Kotlin: collection use

Set is mainly List、Set、Map, They are Java There are interfaces in ,
List --> ArrayList、LinkedList
Set --> HashSet
Map–> HashMap

1、List Set initialization
stay Kotlin Generally, this is the way to initialize a set in ：

val list = ArrayList<String>()
list.add("1")
list.add("2")

This is very cumbersome , So here we will introduce list Several methods of ：listof()、mutableListOf()

val list = listOf("aa", "bb") 
val list = mutableListOf("aa", "bb") 

listof()： What is created is an immutable set , This collection can only be used to read data , Set data cannot be added, deleted or modified .
mutableListOf()： What is created is a variable set , You can add, delete, modify and query .
Traverse List The collection needs to use for-in Loop through , such as ：

fun main(){
    val list1 = listOf("aa", "bb") 
    for (content in list1){
           content.toUpperCase()// Capitalize the traversal content 
    print（content）
    }
    val list2 = mutableListOf("aa", "bb") 
    list2.add("cc")
    for (content in list2){
           content.toUpperCase()// Capitalize the traversal content 
    print（content）
    }
}

2、Set aggregate
Set and List The usage of is actually the same , These methods will be used during initialization ：setof()、mutableSetOf()

val set = setOf("aa", "bb") 
val set = mutableSetOf("aa", "bb") 

setof()： What is created is an immutable set , This collection can only be used to read data , Set data cannot be added, deleted or modified .
mutableSetOf()： What is created is a variable set , You can add, delete, modify and query .
ad locum ,Set aggregate List The difference between sets needs to be noted ,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 .
---- Ergodic and List equally , No explanation

3、Map Set initialization
Map Gather here to focus on ,Map Different from the first two ,Map Is a data structure in the form of key value pairs , Therefore, in usage and List、Set There's a big difference .
General initialization Map You need to create a HashMap

val map = HashMap<String, String>()
map.put("aa", "1")
map.put("bb", "2")

Because in Kotlin in , Not recommended put()、get() Method pair Map Add and read data , Instead, use this subscript like method to add data ：

val map = HashMap<String, String>()
map["aa"] = 1
map["bb"] = 2

When the data is read

val text = map["aa"]

However , The above method of adding data is still very cumbersome , and List、Set equally ,Map It has its own way mapof()、mutableMapOf()

val map1 = mapOf("aa" to 1, "bb" to 2)  // immutable 
val map2 = mutableMapOf("aa" to 1, "bb" to 2)   // variable 

mapof()： What is created is an immutable set , This collection can only be used to read data , Set data cannot be added, deleted or modified .
mutableMapOf()： What is created is a variable set , You can add, delete, modify and query .
Traverse Map aggregate , Also used for-in loop , The only difference is that ,for-in In circulation ,map The key value pair and the variable are declared in a pair of brackets

fun main(){
    val map1 = mapOf("aa" to 1, "bb" to 2) 
    for ((content1, content2) in map1){
        print（content1 + "--" + content2）
    }
}

4、 Set function API（lambda）
First define a set , Then find the element with the longest length inside

val list1 = listOf("aa", "bbbb", "cc") 
var maxLength = ""
for (content in list1){
    if (content.length > maxLength.length){
        maxLength = content
    }
}
print(maxLength)

The traditional way is usually written like this , But for the function of set API, You can make this function more brief

val list1 = listOf("aa", "bbbb", "cc") 
var maxLength = list1.maxBy { it.length }
print(maxLength)

This is the function API Usage of , Just one line of code can handle .
Next, let's take a look at the derivation process ：
First of all, let's get to know lambda Expression structure ：

{ Parameter name 1： Parameter type , Parameter name 2： Parameter type -> The body of the function }
1
The outermost brace , If a parameter value is passed into the expression , You need to declare parameters , End of parameter list -> , Identify the end of the parameter list and the beginning of the function body , Arbitrary code can be written in the function body .
Now go back to the front , First maxBy The working principle of is to traverse according to the incoming conditions , So as to find the maximum value , therefore , To paraphrase Lambda The function expression can be written as ：

val list1 = listOf("aa", "bbbb", "cc") 
var lambda = { content ：String -> content.length }
var maxLength = list1.maxBy ( lambda )
print(maxLength)

maxBy The function essentially receives a lambda Parameters ,
secondly , There is no need to specifically define a lambda Variable , You can directly lambda Expression passed in maxBy Function ,

val list1 = listOf("aa", "bbbb", "cc") 
var maxLength = list1.maxBy( { content ：String -> content.length })
print(maxLength)

stay Kotlin In the regulations , When Lambda When the parameter is the last parameter of a function , Can be Lambda Move the expression outside the parentheses ,

val list1 = listOf("aa", "bbbb", "cc") 
var maxLength = list1.maxBy（） { content ：String -> content.length }
print(maxLength)

If Lambda Parameter is the only parameter of the function , Parentheses can be omitted ,

val list1 = listOf("aa", "bbbb", "cc") 
var maxLength = list1.maxBy { content ：String -> content.length }
print(maxLength)

because Kotlin Type derivation mechanism in ,Lambda Expressions do not have to declare parameter types in most cases ,

val list1 = listOf("aa", "bbbb", "cc") 
var maxLength = list1.maxBy { content  -> content.length }
print(maxLength)

Last , When Lambda When there is only one parameter in the parameter list of the expression , There is no need to declare the parameter name , You can use keywords directly it Instead of ,

val list1 = listOf("aa", "bbbb", "cc") 
var maxLength = list1.maxBy { it.length }
print(maxLength)

5、 Commonly used set function API
（1）、map() function
It is used to map each element to another value , The mapping rule is Lambda The expression specifies , Finally, a new set is generated ,

val list1 = listOf("aa", "bb") 
var newList = list1.map{ it.toUpperCase()}
for (content in list1){
    content.toUpperCase() // Convert to uppercase mode 
}

（2）、filter（） function
Used to filter the data in the collection , Can be used alone , Can also be combined with map（） Use it together

val list1 = listOf("aa", "bbbb", "cc") 
var list2 = list1.filter { it.length <= 2 }
                         .map { it.toUpperCase() }

（3）、any() function
Judge whether at least one element in the set meets the specified conditions

val list1 = listOf("aa", "bbbb", "cc") 
var anyResult = list1.any { it.length <= 2 }

（4）、all() function
Judge whether all elements in the set meet the specified conditions

val list1 = listOf("aa", "bbbb", "cc") 
var allResult = list1.all { it.length <= 2 }