Your lambda needs to be congruent with BiConsumer<String, String>. If you refer to JLS #15.27.3 (Type of a Lambda):
A lambda expression is congruent with a function type if all of the following are true:
- […]
- If the function type’s result is void, the lambda body is either a statement expression (ยง14.8) or a void-compatible block.
So the lambda must either be a statement expression or a void compatible block:
- A constructor invocation is a statement expression so it compiles.
- A string literal isn’t a statement expression and is not void compatible (cf. the examples in 15.27.2) so it does not compile.