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 = mutableListOf(1, 2, 3)
val list2: List = 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()
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()
println(map3.getOrElse("x") { 1 })
map3["x"] = 3
println(map3.getOrElse("x") { 1 })
map3["x"] = null
println(map3.getOrElse("x") { 1 })