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

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

int x = 5;
FunctionalInterface func = (x) -> System.out.println(x);

public abstract void xxx(int value);

Function<T, Stream<T>> of_ref = Stream::of;

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

Object of_ref = Stream::of;

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

Object of_ref = list -> Stream.of(list);

Object obj = (Function<?, ?>) Stream::of;