Day 1 of kotlin learning: simple built-in types of kotlin

Learn video address ：https://coding.imooc.com/class/398.html

1. Realization Java call Kotlin file

1. modify build.gradle Of Project file

Add code ：

classpath 'org.jetbrains.kotlin:kotlin-gradle-plugin:1.3.50'

2. modify build.gradle Of Module file

id 'kotlin-android'

implementation 'org.jetbrains.kotlin:kotlin-stdlib:1.3.50'

So you can do it Java project , Use kotlin 了 .

2. hold java The file is converted to kotlin file

1. Click on code Below connect Java File to kotlin File

2. Conversion success

3.kotlin Basic type

1. Basic types

2. Declare variables

val And var The difference between ：val Read only cannot be modified ,var Readable and modifiable .val And Java Medium final It's kind of like

3.kotlin And Java The difference between ：

1. Don't end with a comma ;
2. Type after , Variable name before ;
3. : The type can be saved .
4. modification Long Type , You can only add Capitalization L, Cannot use lowercase l.

val c=2000L    √
val c=2000l    ×

5.Kotlin Numeric type conversion for
No direct conversion , Need to add to type ().
example :

// take Int Type into Long type 
val e:Int=10
val f:Long=e.toLong()

6.kotlin Unsigned type , And c In language unsigned The same is true .

example ：

fun main()
{
    
    val g:UInt= 2u  // Unsigned type 
    println("$g")
}

7.kotlin Output

val name=" Li Hua "
println("name is :$name")
println("length is :${name.length}")  // When there is a way , Need to add {}
System.out.printf("length is :%d",name.length)

8.kotlin Comparison of address and value in
use === The comparison values are equal , And Java Medium == The effect is the same
use == Compare address equality , And Java Medium equals The effect is the same

9.kotlin Output multiple lines of values , Not like Java Same use \n Line break , And with + To splice .kotlin It is more convenient .

var test=""" 123 123 """.trimIndent()
    println(" Output :\n$test")   //trimIndent Method ： Used to cut the same number of spaces at the beginning of each line 

4. Array

1. Array creation

val c0= intArrayOf(1,2,3,4,5)
val c1=IntArray(5){
    2*(it+1)}
println(c0.contentToString())  //contentToString Method to print the array 
println(c1.contentToString())

2. Length of array

val a=IntArray(5);
println(a.size)

3. Array read and write

val d= arrayOf("Hello","World")
d[1]="Kotlin"
println("${
      d[0]},${
      d[1]}")

4. Traversal of array

val e= floatArrayOf(1f,3f,5f,7f)
// Method 1 
for(value in e)
{
    
    println(value)
}

// Method 2 
e.forEach {
     element->
    println(element)
}


// Method 3 
val array= intArrayOf(1,3,5,7)
for (i in array.indices)  //indices Get the interval [0,4)
{
    
    println(array[i])
}

5. Section

1. The creation of intervals

val intRange=1..10  // Closed interval [1,10]
    val charRange='a' until 'd' // Front closing back opening section [a,d)
    val longRange=10L downTo 1L // Inverted interval 
    val intRangeStep=1..10 step 2 // step step Set to 2

    val floatRange=1f..10f   // The above is discrete value , Here is the continuous value 
    val doubleRange=1.0..10.0

    println(intRange.joinToString())
    println(charRange.joinToString())
    println(longRange.joinToString())
    println(intRangeStep.joinToString())
    println(floatRange.toString())
    println(doubleRange.toString())

2. Simple traversal

Same as array method

6. Collections framework

1. Introduction to collection framework

Collection framework reuse Java API All implementations of

Immutable List Elements cannot be added or deleted .
example ：

val intList: List<Int> = listOf(1,2,3)
val intList2: MutableList<Int> = mutableListOf(1,2,3)
   
val map:Map<String,Any> = mutableMapOf("name" to "luo","age" to 20)//Any Equivalent to Java Of Object
val stringList=ArrayList<String>()

2. Modification of collection framework

// Add data 
for(i in 0..10){
    
     stringList+="num:$i"  // Equivalent to stringList.add("num:$i")
 }
 println(stringList.toString())

 // Delete data 
 for (i in 0..3)
 {
    
     stringList-="num:$i"  // Equivalent to stringList.remove("num:$i")
 }
 println(stringList.toString())

3. The reading and writing of set frame

stringList[5]="HelloWorld" // stay stringList Add data 
val valueAt5=stringList[5] // stay stringList Reading data 
println(valueAt5)

val hashmap=HashMap<String,Int>()
hashmap["Hello"]=10
println(hashmap["Hello"])

4.Pair( Two elements )

// Two ways to create 
val pair ="Hello" to "Kotlin"
val pair2=Pair("Hello","Kotlin")

// Get the corresponding element 
val first=pair.first
val second=pair.second
val (x,y)=pair

5.Triple( Three elements ）

// establish Triple
 val triple=Triple("x",2,3.0)
 
 // Get the corresponding element 
 val first1=triple.first
 val second1=triple.second
 val third1=triple.third
 val (x1,y1,z1)=triple

7. function

1. Function definition

Unit amount to Java Of void, It can be omitted

2. Function reference

(Foo,String,Long)->Any Equivalent to Foo.(String,Long)->Any Equivalent to (Foo,String,Long,Any)

3. Variable parameters

multiParameters(1,2,3,4)  // Set parameters 

//vararg Set variable parameters , The number of parameters is variable 
fun multiParameters(vararg ints:Int)
{
    
    println(ints.contentToString())
}

4. Multiple return values

val (x2,y2,y3)= multiReturnValues() // deconstruction 

fun multiReturnValues():Triple<Int,Long,Double>
{
    
    return Triple(1,3L,4.0)
}

5. The default function

defaultParameter(2)  // among y,z All have default values 

fun defaultParameter(x:Int,y:String=" Xiao Ming ",z:Double=20.0)
{
    
    TODO()
}

6. Named functions

The difference from the default function ：defaultParameter(y=" Li Hua "), You can set the place to assign , Because the default function generally defaults to the first function