java8 stream reduce方法实现归约操作 和SUM的区别介绍

map()和filter()都是Stream的转换方法，而Stream.reduce()则是Stream的一个聚合方法，它可以把一个Stream的所有元素按照聚合函数聚合成一个结果。

Java 利用reduce方法实现归约操作，用户希望通过流操作生成单一值，使用 reduce 方法对每个元素进行累加计算。

Java 的函数式范式经常采用“映射 – 筛选 – 归约”（map-filter-reduce）的过程处理数据。首先，map 操作将一种类型的流转换为另一种类型（如通过调用 length 方法将 String 流转换为 int 流）。接下来，filter 操作产生一个新的流，它仅包含所需的元素（如长度小于某个阈值的字符串）。最后，通过终止操作从流中生成单个值（如长度的总和或均值）。

In Java, both reduce() and sum() are methods that can be used with streams to perform operations on the elements of the stream. However, they serve slightly different purposes:

int sum = list.stream().reduce(0, (a, b) -> a + b);
 

int sum = list.stream().mapToInt(Integer::intValue).sum();
 

In summary, reduce() is a more general-purpose method that can be used for various reduction operations, while sum() is a specialized method designed specifically for summing elements in numeric streams, providing a more concise and direct way to achieve this common operation. Use reduce() when you need to perform custom aggregations or other types of reductions, and use sum() when you want to calculate the sum of numeric elements in a stream.