2.1 programming style

1. Hump nomenclature, without underscores.

For example, public class DataBaseUser

2. The first letter of the class should be capitalized

class People

3. Method and property names should be lowercase

fun hello(){

}

 

2.2 Kotlin constants and variables

Constant: val

Variables: var

2.2.1 constant

val constrain1=3.3

val constrain3:Int=40

fun main(args:Array<String>){
    println(constrain1)
    println(constrain3)

    println("constrain1 The value is"+constrain1)
    println("constrain3 The value is"+constrain3)
}

2.2.2 definition and use of variables

var var1:String="Kotlin"
var var3:Int=10
var var4=5
set(value){
    if(value<=5)
        field=value

}
fun main(args:Array<String>){
    println("var1 Without reassignment"+var1)
    println("var1 The value is"+var1)
    var1=var1+5.3

    println("var1 The value is"+var1)
    var3=var3+5
    println("var3 The value is"+var3)
    var4=var4+5
    println("var4 The value is"+var4)
}

2.2.3 variable scope

class vals(val a:Int=5){
    fun geta():Int = 5

}
fun varfun():Int{
    var b=10
    return b
}
var c=15
fun main(args:Array<String>){
    //println(a)
    println(vals().a)
    //println(b)
    println(varfun())
    println(c)

}

2.3 data type

Basic data type: Boolean, Number,Char

Reference data type: Null, Object, Array

2.3.1 Boolean

var boo: Boolean=true
fun main(args:Array<String>){
    if(boo==true){
        println("boolean To be true")
    }else println("boolean For false")
}

2.3.2 number

var a1:Int=19
var te1:Int=0x88888
var maxint:Int=Int.MAX_VALUE
var minint:Int=Int.MIN_VALUE
var long:Long=128L
var maxlong:Long=Long.MAX_VALUE
var minlong:Long=Long.MIN_VALUE

var double:Double=2.88
var maxdouble:Double=Double.MAX_VALUE
var mindouble:Double= Double.MIN_VALUE

var float:Float=2.0f
var maxfloat:Float=Float.MAX_VALUE
var minfloat:Float=Float.MIN_VALUE

var maxShort:Short=Short.MAX_VALUE
var minShort:Short=Short.MIN_VALUE

var maxbyte:Byte=Byte.MAX_VALUE
var minbyte:Byte=Byte.MIN_VALUE

fun main(args:Array<String>){
    println(a1)
    println(te1)
    println(maxint)
    println(minint)
    println(long)
    println(maxlong)
    println(minlong)
    println(double)
    println(maxdouble)
    println(mindouble)

    println(float)
    println(maxfloat)
    println(minfloat)

    println(maxShort)
    println(minShort)
    
    println(maxbyte)
    println(minbyte)
}

2.3.3 string type (Char)

fun main(args:Array<String>){
    val letterchar:Char='a'
    val nubchar:Char='0'
    val UnicodeChar:Char='\u7231'
    val yi:Char='\n'
    println("Escape character ${yi}Letter, char type ${letterchar},number char type ${nubchar},Unicode Code char Type: ${UnicodeChar}")

}

2.3.4 string type

2.3.5 Tuple type

fun main(args:Array<String>){
    val (status,msg)=Pair(404,"Not Found")
    val (server,script,database)=Triple("Tomcat","PHP","MySQL")

    println("One way:Visit ${server}+${script}+${database}Environmental Science,Get the feedback code ${status},Express ${msg}")

    val http=Pair(404,"Not found")

    val Development=Triple("Tomcat","PHP","MySQL")
    println("Mode two:Visit ${Development.first}+${Development.second}+${Development.third}Environment, feedback to get status code ${http.first},Express ${http.second}")
}

2.3.6 nullable type

fun main(args:Array<String>){
    var address:String?=null
    //Null type
    println(address?.length)
}

2.3.7 object type

class course(var name:String){
    init{
        println("The course name you chose is ${name}")
    }
}
fun main(args:Array<String>){
    val studentobj=course("Kotlin")
}

2.3.8 array type

    val arrayOfInt:IntArray= intArrayOf(1,2,3,4,5)
    val arrayOfChar:CharArray=charArrayOf('H','e','l','l','o')
    val arrayOfString: Array<String> = arrayOf("I","love","Kotlin")

    fun main(args:Array<String>){
        println(arrayOfInt[2])
        for(String in arrayOfString){
            println(String)
        }
    }

2.3.9 check and conversion of array type

fun main(args:Array<String>){
    val a=10
    if(a is Int){
        println(a)
    }
    if(a!is Int){
        println(a)

    }
    val b: String?=a as? String
    println(b)
    val c=a.toDouble()
    println(c)
}