Lambda casting rules

Question

I was curious why a lambda with a return type can not be casted to a Runnable whereas a non void method reference can.

Runnable r1 = () -> 1;         


        

Answer 1

The Runnable interface defines the run method with return type void. In a lambda expression that means that the part following the arrow -> must be a statement. This is explained in JLS §15.27.3:

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

The JLS $14.5 clearly defines the syntax of a statement. As explained above it must be an "ExpressionStatement" (§ 14.8). Looking there, you can find that a simple literal is not an adequate expression, but a method invocation is (even if it returns something).