Java 如何传递一个BiFunction作为参数

Question

I need help figuring out how to pass this BiFunction

BiFunction<Integer, List<Integer>, Integer> func = (a, b) -> {
        int result = 0;
        int temp = 0;
        for(Integer ele : b) {
            temp = b.get(ele);
            result += temp;
        }
        return a + result;
    };

I am using this junit test

void testFoldLeft() {
        LinkedList<Integer> l = new LinkedList<>();
        for(int i = 0; i < 10; i++) l.addFirst(i+1);
        Integer u = fp.foldLeft(0, l, func(0,l));
    }

I am attempting to pass the BiFunction func through foldLeft, which looks like this

static <U,V> V foldLeft(V e, Iterable<U>l, BiFunction<V,U,V> f){
    return null;
}

What func is supposed to do is to take a list, in this case b, sum up all the elements in b then add that number to a and return the result. However, Eclipse gives me an error stating that func is undefined. I'm new to BiFunctions so I'm kind of stuck here. I would appreciate any help.

Answer 1

I think the mismatch is caused by an incorrect declaration of the second parameter of foldLeft . If, in your example, Integer is the Iterable 's argument, then your method should be declared like this:

static <U, V> V foldLeft(V e, Iterable<V> l, BiFunction<V, U, V> f) {
    return null;
}

V is the element type of the iterable, not U (which, in this case, would be equivalent to Iterable<V> ).

Further, your invocation should look like this:

Integer u = foldLeft(0, l, func);

You can't invoke an expression in Java. Unless func is declared as a method in visible scope, func() will always be invalid.

---

Note that you can simplify your generics by getting rid of U (I don't see a specific need for the type variable for the list type here, but I may be missing some use cases):

static <V> V foldLeft(V e, Iterable<V> l, BiFunction<V, Iterable<V>, V> f) {
    return null;
}

You do not show your implementation of foldLeft , but I'm sure it's going to need to iterate over the list, or at least pass it to f.apply . So I suppose you can make the function take a List<V> :

static <V> V foldLeft(V e, List<V> l, BiFunction<V, List<V>, V> f) {
    return f.apply(e, l);
}

Otherwise, calling f.apply(e, l) would fail as Function<Integer, LinkedList<Integer>, Integer> and Function<Integer, Iterable<Integer>, Integer> are incompatible.

Answer 2

There are two issues in your code:

1. func(0,l) is a syntax error, just pass func variable as the BiFunction . No need for providing arguments:

    Integer u = foldLeft(0, l, func);

2. The generics signature of foldLeft does not match the BiFunction . Given the current way it is written, Iterable<U> l makes the compiler infer U as Integer , hence the compiler expects the third argument to be a BiFunction<Integer, Integer, Integer> instead of BiFunction<Integer, List<Integer>, Integer> . To make U match the LinkedList<Integer> , declare it as follows:

    static <U extends Iterable<V>, V> V foldLeft(V e, U l, BiFunction<V, U, V> f){ return null; }

Side note: I wonder what the function code is doing:

for(Integer ele : b) {
    temp = b.get(ele);
    result += temp;
}

This is looping over elements of b and treating them as indices. I doubt this is what you want.