Why do Consumers accept lambdas with statement bodies but not expression bodies?

Question

The following code surprisingly is compiling successfully:

Consumer p = \"\"::equals;

This too:

p = s -> \         


        

Answer 1

I think the other answers complicate the explanation by focusing on lambdas whereas their behavior in this case is similar to the behavior of manually implemented methods. This compiles:

new Consumer() {
    @Override
    public void accept(final String s) {
        "".equals(s);
    }
}

whereas this does not:

new Consumer() {
    @Override
    public void accept(final String s) {
        true;
    }
}

because "".equals(s) is a statement but true is not. A lambda expression for a functional interface returning void requires a statement so it follows the same rules as a method's body.

Note that in general lambda bodies don't follow exactly the same rules as method bodies - in particular, if a lambda whose body is an expression implements a method returning a value, it has an implicit return. So for example, x -> true would be a valid implementation of Function, whereas true; is not a valid method body. But in this particular case functional interfaces and method bodies coincide.