Kotlin 中list set map

给定一个字符串 ，转成想要的List 关键字：filterTo 和 -=

fun main(args: Array<String>) {
    val words = "A long time ago in a galaxy far far away".split(" ")
    val shortWords = mutableListOf<String>()
    words.getShortWordsTo(shortWords, 3)
    println(shortWords)
}
fun List<String>.getShortWordsTo(shortWords: MutableList<String>, maxLength: Int) {
    this.filterTo(shortWords) { it.length <= maxLength }
    val articles = setOf("a", "A", "an", "An", "the", "The")
    shortWords -= articles
}

输出结果：[ago, in, far, far]
衍生：
***+=***  输出结果：[A, ago, in, a, far, far, a, A, an, An, the, The]
***==***  输出结果：[A, ago, in, a, far, far]

List 元素怎么才算相等

    val bob = Person("Bob", 31)
    val people = listOf(Person("Adam", 20), bob, bob)
    val people2 = listOf(Person("Adam", 20), Person("Bob", 31), bob)
    println(people == people2)
    bob.age = 32
    println(people == people2)

输出结果：
true
false

MutableList 是可以进行写操作的 List ，关键词 shuffle() 大打乱次序

 val numbers = mutableListOf(1, 2, 3, 4)
    numbers.add(5)
    numbers.removeAt(1)
    numbers[0] = 0
    numbers.shuffle()
    println(numbers)

输出结果：
[4, 0, 3, 5]

set

 val numbers = setOf(1, 2, 3, 4)
 println("Number of elements: ${numbers.size}")
    if (numbers.contains(1)) println("1 is in the set")

    val numbersBackwards = setOf(4, 3, 2, 1)
    println("The sets are equal: ${numbers == numbersBackwards}")

 输出结果：
 Number of elements: 4
 1 is in the set
 The sets are equal: true

Set的默认实现 - LinkedHashSet—— 保留元素插入的顺序

    val numbers = setOf(1, 2, 3, 4)  // LinkedHashSet is the default implementation
    val numbersBackwards = setOf(4, 3, 2, 1)

    println(numbers.first() == numbersBackwards.first())
    println(numbers.first() == numbersBackwards.last())

输出结果：
false
true

Map 的默认实现 – LinkedHashMap—— 迭代 Map 时保留元素插入的顺序。 反之，另一种实现 – HashMap—— 不声明元素的顺序。

  val numbersMap = mapOf("key1" to 1, "key2" to 2, "key3" to 3, "key4" to 1)

    println("All keys: ${numbersMap.keys}")
    println("All values: ${numbersMap.values}")
    if ("key2" in numbersMap) println("Value by key \"key2\": ${numbersMap["key2"]}")
    if (1 in numbersMap.values) println("The value 1 is in the map")
    if (numbersMap.containsValue(1)) println("The value 1 is in the map") // 同上

输出结果：
All keys: [key1, key2, key3, key4]
All values: [1, 2, 3, 1]
Value by key "key2": 2
The value 1 is in the map
The value 1 is in the map

MutableMap 是一个具有写操作的 Map 接口，可以使用该接口添加一个新的键值对或更新给定键的值。

val numbersMap = mutableMapOf("one" to 1, "two" to 2)
numbersMap.put("three", 3)
numbersMap["one"] = 11

println(numbersMap)

输出结果：
{one=11, two=2, three=3}