Basic Kotlin Language Advanced

Classes 类

类的声明: 类名称，类头(可选)，类身（可选）

class Customer                               class Contact(val id: Int, var email: String)

Collections 集合

主要集合: List, Set, Map

需创建可变集合时，添加mutable

val list1: MutableList<Int> = mutableListOf(1, 2, 3)val list2: List<Int> = ListOf(1, 2, 3)

集合函数

    val numbers = listOf(1, -2, 3, -4, 5, -6)    // filter 截取    val positives = numbers.filter { x -> x > 0 }     val negatives = numbers.filter { it < 0 }    // map 映射    val doubled = numbers.map { x -> x * 2 }    val tripled = numbers.map { it * 3 }    // any 存在    val anyNegative = numbers.any { it < 0 }                 // all 存在    val allEven = numbers.all { it % 2 == 0 }     // none 存在    val allEven2 = numbers.none { it % 2 == 1 }     // find 查找    val words = listOf("Lets", "find", "something", "in", "collection", "somehow")  // 1    val first = words.find { it.startsWith("some") }                                // 2    val last = words.findLast { it.startsWith("some") }                             // 3    val nothing = words.find { it.contains("nothing") }    // first 查找    val firstNum = numbers.first()    val lastNum = numbers.last()    val firstEven = numbers.first { it % 2 == 0 }        // 4    val lastOdd = numbers.last { it % 2 != 0 }    // 数量    val totalCount = numbers.count()                     // 2    val evenCount = numbers.count { it % 2 == 0 }        data class Person(val name: String, val city: String, val phone: String) // 1        val people = listOf(                                                     // 2            Person("John", "Boston", "+1-888-123456"),            Person("Sarah", "Munich", "+49-777-789123"),            Person("Svyatoslav", "Saint-Petersburg", "+7-999-456789"),            Person("Vasilisa", "Saint-Petersburg", "+7-999-123456"))        val phoneBook = people.associateBy { it.phone }                          // 3        val cityBook = people.associateBy(Person::phone, Person::city)           // 4        val peopleCities = people.groupBy(Person::city, Person::name)            // 5    // filter 分割    val evenOdd = numbers.partition { it % 2 == 0 }  // ([-2, -4, -6], [1, 3, 5])    val (positives2, negatives2) = numbers.partition { it > 0 }        // flatMap 映射    val tripled2 = numbers.flatMap { listOf(it, it, it) } // []    println(numbers.flatMap { listOf(it, it *it) }) // [1, 1, -2, 4, 3, 9, -4, 16, 5, 25, -6, 36]    // min and max functions 最大最小    val empty = emptyList<Int>()    println("Numbers: $numbers, min = ${numbers.min()} max = ${numbers.max()}") // 1    println("Empty: $empty, min = ${empty.min()}, max = ${empty.max()}") // For empty collections both functions return null.    // sorted 排序    println("Soroted: ${ numbers.sorted()}")    val inverted = numbers.sortedBy { -it }    // 获取map的值    val map = mapOf("key" to 42)    val value1 = map["key"]    val value2 = map["key2"]   // 返回null    val value3: Int = map.getValue("key")  // 存在正常，不存在异常    val mapWithDefault = map.withDefault { k -> k.length } // 设置默认值    val value4 = mapWithDefault.getValue("key2")    try {       map.getValue("anotherKey")                              // 4    } catch (e: NoSuchElementException) {        println("Message: $e")    }    // zip 压缩    val A = listOf("a", "b", "c")                  // 1    val B = listOf(1, 2)                     // 1    // zip 不能变长    val resultPairs = A zip B                         // [(a, 1), (b, 2)],             val resultReduce = A.zip(B) { a, b -> "$a$b" }    //[a1, b2],     val resultlist = A.zip(B) { a, b -> listOf(a,b) } // [[a, 1], [b, 2]]    // getOrElse 获取    val list = listOf(0, 10, 20)    println(list.getOrElse(1) { 42 })        println(list.getOrElse(10) { 42 })    val map3 = mutableMapOf<String, Int?>()    println(map3.getOrElse("x") { 1 })           map3["x"] = 3    println(map3.getOrElse("x") { 1 })           map3["x"] = null    println(map3.getOrElse("x") { 1 })

Reference 参考

  • kotlin Example