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

This is not really a “special” rule for Lambdas. It has always been acceptable to use a non-void method in a statement context (in which case, the return value is thrown away), but not so with a simple expression. For instance “String.valueOf(true);” is a valid java statement, that may occupy a line by itself, whereas “true;” is not and may not.