java - Why can I not assign a method reference directly to a variable of Object type?

Question

Simple question about java-8 syntax. Why does JLS-8 restrict such expressions like:

Object of_ref = Stream::of;  // compile-time error

and allow only something like:

java.util.function.Function of_ref = Stream::of;
Object obj = of_ref; // compiles ok

?

Answer 1

Object is not a functional interface and a method reference can only be assigned to a functional interface. See for example JLS #15.13.2

A method reference expression is compatible in an assignment context, invocation context, or casting context with a target type T if T is a functional interface type (§9.8) and the expression is congruent with the function type of the ground target type derived from T.