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 er         


        

Answer 1

The key point is that there are no "function types" in Java. A lambda expression doesn't have a "type" by itself -- it can be typed to any functional interface whose sole method's signature matches the lambda. Therefore, a lambda's type is based on the type provided by its context. You must provide a functional interface as the context for it to get a type.

It is instructive to consider the same issue but for anonymous classes. Although there are implementation differences between lambdas and anonymous classes, semantically, lambdas are essentially equivalent to a subset of anonymous classes, and a lambda expression can always be converted to an equivalent anonymous class creation expression.

When you write:

Function> of_ref = Stream::of;

it is equivalent to something like the following using anonymous classes:

Function> of_ref = new Function>() {
    Stream apply(T t) {
        return Stream.of(t);
    }
};

Now consider

Object of_ref = Stream::of;

what is the equivalent with anonymous classes?

Object of_ref = new [**What goes here?**]() {
    [**What method signature goes here?**] {
        return Stream.of(t);
    }
};

You see why it doesn't make sense -- we don't know what type to use as the base class of the anonymous class.