Method reference and Lambda expression

Question

I´m currently reading through someone else´s code and he has the following two methods:

public static double fac(double d) {
    return d <= 0 ? 1 : d * fac(d - 1);
}

public static DoubleUnaryOperator getfun() {
    return LamdaTests::fac; // LamdaTests is the classname of this class
}

First of all, i´m currently trying to understand lambda expressions/method references.

I have 2 questions:

1) What does the getFun() method exactly do? It should call the fac(double d) method in this class LambdaTests right? But with which argument and how can the return value be a DoubleUnaryOperator, shouldn´t "LamdaTests:fac" return a double?

2) What would be the equivalent Lamda expression for Lamdatests::fac in this case?

Edit: As far as i know

return LamdaTests::fac; 

should be equal to

return x -> fac(x);

But i just dont understand where it gets the argument x from.

Answer 1

getFun doesn't call anything. It just returns a reference to the fac function. You need to call the returned function still to get a result. getFac isn't very useful here, so that may be what's confusing things. In reality, you would just use LamdaTests::fac directly, unless that method was private.

For the latter question, think of it this way:

x -> fac(x)

is a function that takes a double and returns a double. What is fac ? It's the same thing: a function that takes a double and returns a double. The argument is just implicit here.