Java8中findAny和findFirst的区别

痛定思痛。 2024-02-05 17:26 54阅读 0赞

试验了一下java [stream][]中的findAny和findFirst，发现都返回了列表中的第一个元素。那么，这两种方法的区别是什么呢？

查看了一下Java API document：

findFirst：

> findFirst
> 
>     Optional<T> findFirst()
> 
> Returns an [`Optional`][Optional] describing the first element of this stream, or an empty `Optional` if the stream is empty. If the stream has no encounter order, then any element may be returned.
> 
> This is a [short-circuiting terminal operation][].
> 
> Returns:
> 
> an `Optional` describing the first element of this stream, or an empty `Optional` if the stream is empty
> 
> Throws:
> 
> `[NullPointerException](https://docs.oracle.com/javase/8/docs/api/java/lang/NullPointerException.html)` \- if the element selected is null

顾名思义，即返回列表中的第一个元素。

这里的short-circuiting是指：有时候需要在遍历中途停止操作，比如查找第一个满足条件的元素或者limit操作。在Stream中short-circuiting操作有：anyMatch、allMatch、noneMatch、findFirst、findAny、limit，这些操作在Sink中都有一个变量来判断是否短路，比如limit用的是m，match用的是stop，find用的是hasValue。

这里的terminal operation是指：一个终结操作，比如foreach，IntStream.sum

那么，findAny是什么呢？

> findAny
> 
>     Optional<T> findAny()
> 
> Returns an [`Optional`][Optional] describing some element of the stream, or an empty `Optional` if the stream is empty.
> 
> This is a [short-circuiting terminal operation][].
> 
> The behavior of this operation is explicitly nondeterministic(不确定的); it is free to select any element in the stream. This is to allow for maximal performance in parallel operations; the cost is that multiple invocations(调用) on the same source may not return the same result. (If a stable result is desired, use [`findFirst()`][findFirst] instead.)
> 
> Returns:
> 
> an `Optional` describing some element of this stream, or an empty `Optional` if the stream is empty
> 
> Throws:
> 
> `[NullPointerException](https://docs.oracle.com/javase/8/docs/api/java/lang/NullPointerException.html)` \- if the element selected is null
> 
> See Also:
> 
> [`findFirst()`][findFirst]

可以看到findAny()操作，返回的元素是不确定的，对于同一个列表多次调用findAny()有可能会返回不同的值。使用findAny()是为了更高效的性能。如果是数据较少，串行地情况下，一般会返回第一个结果，如果是并行的情况，那就不能确保是第一个。比如下面的例子会随机地返回OptionalInt\[50\]。

System.out.println(IntStream.range(0, 100).parallel().findAny());

让我们来举另外一个例子：

List<String> lst1 = Arrays.asList("Jhonny", "David", "Jack", "Duke", "Jill","Dany","Julia","Jenish","Divya");
    List<String> lst2 = Arrays.asList("Jhonny", "David", "Jack", "Duke", "Jill","Dany","Julia","Jenish","Divya");
     
    Optional<String> findFirst = lst1.parallelStream().filter(s -> s.startsWith("D")).findFirst();
    Optional<String> fidnAny = lst2.parallelStream().filter(s -> s.startsWith("J")).findAny();
     
    System.out.println(findFirst.get()); //总是打印出David
    System.out.println(fidnAny.get()); //会随机地打印出Jack/Jill/Julia

参考链接：

1.  [https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html\#findFirst–][findFirst]
2.  [https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html\#findAny–][https_docs.oracle.com_javase_8_docs_api_java_util_stream_Stream.html_findAny]
3.  [https://stackoverflow.com/questions/35359112/difference-between-findany-and-findfirst-in-java-8][https_stackoverflow.com_questions_35359112_difference-between-findany-and-findfirst-in-java-8]

[stream]: https://so.csdn.net/so/search?q=stream&spm=1001.2101.3001.7020
[Optional]: https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html
[short-circuiting terminal operation]: https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html#StreamOps
[findFirst]: https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#findFirst--
[https_docs.oracle.com_javase_8_docs_api_java_util_stream_Stream.html_findAny]: https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#findAny--
[https_stackoverflow.com_questions_35359112_difference-between-findany-and-findfirst-in-java-8]: https://stackoverflow.com/questions/35359112/difference-between-findany-and-findfirst-in-java-8