Transformation function map and flatmap in kotlin

//Kotlin Transformation function in map And flatmap
fun main() {
    val list = listOf(" Zhang San ", " Li Si ", " Wang Wu ")
    // principle ： Add the return value of the last line of the anonymous function to a new set , The generic type of the new set is R, And return a new set 
    val list2: List<Int> = list.map {
        //it == T ==  Elements  == String
        "[$it]"
        88
    }
    println(list2)
    // purpose ： and RxJava The same way of thinking 
    val list3: List<String> = list.map {
        " Name is :$it"
    }.map {
        "$it, Text length is ：${it.length}"
    }.map {
        "[$it]"
    }
    for (s in list3) {
        println("$s ")
    }

    //map{ Return type ：T String Int Boolean...  Add each element to the new set , Finally back to List<String>}
    //flatMap{ Return type ： Every element T  aggregate 1  aggregate 2...  Put every element （ aggregate ） Add to new collection , Finally, return the new set List<List<String>>
    // Finally, the internal will be processed into List<String>
    //flatMap  amount to List<List<String>> A collection of collections , There are nested relationships 
    val list4: List<String> = listOf(" Zhang San ", " Li Si ", " Wang Wu ")
    val newList: List<String> = list4.map {
        " The content is :$it"  // One at a time  String
    }.map {
        "$it,  The length is ：${it.length}"
    }.flatMap {
        listOf("$it, I'm learning C++", "$it, I'm learning Java", "$it, I'm learning Kotlin")
    }
    println(newList)
    val newList2: List<String> = list.flatMap {
        listOf("$it, I'm learning C++", "$it, I'm learning Java", "$it, I'm learning Kotlin")
    }
    println(newList2)
}