Collections

Kotlin lets you manipulate collections independently of the exact type of objects stored in them. In other words, you add a String to a list of Strings the same way as you would do with Ints or a user-defined class. So, the Kotlin Standard Library offers generic interfaces, classes, and functions for creating, populating, and managing collections of any type.

val numbers = mutableListOf("one", "two", "three", "four")
numbers.add("five")   // this is OK    
//numbers = mutableListOf("six", "seven")   

只读集合是型变的,可变集合不是型变的
在 Kotlin 中，List 的默认实现是 ArrayList，可以将其视为可调整大小的数组

• 注意，to 符号创建了一个短时存活的 Pair 对象，因此建议仅在性能不重要时才使用它。 为避免过多的内存使用，请使用其他方法

可以通过其他集合各种操作的结果来创建集合

1. 过滤filter() filterIndexed() filterNot() filterIsInstance() filterNotNull() partition() 检验谓词 any() none() all()

val numbers = listOf("one", "two", "three", "four")
println(numbers.any { it.endsWith("e") })
println(numbers.none { it.endsWith("a") })
println(numbers.all { it.endsWith("e") })

println(emptyList<Int>().all { it > 5 })   // vacuous truth

2. 映射 map mapIndexed mapNotNull mapIndexedNotNull mapKeys mapValues
3. 关联 associateWith associateBy() 区别:building maps with collection elements as keys/values
4. 分组 groupBy()
   5 slice() take() drop() takeWhile()
   6.取单个元素 elementAt() first() last() elementAtOrNull()

双路合并(zip)

val colors = listOf("red", "brown", "grey")
val animals = listOf("fox", "bear", "wolf")
println(colors zip animals)

val twoAnimals = listOf("fox", "bear")
println(colors.zip(twoAnimals))

集合排序
Most built-in types are comparable: 自定义类型如何排序?
A shorter way to define a Comparator is the compareBy() standard library.
The Kotlin collections package provides functions for sorting collections in natural, custom, and even random orders.
sort read-only collections and sorting mutable collections difference?
sortedBy() sortedWith()
reversed() 返回一个集合(原元素的拷贝),偏好使用asReversed(),如果原集合不变的话.但是,如果如果list的可变行不知道或者原集合就不是list,推荐使用reversed().
随机顺序 shuffled()

集合写操作
add() addAll() += remvove() removeAll() retainAll() clear() -=(single instance remove the first occurrence of it)

val numbers = mutableListOf("one", "two", "three", "three", "four")
numbers -= "three"
println(numbers)
numbers -= listOf("four", "five")    
//numbers -= listOf("four")    // does the same as above
println(numbers)    

list的操作
getOrElse() getOrNull subList() indexOf() indexOfFirst() binarySearch()
add() addAll() set() fill() removeAt()
set
map plus minus操作