Instructions for using common symbols in Kotlin

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• What is Kotlin?

Kotlin [Kotlin] is a static programming language for modern multi-platform applications, developed by JetBrains;
Kotlin is a static language that supports multiple platforms, including mobile, server, and browser;
Kotlin is also a A language that combines object-oriented and functional programming, supports generics, safe null judgments, and Kotlin and Java can achieve complete interaction.

In Google I/O 2017, Google announced Kotlin as the official Android development language.

The role of Kotlin:

① Conciseness: Greatly reduce the amount of boilerplate code.
②Security: Avoid the entire class of errors such as null pointer exceptions.
③Interoperability: Make full use of the existing libraries of JVM, Android and browsers.
④Tool-friendly: It can be built with any Java IDE or using the command line.

Some compound symbols commonly used in Kotlin ( '?' '?:' '!!' '::' '. .' 'as?' )

1. [ : ] ------ colon operator

" : "

Operators are used to define variables, class inheritance, etc.

" : " colon followed by reference type.

• " : " Operators are used to define variables, class inheritance, etc.

//定义变量
var name: String
//继承类
class MainActivity : AppCompatActivity()

2. [ ? ] ------ question mark

" ？"

Modified after the type of the member variable, indicating this variablecan be empty(null), the system will not report its null pointer exception under any circumstances;
" ? " modified behind the object means that if the object is empty (null), the subsequent logic will not be processed;

If the variable we define is a nullable type, we need to use 【String?】.

• " ？" Indicates that this object can be empty

//在变量类型后面加上 ? ，代表该变量是可为空
var age: String? = "23" 

// 如果 str 不能转为 Int 类型，则返回 null
fun parseInt(str: String): Int? {
    
     
 // nothing
}

3. [ !! ] ------ double exclamation mark operator

" !! "

Put it after the object or method passing the actual parameter, which meansCan not be empty(null), if it is null, an exception is thrown;
Kotlin does not recommend the use of non-null assertions, usually we use "?: " to prevent the program from crashing due to a null pointer exception when it is running;

The usage of " !! " is equivalent to if else in Java to judge whether it is null.

• " !! " Null check mechanism specifier

//非空 b,如果 b 为空，则抛出空指针
val l = b!!.length

4. [ ?: ] ------ exclamation point colon operator (Elvis)

" ?: "

Nullable variables can be specifiedempty time, call the return value of the member method or property in the variable;
its syntax format is: "expression A ?: expression B" .
Returns the value of left-hand expression A if left-hand expression A is non-null, otherwise returns the value of right-hand expression B. The expression B on the right is evaluated if and only if the left is empty.

The use of " ?: " is similar to using the ternary operator in Java.

• " ?：" If it is not empty, use it, otherwise return the specified value

//a 不为空，返回 a 的长度，否则返回 -1
val 1ength==a?.length?:-1
//相当于
val length:Int=if(a!=null)a.length else -1

5. [ :: ] ------ double colon operator

" :: "

Indicates that a method is used as a parameter and passed to another method for use. Generally speaking, it refers to a method; it can be used to obtain the Class object of the class;

" :: " The function of the double colon and the method name is to pass the method as a parameter.

• " :: " Can be used to get the Class object of the class

//得到 SecondActivity 对象
startActivity(Intent(this@MainActivity, SecondActivity::class.java))

6. [ as ] ------ type conversion operator

" as "

Indicates that if the type conversion is unsuccessful, the type conversion operator usually throws an exception, so the unsafe type conversion uses the operator as;
when the type conversion fails, it returns null, in order to avoid throwing an exception, you can use the safe type conversion operator as?;

" as " converts the value to the given type, or returns null if the type is inappropriate.

7. [ . . ] ------ double dot interval operator

" . . "

Expressions representing ranges (or ranges) are formed by the rangeTo function of the operator form ... supplemented by in and !in.

The in operator means that i is in a certain interval class; !in means that i is not in
a certain interval class ; . The interval from a to b includes a and does not include b; downTo() function: loop in the flashback interval, and iterates the numbers in the interval from a to b; step() function: jumping function, parity judgment selection, you can specify any step size.

// 输出数字 “ 1 到 10 ”
if (i in 1..10) {
    
     print(i)} 	
// 输出数字 “ 1 到 9 ”同[1,10)
for (i in 1 until 10) {
    
     print(i)} 	
// 输出数字 “54321”
for (i in 5 downTo 1) print(i) 
// 输出数字 “ 135 ”
for (i in 1..5 step 2) {
    
     print(i)} 	
// 输出数字 “ 531 ”
for (i in 5 downTo 1 step 2) {
    
     print(i)} 	

// 如果 i 在 1 到 10 十个数之间，就输出 i
if(i in 1..10){
    
     
    print (i)
}
// 如果 i 不在 1 到 10 十个数之间，就输出 i
if(i !in 1..10){
    
     
    print (i)
}
// 什么都不输出,注意这种递减情况建议使用 until 或 downTo 
for (i in 4..1){
    
    
	 print(i)
 }
// 如果 i 在 1 到 10 九个数之间（不包含 10），就输出 i
if(i in 1 until 10){
    
     
    print (i)
}

8. [ -> ] ------ middle bar greater than sign operator

" -> "

From a morphological point of view, it is aflow and correspondenceThe relationship;
means that after the previous statement is executed, the execution flow will be transferred to the pointed statement, and it is corresponding.

9. [ == and === ] ------ equal sign operator

== Judgmentvalueis equal, compares twovaluesize.
=== Judgmentvalue and referenceare exactly equal, compare twoobjectaddress.

var a:Int = 10
print(a===a)	//结果：true
var b:Int =a
var c:Int =a
print(b===c)	//结果：false
print(b==c)		//结果：true

10. [ is ] ------ belongs to the operator

" is "

This operator can be used to determine whether an instancebelonganother instance

//name 是否是 String 类型
if (name is String){
    
     
    print(true)
}

11. [ $ ] ------ template operator

" $ "

This template operator canoutputA variable value, which is equivalent to concatenating strings.

// 输出结果为 My name is GuYu
var name="GuYu"
print("My name is $name")

Kotlin files are suffixed with .kt; its basic mind map is as follows:

• Recommended reference link:

  https://www.cnblogs.com/shen-hua/category/1141903.html
  https://jetictors.github.io/categories/Kotlin/page/2/

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