Java 8 mapToInt ( mapToInt(e -> e) ) 如何准确提高性能？
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How does Java 8 mapToInt ( mapToInt(e -> e) )improves performance exactly?
提问by adragomir
I'm reading the book "Java 8 Lambdas", and at some point the author says "It's a good idea to use the primitive specialized functions wherever possible because of the performance benefits.".
我正在阅读“Java 8 Lambdas”一书，在某些时候作者说“由于性能优势，尽可能使用原始的专用函数是个好主意。”。
He is referring here to mapToInt, mapToLong, etc.
他在这里指的是 mapToInt、mapToLong 等。
The thing is I don't know where the performance comes from to be honest.
问题是老实说我不知道性能来自哪里。
Let's consider an example:
让我们考虑一个例子：
// Consider this a very very long list, with a lot of elements
List<Integer> list = Arrays.asList(1, 2, 3, 4);
//sum it, flavour 1
int sum1 = list.stream().reduce(0, (acc, e) -> acc + e).intValue();
//sum it, flavour 2
int sum2 = list.stream().mapToInt(e -> e).sum();
System.out.println(sum1 + " " + sum2);
So, in the first case I use reduce to sum the values, so the BinaryOperator function will receive all the time an int ( acc ) and an Integer ( the current element of the collection ) and then will do an unboxing of the Integer to the int ( acc + e)
因此，在第一种情况下，我使用 reduce 对值求和，因此 BinaryOperator 函数将始终接收一个 int ( acc ) 和一个 Integer ( 集合的当前元素)，然后将 Integer 拆箱到int (acc + e)
In the second case, I use mapToInt, which unboxes each Integer into an int, and then sums it.
在第二种情况下，我使用 mapToInt，它将每个 Integer 拆箱为一个 int，然后对它求和。
My question is, is there any advantage of the second approach? Also what's the point of map to int, when I could have used map?
我的问题是，第二种方法有什么好处吗？另外，当我可以使用 map 时，map to int 的意义何在？
So yeah, is it all just sugar syntax or does it has some performance benefits? In case it does, please offer some information
所以是的，这只是糖语法还是它有一些性能优势？如果有，请提供一些信息
Regards,
问候，
回答by Ian Roberts
There's an extra level of boxing going on in
有一个额外的拳击水平在进行中
int sum1 = list.stream().reduce(0, (acc, e) -> acc + e).intValue();
as the reduction function is a BinaryOperator<Integer>- it gets passed two Integervalues, unboxes them, adds them, and then re-boxes the result. The mapToIntversion unboxes the Integerelements from the list onceand then works with primitive intvalues from that point on as an IntStream.
因为reduce函数是a BinaryOperator<Integer>- 它传递了两个Integer值，将它们拆箱，添加它们，然后重新装箱结果。该mapToInt版本Integer将列表中的元素拆箱一次，然后从那时起将原始int值作为IntStream.
