映射到Java 8中的运行总和 [英] Map to a running sum in Java 8

问题描述

如果我有一个集合：

List<Long> numbers = asList(2, 2, 4, 5);

如何映射/处理这些以构建运行总计。产生类似的东西：

How can I map/process these to build up a running total. To produce something like:

List<Long> runningTotals = asList(2, 4, 8, 13);

更好的是，我如何建立一个东西的列表（如元组），这样我就可以保留orignals：

Even better, how can I build a list of something (like a tuple) so I can preserve the orignals:

((2 -> 2), (2 -> 4), (4 -> 8), (5 -> 13));

推荐答案

更新：作为Holger使用 Stream.reduce（） 为此目的不正确。请参阅减少和可变减少或 Java 8 Streams - collect vs reduce 以获取更多信息。

Update: As Holger pointed out in the comments using Stream.reduce() for this purpose is not correct. See Reduction and Mutable Reduction or Java 8 Streams - collect vs reduce for more information.

您可以使用Java Stream.collect（） 而不是生成你的清单总和：

You can use Java Stream.collect() instead to generate your list with sums:

List<Long> numbers = Arrays.asList(2L, 2L, 4L, 5L);
List<Pair> results = numbers.stream()
        .collect(ArrayList::new, (sums, number) -> {
            if (sums.isEmpty()) {
                sums.add(new Pair(number, number));
            } else {
                sums.add(new Pair(number, number + sums.get(sums.size() - 1).getSum()));
            }
        }, (sums1, sums2) -> {
            if (!sums1.isEmpty()) {
                long sum = sums1.get(sums1.size() - 1).getSum();
                sums2.forEach(p -> p.setSum(p.getSum() + sum));
            }
            sums1.addAll(sums2);
        });

这组合了所有数字，并为每个数字创建一对，并加上前一个总和。它使用以下 Pair 类作为帮助程序：

This combines all the numbers and creates a pair for each number with the addition to the previous sum. It uses the following Pair class as helper:

public class Pair {
    private long number;
    private long sum;

    public Pair(long number, long sum) {
        this.number = number;
        this.sum = sum;
    }

    public long getNumber() {
        return number;
    }

    public void setSum(long sum) {
        this.sum = sum;
    }

    public long getSum() {
        return sum;
    }
}

如果您愿意，可以轻松更改助手类添加更多信息。

You can easily change that helper class if you want to add some more information.

最后的结果是：

[
    Pair{number=2, sum=2}, 
    Pair{number=2, sum=4}, 
    Pair{number=4, sum=8}, 
    Pair{number=5, sum=13}
]

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