Java 8 Mapping multiple optional parameters into a function

Question

Let's say that I have function Object f(String a, String b) and I want to call two different functions that return Optional Strings to get the parameters for f Optional<String> getA() and Optional<String> getB(). I can think of two solutions but neither look all that clean, especially when you have even more parameters:

1:

return getA().flatMap(
    a -> getB().map(
        b -> f(a,b)).get()

2:

Optional<String> a = getA();
Optional<String> b = getB();
if(a.isPresent() && b.isPresent()) {
    return f(a.get(), b.get());
}

Is there a cleaner way to do this?

Answer 1

You've just stumbled upon a concept called lifting in functional programming, that enables you to lift regular functions (e.g. A -> B) into new domains (e.g. Optional<A> -> Optional<B>).

There's also a syntactic sugar for flatMapping and mapping more comfortably called the do notation in Haskell and similar languages, and for comprehension in Scala. It gives you a way to keep the flow linear and avoid nesting (that you were forced to go through in your example 1).

Java, unfortunately has nothing of the sort, as its functional programming capabilities are meager, and even Optional isn't really a first-class citizen (no standard API actually uses it). So you're stuck with the approaches you've already discovered.

In case you're curious about the concepts mentioned above, read on.

Lifting

Assuming you have:

public String f(A a, B b) {
    return b + "-" + a;
}

With its Scala equivalent:

def f(a: A, b: B) = b + "-" + a

Lifting f into Option (same as Optional in Java) would look like this (using Scalaz library, see here for Cats)

val lifted = Monad[Option].lift2(f)

lifted is now a function equivalent to:

public Optional<String> f(Optional<A> a, Optional<B> b) {
    if(a.isPresent() && b.isPresent()) {
        return Optional.of(b + "-" + a);
    }
    return  Optional.empty;
}

Exactly what you're looking for, in 1 line, and works for any context (e.g. List, not just Option) and any function.

For comprehension / Do notation

Using for comprehension, your example would look like this (I think, my Scala is weak):

for { 
    a <- getA();
    b <- getB();
} yield f(a, b)

And again, this is applicable to anything that can be flatMapped over, like List, Future etc.