Kotlin基础知识四：top-level函数和top-leve

一、top-level函数和top-level属性

1. top-level函数

不依赖于任何类的静态函数，经Kotlin编译成Java文件后，成为:<静态函数所在的文件名> + "Kt"的Java类的静态成员函数。如果原kotlin文件的文件名首字母为小写时，转换成大写。
举例:

/** joinsample.kt */
package com.example.kotlin

import java.lang.StringBuilder

fun <T> joinToString(
    collection: Collection<T>,
    separator: String = ", ",
    prefix: String = "",
    postfix: String = ""
): String {
    val result = StringBuilder(prefix)
    for ((index, element) in collection.withIndex()) {
        if (index > 0) {
            result.append(separator)
        }

        result.append(element)
    }
    result.append(postfix)

    return result.toString()
}

如果需要在Java中调用该方法时:

import com.example.kotlin.JoinsampleKt;
import java.util.Arrays;
import java.util.List;

public class JavaSample {
    public static void main(String[] args) {
        List<String> list = Arrays.asList("hello", "world");
        JoinsampleKt.joinToString(list, ", ", "", "");
    }
}

如果想要改变top-level的默认Java类名，可通过注解的方式修改：

@file:JvmName("StringFunctions")
package com.example.kotlin
import java.lang.StringBuilder
fun <T> joinToString(
    collection: Collection<T>,
    separator: String = ", ",
    prefix: String = "",
    postfix: String = ""
): String {
    val result = StringBuilder(prefix)
    for ((index, element) in collection.withIndex()) {
        if (index > 0) {
            result.append(separator)
        }
        result.append(element)
    }
    result.append(postfix)
    return result.toString()
}