Kotlin basic grammar

One .Kotlin Basic data type

1.Kotlin The basic numerical types of are Byte、Short、Int、Long、Float、Double etc. . differ Java Yes. , Character is not of numerical type , Is a separate data type ,eg: The following table

type	Bit width	Bytes occupied
Double	64	8
Float	32	4
Long	64	8
Int	32	4
Short	16	2
Byte	8	1

Data type conversion method ,eg：

toByte(): The switch to Byte type
toShort(): The switch to Short type
toInt(): The switch to Int type
toLong(): The switch to Long type
toFloat(): The switch to Float type
toDouble(): The switch to Double type
toChar(): The switch to Char type

2. Defining some constants and variables

1). Variable definition ：var keyword

var < identifier > : < type > = < initialize value >

2). Definition of immutable variable ：val keyword , Variables that can only be assigned once ( similar Java in final Decorated variable )

val < identifier > : < type > = < initialize value >

    var m: Int=30
    var flag:Boolean=true
    val value1 : Int=200
    val value2: Double=30.5

remarks ： java and kotlin The difference between variables

1'. Different location ,kotlin Put the data type in after the variable , And use a colon (:) Separate , And java contrary

2'. The key is different ,kotlin To define variables in var start , Define constants with val start

3'. The first letter of the number type has different case ,kotlin All data types in begin with capital letters ,java There are only classes in 、 The names of complex data types such as interfaces are capitalized

4'.kotlin Both constants and variables in can have no initialization value , But you must initialize... Before referencing , The compiler supports automatic type determination , That is, you can declare without specifying the type , Judged by the compiler

Two . Package declaration , and java equally , The beginning of the code file is usually the declaration of the package , And Kotlin Document to  .kt  For the suffix ,eg:

package com.example.kotlin

class PersonTest(val name:String) {
    fun greet(){
        println("Hi,$name")
    }

    var m: Int=30
    var flag:Boolean=true
    val value1 : Int=200
    val value2: Double=30.5
}

fun main(args:Array<String>){
    PersonTest("kotlin!").greet()
}

If there is no specified package , The default is default package , Multiple packages will be imported into each by default Kotlin In file ：

kotlin.*
kotlin.annotation.*
kotlin.collections.*
kotlin.comparisons.*
kotlin.io.*
kotlin.ranges.*
kotlin.sequences.*
kotlin.text.*

3、 ... and . Function definition , The function definition uses the keyword fun, The parameter format is ： Parameters : type , The variable length parameter of the function can be  vararg  Keywords to identify ,eg:

fun sum(a:Int,b:Int):Int{
    return a+b
}

fun getName():String{
    return "jon"
}

fun vars(vararg values:Int){
    for (value in values){
        println(value)
    }
}

Four . Annotation mode , Support single line annotation and block annotation

 Single-line comments 
//  This is a single line comment 

 Block annotation 
/* Hi kotlin */

5、 ... and . String template , The template uses the dollar sign （$） start ,eg:"i=$i“ Medium $i It's a placeholder , among $ hinder i It's a variable. , With i change ,eg:

 val i=100
    val j="i=$i" // amount to "i=100"
    println(j)

    val k="1234"
    var str="$k length is ${k.length}" // Use string to get k The length of 
    println(str)

6、 ... and . Array

  // Using functions arrayOf()
    val arr1= arrayOf(1,2,3,'a',4)
    println(arr1[3])
    arr1[2]='b'
    println(arr1[2])

    var arr2=Array(5,{i->(i*i).toString()})
    println(arr2[3])

    // Use IntArray Array , No packing operation , So it's more efficient , Its usage is the same as Array equally , You can also define ByteArray, ShortArray and IntArray The definition is similar to 
    var arr3:IntArray= intArrayOf(2,4,6,8,10)
    println("arr3[2]="+arr3[2])