Why does this Java 8 lambda fail to compile?

Question

The following Java code fails to compile:

@FunctionalInterface
private interface BiConsumer {
    void accept(A a, B b);
}

private static void t         


        

Answer 1

Your lambda needs to be congruent with BiConsumer. 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 (§14.8) 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.