and using up :: here is far better than using up #. (And, while this might look a little weird to C++ programmers, the overlap between the Java and C++ developer bases at this point is small enough that I don't think we should be too worried about that.)

Why was the double colon operator chosen for Java 8?

1 Answers1