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

The JLS specify that

If the function type's result is void, the lambda body is either a statement expression (§14.8) or a void-compatible block.

Now let's see that in detail,

Since your takeBiConsumer method is of void type, the lambda receiving new String("hi") will interpret it as a block like

{
    new String("hi");
}

which is valid in a void, hence the first case compile.

However, in the case where the lambda is -> "hi", a block such as

{
    "hi";
}

is not valid syntax in java. Therefore the only thing to do with "hi" is to try and return it.

{
    return "hi";
}

which is not valid in a void and explain the error message

incompatible types: bad return type in lambda expression
    java.lang.String cannot be converted to void

For a better understanding, note that if you change the type of takeBiConsumer to a String, -> "hi" will be valid as it will simply try to directly return the string.

---

Note that at first I tought the error was caused by the lambda being in a wrong invocation context, so I'll share this possibility with the community :

JLS 15.27

It is a compile-time error if a lambda expression occurs in a program in someplace other than an assignment context (§5.2), an invocation context (§5.3), or a casting context (§5.5).

However in our case, we are in an invocation context which is correct.