【Kotlin】Kotlin笔记7-Lambda编程2(List,Map的函数式API)

Kotlin笔记7-Lambda编程2-List,Map的函数式API

4.2 Lambda编程2

• Lambda表达式

​​Example:​​

//list
//最长水果单词
fun findLongestFruit() {
    val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")
    var maxLengthFruit = ""
    for (fruit in list) {
        if (fruit.length > maxLengthFruit.length)
            maxLengthFruit = fruit
    }
    println("max length fruit is $maxLengthFruit")
}

集合的函数式API

fun findLongestFruit1() {
    //优化
    val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")
    val maxLengthFruit = list.maxByOrNull { it.length }
    println("max length fruit is $maxLengthFruit")
}

Lambda表达式的语法结构

{ 参数名1: 参数类型, 参数名2: 参数类型 -> 函数体 }

​​推演过程:​​

val lambda = { fruit: String -> fruit.length }
val maxLengthFruit = list.maxByOrNull(lambda)

​​直接传入:​​

val maxLengthFruit = list.maxByOrNull({ fruit: String -> fruit.length })

​​移到括号外:​​

val maxLengthFruit1 = list.maxByOrNull() { fruit: String -> fruit.length }

​​lambda是函数唯一参数,省略括号:​​

val maxLengthFruit2 = list.maxByOrNull { fruit: String -> fruit.length }

​​类型推导:​​

val maxLengthFruit3 = list.maxByOrNull { fruit -> fruit.length }

​​唯一参数,it代替:​​

it: 唯一参数,it代替

val maxLengthFruit4 = list.maxByOrNull { it.length }

• Map函数API

​​Example:​​

fun getUpperFruits() {
    val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")
    val newlist = list.map { it.toUpperCase() }
    for (fruit in newlist) {
        println(fruit)
    }
}

filter:

​​Example:​​

fun filterFruits() {
    val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")
    val newlist = list.filter { it.length < 5 }
        .map { it.toUpperCase() }
    for (fruit in newlist) {
        println(fruit)
    }
}

any: 是否存在一个元素满足指定条件

val anyResult = list.any { it.length <= 5 }

all: 是否所有元素都满足指定条件

val allResult = list.all { it.length <= 5 }

​​Example:​​

//any&all
fun anyAndAll() {
    val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")
    val anyResult = list.any { it.length <= 5 }
    val allResult = list.all { it.length <= 5 }
    println("anyResult is $anyResult, allResult is $allResult")
}