英语里函数的 identity 是啥？ Java stream api 的 reduce 文档看不懂

[Leviathann]

Performs a reduction on the elements of this stream, using the provided identity, accumulation and combining functions. This is equivalent to:

U result = identity;
for (T element : this stream)
result = accumulator.apply(result, element)
return result;

but is not constrained to execute sequentially.
The identity value must be an identity for the combiner function. This means that for all u, combiner(identity, u) is equal to u.

搜了下大概知道这个 combiner 是并行流合并子流 reduce 的结果用的，但是那个 combiner(identity, u) is equal to u.是啥意思

[sosilver]

wiki/Identity_element

[Leviathann]

@sosilver 原来如此，吃了数学的亏
我一直以为物理数学里的 单位 都是 unit 。。