问题
I tried naming a lambda parameter _, e.g. (a cut down version):
Consumer<Object> c = _ -> {};
as I wanted to signify that a parameter was being ignored, but I got the following compiler error:
use of '_' as an identifier is forbidden for lambda parameters
This was a surprise for me. Interestingly, two underscores is OK:
Consumer<Object> c = __ -> {}; // no compile error
So it's not the underscore character in general, but a single one.
Why is the single-underscore name specifically forbidden?
回答1:
The reason is expressed in this post from Brian Goetz himself:
We are "reclaiming" the syntactic real estate of "_" from the space of identifiers for use in future language features. However, because there are existing programs that might use it, it is a warning for identifiers that occur in existing syntactic positions for 8, and an error for lambda formals (since there is no existing code with lambdas.)
来源：https://stackoverflow.com/questions/34521690/why-is-a-single-underscore-character-an-illegal-name-for-a-lambda-parameter