在Java中使用Lambda表达式查找Max [英] Finding Max with Lambda Expression in Java

问题描述

这是我的代码

    List<Integer> ints = Stream.of(1,2,4,3,5).collect(Collectors.toList());
    Integer maxInt = ints.stream()
                              .max(Comparator.comparing(i -> i))
                              .get();

    System.out.println("Maximum number in the set is " + maxInt);

输出：

Maximum number in the set is 5

我无法在两个<$ c之间进行distingues $ c>我在我的代码的下一部分

I cannot make distingues between two i in below section of my code

Comparator.comparing(i -> i)

任何人都可以善待并解释两个之间的区别我？

can anyone be kind and explain the difference between two i?

推荐答案

方法 Comparator.comparing（...）旨在创建一个 Comparator ，它使用基于对象属性的订单进行比较。使用lambda表达式时 i - >我，这是（int i）的简写 - > {return i; } 这里，作为属性提供程序函数，生成的 Comparator 将比较值本身。当要比较的对象具有自然顺序为整数具有此功能时，此方法有效。

The method Comparator.comparing(…) is intended to create a Comparator which uses an order based on a property of the objects to compare. When using the lambda expression i -> i, which is a short writing for (int i) -> { return i; } here, as a property provider function, the resulting Comparator will compare the values itself. This works when the objects to compare have a natural order as Integer has.

所以

Stream.of(1,2,4,3,5).max(Comparator.comparing(i -> i))
.ifPresent(maxInt->System.out.println("Maximum number in the set is " + maxInt));

与

Stream.of(1,2,4,3,5).max(Comparator.naturalOrder())
.ifPresent(maxInt->System.out.println("Maximum number in the set is " + maxInt));

虽然后者效率更高，因为它对所有具有自然顺序的类型实施为单身（并实现 Comparable ）。

though the latter is more efficient as it is implemented as singleton for all types which have a natural order (and implement Comparable).

max 的原因>需要一个 Comparator ，因为你使用的泛型类 Stream 可能包含任意对象。

The reason why max requires a Comparator at all, is because you are using the generic class Stream which might contain arbitrary objects.

这允许，例如使用它像 streamOfPoints.max（Comparator.comparing（p-> px））来查找最大 x 值而点本身没有自然顺序。或者做类似 streamOfPersons.sorted（Comparator.comparing（Person :: getAge））。

This allows, e.g. to use it like streamOfPoints.max(Comparator.comparing(p->p.x)) to find the point with the largest x value while Point itself does not have a natural order. Or do something like streamOfPersons.sorted(Comparator.comparing(Person::getAge)).

使用时专门的 IntStream 您可以直接使用自然订单，这可能更有效：

When using the specialized IntStream you can use the natural order directly which is likely to be more efficient:

IntStream.of(1,2,4,3,5).max()
.ifPresent(maxInt->System.out.println("Maximum number in the set is " + maxInt));

---

说明自然顺序与基于房产的订单：

---

To illustrate the difference between "natural order" and a property based order:

Stream.of("a","bb","aaa","z","b").max(Comparator.naturalOrder())
.ifPresent(max->System.out.println("Maximum string in the set is " + max));

这将打印

集合中的最大字符串是z

Maximum string in the set is z

作为字符串的自然顺序 s是字典顺序，其中 z 大于 b ，大于 a

as the natural order of Strings is the lexicographical order where z is greater than b which is greater than a

另一方面

Stream.of("a","bb","aaa","z","b").max(Comparator.comparing(s->s.length()))
.ifPresent(max->System.out.println("Maximum string in the set is " + max));

将打印

集合中的最大字符串是aaa

Maximum string in the set is aaa

as aaa 具有流中所有字符串的最大长度。这是 Comparator.comparing 的预期用例，在使用方法引用时可以使其更具可读性，即 Comparator.comparing（String :: length ）这几乎说不出话来......

as aaa has the maximum length of all Strings in the stream. This is the intended use case for Comparator.comparing which can be made even more readable when using method references, i.e. Comparator.comparing(String::length) which almost speaks for itself…

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