如何在 Kotlin 的 Java 8 流上调用 collect(Collectors.toList())?

How can I call collect(Collectors.toList()) on a Java 8 Stream in Kotlin?(如何在 Kotlin 的 Java 8 流上调用 collect(Collectors.toList())?)

问题描述

我有一些代码:

directoryChooser.title = "Select the directory"
val file = directoryChooser.showDialog(null)
if (file != null) {
    var files = Files.list(file.toPath())
            .filter { f ->
                f.fileName.endsWith("zip") && f.fileName.endsWith("ZIP")
                        && (f.fileName.startsWith("1207") || f.fileName.startsWith("4407") || f.fileName.startsWith("1507") || f.fileName.startsWith("9007") || f.fileName.startsWith("1807"))
            }
    for (f in files) {
        textArea.appendText(f.toString() + "
")
    }
}

如果我在过滤器末尾调用 collect(Collectors.toList())，我会得到:

If I call collect(Collectors.toList()) at the end of filter, I get:

Error:(22, 13) Kotlin: [Internal Error] org.jetbrains.kotlin.codegen.CompilationException: Back-end (JVM) Internal error: no descriptor for type constructor of ('Captured(in ('Path'..'Path?'))'..'CapturedTypeConstructor(in ('Path'..'Path?'))?')
Cause: no descriptor for type constructor of ('Captured(in ('Path'..'Path?'))'..'CapturedTypeConstructor(in ('Path'..'Path?'))?')
File being compiled and position: (22,13) in D:/My/devel/ListOfReestrs/src/Controller.kt
PsiElement: var files = Files.list(file.toPath())
                    .filter { f ->
                        f.fileName.endsWith("zip") && f.fileName.endsWith("ZIP")
                                && (f.fileName.startsWith("1207") || f.fileName.startsWith("4407") || f.fileName.startsWith("1507") || f.fileName.startsWith("9007") || f.fileName.startsWith("1807"))
                    }.collect(Collectors.toList())
The root cause was thrown at: JetTypeMapper.java:430

如果我不这样做，我会在我的 for 循环中获得类型为 [error: Error] 的 f.

If I don't do this, I get the f with the type [error: Error] in my for-loop.

推荐答案

更新: 这个问题现在在 Kotlin 1.0.1 中得到修复(之前是 KT-5190).不需要变通.

UPDATE: This issue is now fixed in Kotlin 1.0.1 (previously was KT-5190). No work around is needed.

解决方法 #1:

创建这个扩展函数，然后在 Stream 上将其简单地用作 .toList():

Create this extension function, then use it simply as .toList() on the Stream:

fun <T: Any> Stream<T>.toList(): List<T> = this.collect(Collectors.toList<T>())

用法:

Files.list(Paths.get(file)).filter { /* filter clause */ }.toList()

这为 Collectors.toList() 调用添加了一个更明确的泛型参数，从而防止在泛型推断期间发生的错误(对于该方法返回类型 Collector<T, ?, List<T>>, eeeks!?!).

This adds a more explicit generic parameter to the Collectors.toList() call, preventing the bug which occurs during inference of the generics (which are somewhat convoluted for that method return type Collector<T, ?, List<T>>, eeeks!?!).

解决方法 #2:

将正确的类型参数添加到您的调用中作为 Collectors.toList<Path>() 以避免该参数的类型推断:

Add the correct type parameter to your call as Collectors.toList<Path>() to avoid type inference of that parameter:

Files.list(Paths.get(file)).filter { /* filter clause */ }.collect(Collectors.toList<Path>())

但解决方法 #1 中的扩展功能更可重用且更简洁.

But the extension function in workaround #1 is more re-usable and more concise.

解决该错误的另一种方法是不收集 Stream.你可以保持懒惰，将 Stream 转换为 Kotlin Sequence 或 Iterator，这是一个用于制作 Sequence<的扩展函数/代码>:

fun <T: Any> Stream<T>.asSequence(): Sequence<T> = this.iterator().asSequence()

有了这个，我将创建一个用于列出文件的扩展，以避免 Paths、Path、Files、<代码>文件:

And with this, I would create an extensions for listing files to avoid the bad API design of Paths, Path, Files, File:

fun Path.list(): Sequence<Path> = Files.list(this).iterator().asSequence()

至少会从左到右很好地流动:

which would at least flow nicely from left to right:

File(someDir).toPath().list().forEach { println(it) }
Paths.get(dirname).list().forEach { println(it) }

<小时>

使用 Java 8 流的替代方案:

我们可以稍微更改您的代码以从 File 获取文件列表，而您只需在末尾使用 toList():

Alternatives to using Java 8 Streams:

We can change your code slightly to get the file list from File instead, and you would just use toList() at the end:

file.listFiles().filter { /* filter clause */ }.toList()

或

file.listFiles { file, name ->  /* filter clause */ }.toList()

不幸的是，您最初使用的 Files.list(...) 返回一个 Stream 并且没有给您使用传统集合的机会.此更改通过从返回数组或集合的函数开始来避免这种情况.

Unfortunately the Files.list(...) that you originally used returns a Stream and doesn't give you the opportunity to use a traditional collection. This change avoids that by starting with a function that returns an Array or collection.

一般:

在大多数情况下，您可以避免使用 Java 8 流，并使用本机 Kotlin 标准库函数和对 Java 集合的扩展.Kotlin 确实通过编译时只读和可变接口使用 Java 集合.但随后它添加了扩展功能以提供更多功能.因此，您具有相同的性能，但具有更多功能.

In most cases you can avoid Java 8 streams, and use native Kotlin stdlib functions and extensions to Java collections. Kotlin does indeed use Java collections, via compile-time readonly and mutable interfaces. But then it adds extension functions to provide more functionality. Therefore you have the same performance but with many more capabilities.

另请参阅:

• 标准 Kotlin 库中有哪些 Java 8 Stream.collect 等效项? - 你会发现它更简洁使用 Kotlin 标准库函数和扩展.
• Kotlin 集合和扩展函数 API 文档
• Kotlin 序列 API 文档
• Kotlin 成语

• What Java 8 Stream.collect equivalents are available in the standard Kotlin library? - You will see it is more concise to use Kotlin stdlib functions and extensions.
• Kotlin Collections, and Extension Functions API docs
• Kotlin Sequences API docs
• Kotlin idioms

您应该查看 API 参考以了解什么在标准库中可用.

You should review the API reference for knowing what is available in the stdlib.

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