kotlin - Java Lambda类型推断无法按预期在Kotlin中运行

Question

如果没有Collectors.toList<String>()中的显式类型参数，为什么这段Java代码无法在Kotlin中编译？有没有更惯用的方法来做到这一点？

// works
List<String> folders = Files.walk(Paths.get(args[0]))
            .filter(it -> it.toFile().isDirectory())
            .map(it -> it.toAbsolutePath().toString())
            .collect(Collectors.toList());

// does not compile - resulting type is `MutableList<in String!>..List<Any?>?` which is not compatible to `List<String>`
val folders: List<String> = Files.walk(Paths.get(args[0]))
            .filter { it.toFile().isDirectory }
            .map { it.toAbsolutePath().toString() }
            .collect(Collectors.toList())

// compiles
val folders: List<String> = Files.walk(Paths.get(args[0]))
            .filter { it.toFile().isDirectory }
            .map { it.toAbsolutePath().toString() }
            .collect(Collectors.toList<String>())

Answer 1

Why does this piece of Java code not compile in Kotlin without the explicit type parameter in Collectors.toList<String>()?

在我看来，这似乎是一个编译器错误。我建议在 Kotlin (KT) | YouTrack中创建一个问题。

Is there a more idiomatic way to do this?

是。与 Kirill Rakhman comments一样，“Kotlin有其自己的 File.walk扩展方法。”例如。:

val folders: List<String> = File(args[0]).walk()
        .filter(File::isDirectory)
        .map(File::getAbsolutePath)
        .toList()

如果您更喜欢使用Java 8流，请 checkout Kotlin/kotlinx.support: Extension and top-level functions to use JDK7/JDK8 features in Kotlin 1.0。它定义了一个 Stream<T>.toList()函数:

val folders: List<String> = Files.walk(Paths.get(args[0]))
        .filter { it.toFile().isDirectory }
        .map { it.toAbsolutePath().toString() }
        .toList()