Kotlin introductory notes (VIII) collection and traversal

Preface : This tutorial is best done with JAVA Study on the basis of

One 、List aggregate

ListOf()

Press Java Of List How to write it ,Kotlin should ：

val list = ArrayList<String>()
list.add("Apple")
list.add("Banana")
list.add("Orange")
list.add("Pear")
list.add("Grape")

however Kotlin Specially built-in listOf() Function to simplify the writing of the initialization set ：

val list = listOf("Apple","Banana","Orange","Pear","Grape")

It can be used for-in loop Traversal .

fun main() {
    val list = listOf("Apple","Banana","Orange","Pear","Grape")
    for (fruit in list) {
        println(fruit)
    }
}

But here's the thing ,listOf() Function creates an immutable collection .（ Unable to add collection 、 Modify or delete operations ）

mutableListOf()

This design and val keyword 、 Class is not inheritable   The original intention of the design is similar ,Kotlin In terms of immutability, the control is extremely strict . If you create a variable set , have access to mutableListOf() Function is OK , Examples are as follows ：

fun main() {
    val list = mutableListOf("Apple","Banana","Orange","Pear","Grape")
    list.add("Watermenol")
    for (fruit in list) {
        println(fruit)
    }
}

  So let's use this mutableListOf() Function to create a mutable collection , Then a new fruit is added to the collection , Final use for-in loop The collection is traversed .

(2) Set aggregate

setOf()

val set = setOf("Apple","Banana","Orange","Pear","Grape")
for (fruit in set) {
    println(fruit)
}

mutableSetOf()

        Set Of  setOf() and mutableSetOf()  And  List A collection of  listOf() and mutableListOf() The general usage is the same , But tell me more .

It should be noted that , Set The bottom layer is to use hash Mapping mechanism to store data , Therefore, the elements in the collection cannot be guaranteed to be orderly , This is the List The biggest difference .

Map aggregate

Map It's a kind of “ Key value pair ” Data structure in form , Therefore, the usage is similar to List、Set Sets differ greatly .

according to Java Writing ：

val map = HashMap<String , Int>()
map.put("Apple",1)
map.put("Banana",2)
map.put("Orange",3)
map.put("Pear",4)
map.put("Grape",5)

But in Kotlin It is more recommended to use a syntax structure similar to array subscript ,

For example Map You can add a piece of data in ：

map["Apple"] = 1

Read  Map The structure in can be written like this ：

val num = map["Apple"]

therefore , The above code can be written as ：

val map = HashMap<String , Int>()
map["Apple"] = 1
map["Banana"] = 2
map["Orange"] = 3
map["Pear"] = 4
map["Grape"] = 5

however , according to Kotlin The usual style . . . . you 're right , It provides a pair of mapOf() And mutableMapOf() Function to continue simplification Map Usage of . stay mapOf() in , We can directly pass in the initialized key value pair combination to complete the pair Map Set creation ：

val map = mapOf ("Apple" to 1 , "Banana" to 2 , "Orange" to 3 , "Pear" to 4 , "Grape" to 5)

The key value pair combination here looks like to This keyword is used for association , But in fact to It's not a keyword , It is a infix function （ Put this temporarily , Will be discussed later ）

fun main() {
    val map = mapOf("Apple" to 1 , "Banana" to 2 , "Orange" to 3 , "Pear" to 4 , "Grape" to 5)
    for((fruit , number) in map) {
        println("fruit is " + fruit + ", number is " + number)
    }
}

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Reference resources :

        《 First line of code Android ( The third edition )》   --- Guo Lin