lambda - 试图了解Kotlin示例

Question

我想学习Kotlin，并正在通过
try.kotlinlang.org

我很难理解一些示例，尤其是Lazy属性示例:https://try.kotlinlang.org/#/Examples/Delegated%20properties/Lazy%20property/Lazy%20property.kt

/**
 * Delegates.lazy() is a function that returns a delegate that implements a lazy property:
 * the first call to get() executes the lambda expression passed to lazy() as an argument
 * and remembers the result, subsequent calls to get() simply return the remembered result.
 * If you want thread safety, use blockingLazy() instead: it guarantees that the values will
 * be computed only in one thread, and that all threads will see the same value.
 */

class LazySample {
    val lazy: String by lazy {
        println("computed!")
        "my lazy"
    }
}

fun main(args: Array<String>) {
    val sample = LazySample()
    println("lazy = ${sample.lazy}")
    println("lazy = ${sample.lazy}")
}

输出:

computed!
lazy = my lazy
lazy = my lazy

我不明白这里发生了什么。 (可能是因为我不太熟悉lambda)

为什么println()只执行一次？

我也对“我的懒惰”这一行感到困惑
字符串未分配给任何东西(字符串x =“my lazy”)或未在返回中使用
(返回“我的懒惰”)

有人可以解释吗？ :)

Answer 1

Why is the println() only executed once?

发生这种情况是因为您是第一次访问它，它已创建。要创建，它将调用仅传递一次的lambda并分配值 "my lazy"。
您在 Kotlin中编写的代码与以下Java代码相同:

public class LazySample {

    private String lazy;

    private String getLazy() {
        if (lazy == null) {
            System.out.println("computed!");
            lazy = "my lazy";
        }
        return lazy;
    }
}

I am also confused about the line "my lazy" the String isn't assigned to anything (String x = "my lazy") or used in a return (return "my lazy)

Kotlin支持 implicit returns用于lambda。这意味着将lambda的最后一条语句视为其返回值。
您还可以使用 return@label指定显式返回。
在这种情况下:

class LazySample {
    val lazy: String by lazy {
        println("computed!")
        return@lazy "my lazy"
    }
}