_ (underscore) is a reserved keyword
java, java-8, lambda
Solution
The place to look is JLS §15.27.1. Lambda Parameters
It is a compile-time error if a lambda parameter has the name _ (that is, a single underscore character).
The use of the variable name _ in any context is discouraged. Future versions of the Java programming language may reserve this name as a keyword and/or give it special semantics.
So the Eclipse message is misleading, especially as the same message is used for both cases, when an error is generated for a lambda parameter or when a warning is generated for any other `_` identifier.
Problem
I've just replaced `s` in the following lambda expression by `_`: ``` s -> Integer.parseInt(s) ``` Eclipse compiler says: '_' should not be used as an identifier, since it is a reserved keyword from source level 1.8 on. I haven't found any explanation in the JLS §3.9 Lexical Structure / Keywords.