joinTo所在位置是kotlin.collections.joinTo，其相关用法介绍如下。

用法:

fun <T, A : Appendable> Array<out T>.joinTo(
    buffer: A, 
    separator: CharSequence = ", ", 
    prefix: CharSequence = "", 
    postfix: CharSequence = "", 
    limit: Int = -1, 
    truncated: CharSequence = "...", 
    transform: ((T) -> CharSequence)? = null
): A
fun <A : Appendable> ByteArray.joinTo(
    buffer: A, 
    separator: CharSequence = ", ", 
    prefix: CharSequence = "", 
    postfix: CharSequence = "", 
    limit: Int = -1, 
    truncated: CharSequence = "...", 
    transform: ((Byte) -> CharSequence)? = null
): A
fun <A : Appendable> ShortArray.joinTo(
    buffer: A, 
    separator: CharSequence = ", ", 
    prefix: CharSequence = "", 
    postfix: CharSequence = "", 
    limit: Int = -1, 
    truncated: CharSequence = "...", 
    transform: ((Short) -> CharSequence)? = null
): A
fun <A : Appendable> IntArray.joinTo(
    buffer: A, 
    separator: CharSequence = ", ", 
    prefix: CharSequence = "", 
    postfix: CharSequence = "", 
    limit: Int = -1, 
    truncated: CharSequence = "...", 
    transform: ((Int) -> CharSequence)? = null
): A
fun <A : Appendable> LongArray.joinTo(
    buffer: A, 
    separator: CharSequence = ", ", 
    prefix: CharSequence = "", 
    postfix: CharSequence = "", 
    limit: Int = -1, 
    truncated: CharSequence = "...", 
    transform: ((Long) -> CharSequence)? = null
): A
fun <A : Appendable> FloatArray.joinTo(
    buffer: A, 
    separator: CharSequence = ", ", 
    prefix: CharSequence = "", 
    postfix: CharSequence = "", 
    limit: Int = -1, 
    truncated: CharSequence = "...", 
    transform: ((Float) -> CharSequence)? = null
): A
fun <A : Appendable> DoubleArray.joinTo(
    buffer: A, 
    separator: CharSequence = ", ", 
    prefix: CharSequence = "", 
    postfix: CharSequence = "", 
    limit: Int = -1, 
    truncated: CharSequence = "...", 
    transform: ((Double) -> CharSequence)? = null
): A
fun <A : Appendable> BooleanArray.joinTo(
    buffer: A, 
    separator: CharSequence = ", ", 
    prefix: CharSequence = "", 
    postfix: CharSequence = "", 
    limit: Int = -1, 
    truncated: CharSequence = "...", 
    transform: ((Boolean) -> CharSequence)? = null
): A
fun <A : Appendable> CharArray.joinTo(
    buffer: A, 
    separator: CharSequence = ", ", 
    prefix: CharSequence = "", 
    postfix: CharSequence = "", 
    limit: Int = -1, 
    truncated: CharSequence = "...", 
    transform: ((Char) -> CharSequence)? = null
): A
fun <T, A : Appendable> Iterable<T>.joinTo(
    buffer: A, 
    separator: CharSequence = ", ", 
    prefix: CharSequence = "", 
    postfix: CharSequence = "", 
    limit: Int = -1, 
    truncated: CharSequence = "...", 
    transform: ((T) -> CharSequence)? = null
): A

附加使用separator 分隔的所有元素的字符串，并使用给定的prefix 和postfix(如果提供)。

如果集合可能很大，您可以指定一个非负值 limit ，在这种情况下，只会附加第一个 limit 元素，然后是 truncated 字符串(默认为 "...")。

例子：

import kotlin.test.*

fun main(args: Array<String>) {
//sampleStart
val sb = StringBuilder("An existing string and a list: ")
val numbers = listOf(1, 2, 3)
println(numbers.joinTo(sb, prefix = "[", postfix = "]").toString()) // An existing string and a list: [1, 2, 3]

val lotOfNumbers: Iterable<Int> = 1..100
val firstNumbers = StringBuilder("First five numbers: ")
println(lotOfNumbers.joinTo(firstNumbers, limit = 5).toString()) // First five numbers: 1, 2, 3, 4, 5, ...
//sampleEnd
}

输出：

An existing string and a list: [1, 2, 3]
First five numbers: 1, 2, 3, 4, 5, ...