仅用reduce和lambdas解释Java流映射函数

Question

我目前正在阅读一本Java8Lambdas的书（相当受欢迎），我对其中一个高级问题的答案中的一些语法感到困惑。

问题提出如下问题：

只使用reduce和lambda表达式编写stream函数'map'的实现。您可以返回列表而不是流。

public static <I, O> List<O> map(Stream<I> stream, Function<I, O> mapper) {
    return stream.reduce(new ArrayList<O>(), (acc, x) -> {
        // We are copying data from acc to new list instance. It is very inefficient,
        // but contract of Stream.reduce method requires that accumulator function does
        // not mutate its arguments.
        // Stream.collect method could be used to implement more efficient mutable reduction,
        // but this exercise asks to use reduce method.
        List<O> newAcc = new ArrayList<>(acc);
        newAcc.add(mapper.apply(x));
        return newAcc;
    }, (List<O> left, List<O> right) -> {
        // We are copying left to new list to avoid mutating it. 
        List<O> newLeft = new ArrayList<>(left);
        newLeft.addAll(right);
        return newLeft;
    });
}

, (List<O> left, List<O> right) -> {
        // We are copying left to new list to avoid mutating it. 
        List<O> newLeft = new ArrayList<>(left);
        newLeft.addAll(right);
        return newLeft;
    });

到目前为止，Java8 lambdas已经非常简单了，我觉得我似乎理解了书中的所有理论，但也许我误解了什么？我想知道我在这里错过了什么？

Answer 1

最后一个部分称为组合器，如果流是并行的，它就很有用。

它将创建最终需要放在一起的多个中间结果。这正是这个lambda正在做的事情。

您可以先执行下面的代码，它将通过您的函数运行一个顺序的流。注意我是如何在组合器中添加system.out.println（“combinating...”）的。

public static void main(String[] args) {
    Stream<Integer> boxed = IntStream.rangeClosed(1, 10).limit(25).boxed();
    List<String> map = map(boxed, String::valueOf);
    System.out.println(map);
}

public static <I, O> List<O> map(Stream<I> stream, Function<I, O> mapper) {
    return stream.reduce(new ArrayList<O>(), (acc, x) -> {
        // We are copying data from acc to new list instance. It is very inefficient,
        // but contract of Stream.reduce method requires that accumulator function does
        // not mutate its arguments.
        // Stream.collect method could be used to implement more efficient mutable reduction,
        // but this exercise asks to use reduce method.
        List<O> newAcc = new ArrayList<>(acc);
        newAcc.add(mapper.apply(x));
        return newAcc;
    }, (List<O> left, List<O> right) -> {
        System.out.println("Combining...");
        // We are copying left to new list to avoid mutating it.
        List<O> newLeft = new ArrayList<>(left);
        newLeft.addAll(right);
        return newLeft;
    });
}

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

public static void main(String[] args) {
    Stream<Integer> boxed = IntStream.rangeClosed(1, 10).parallel().limit(25).boxed();
    List<String> map = map(boxed, String::valueOf);
    System.out.println(map);
}

Combining...
Combining...
Combining...
Combining...
Combining...
Combining...
Combining...
Combining...
Combining...
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]