The parameter name '_' is prohibited by the parser for lambdas. This is correct behavior, but the error message is very unhelpful:

UnderscoreName.java:7: error: <identifier> expected
  java.util.function.Function<String,String> f = _ -> "x";
                                                 ^
UnderscoreName.java:7: error: illegal start of expression
  java.util.function.Function<String,String> f = _ -> "x";
                                                   ^
UnderscoreName.java:7: error: ';' expected
  java.util.function.Function<String,String> f = _ -> "x";
                                                     ^
A better approach would be to parse the name as is, and provide a descriptive error message in a later phase.