Java 8将多个可选参数映射到函数中

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 could stream the arguments and apply the condition only once, but whether or not this is more elegant than your solutions is in the eye of the beholder:

if (Stream.of(a, b).allMatch(Optional::isPresent)) {
    return f(a.get(), b.get());
}

Answer 2

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

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 (eg 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.

Answer 3

I'm of the opinion that if there is no good way to use Optional , then there is no reason to try to use it anyway.

I find this to be cleaner and simpler than your option 2:

String a = getA().orElse(null);
String b = getB().orElse(null);
if(a != null && b != null) {
    return f(a, b);
}

Answer 4

If you are sure that a and b are both present (as your final call to get in solution 1 seems to suggest), I think it is pretty straightforward:

    return f(getA().orElseThrow(() -> new NoSuchElementException("a not present")),
             getB().orElseThrow(() -> new NoSuchElementException("b not present")));

If you aren't sure that both are present, I would prefer your solution 1. It exploits Optional the best. Only I would not call get at the end, but rather orElse or what makes sense in your situation, for example:

    return getA()
            .flatMap(a -> getB().map(b -> f(a,b)))
            .orElse("Not both present");