何时使用专门的流方法而不是Collectors类中的方法，反之亦然？ [英] When to use specialised stream methods over methods in the Collectors class or vice versa?

问题描述

我们可以看到收藏家 class包含许多常见聚合操作的工厂方法。列举一些 summingXXX ， averagingXXX ， maxBy ， minBy 。

We can see that the Collectors class contains factory methods for many common aggregation operations. to name a few summingXXX, averagingXXX, maxBy, minBy.

这些方法实际上是与专用流上的调用方法相比的重复功能。以 IntStream 为例有一个 sum（）， average（）， max（）和 min（）。

These methods are actually duplicate functionality over calling methods on specialised streams. take for example IntStream, this has a sum(), average() , max() and min() respectively.

DoubleStream 和 LongStream 。

时不时我看到人们可以互换地使用属于Collectors类和专用流方法的方法。

Every now and then I see people using the methods which belong to the Collectors class and specialized stream methods interchangeably.

例如，假设我们有一个 List< Employee> 并且想要获得平均工资，你可以使用专业原始流中的方法。

For example, assuming we have a List<Employee> and one wants to get the average salary, you can either use the methods in the specialised primitive streams.

OptionalDouble averageUsingIntStream =
             employees.stream()
                      .mapToInt(Employee::getSalary)
                      .average();

或使用收藏家中的工厂方法class。

or use the factory methods in the Collectors class.

Double averageUsingCollectors =
             employees.stream()
                      .collect(Collectors.averagingInt(Employee::getSalary));

现在，我的问题是这个重复功能的动机是什么，在哪种情况下它更好使用第一种方法还是第二种方法？

Now, my question is what is the motivation behind this duplicate functionality and in which situation is it better to use the first approach or the second?

推荐答案

收集器类中所述工厂方法存在的真正动机将用作下游收集器，即Java运行时应用于另一个收集器的结果的收集器。这使得人们可以组合在一起并执行更复杂的聚合。

The real motivation for the said factory methods in the Collectors class to exist is to be used as downstream collectors i.e. a collector that the Java runtime applies to the results of another collector. This enables one to compose together and perform more complex aggregations.

例如，有 groupingBy 接受下游收集器的收集器 collectAndThen ， partitioningBy ， flatMapping （java-9）等。

For example, there are groupingBy collectors that accept a downstream collector, collectingAndThen, partitioningBy, flatMapping (java-9) et al.

因此，经验法则是只将问题中的上述方法用作下游收集器。

So the rule of thumb is to only use the aforementioned methods in the question as downstream collectors.

专用原始流也更有效，所以如果你想做的就是计算 sum（）， average（） ， max（）或 min（）然后我会坚持原始专业化。

The specialised primitive streams are also more efficient so if all you want to do is compute the sum(), average() , max() or min() then I'd stick with the primitive specialisation.

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