Code Kotlin学习笔记0x02

Kotlin学习笔记0x02

变量类型推断

  • 例子1
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    fun main(args:Array<String>){
        val str = "A"
        println(str)
        println(str is String)
        println(str::class)
        println(str::class.java)
    }
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    A
    true
    class kotlin.String
    class java.lang.String
  • 例子2
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    import java.util.Date

    fun main(args:Array<String>){
        val d = Date()
        println(d)
        println(d is Date)
        println(d::class)
        println(d::class.java)
    }
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    Fri Oct 27 09:34:03 UTC 2017
    true
    class java.util.Date
    class java.util.Date
  • 例子2
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    fun main(args:Array<String>){
        val bool = true;
        println(bool)
        println(bool::class)
        println(bool::class.java)
    }
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    true
    class kotlin.Boolean
    boolean
  • 例子3
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    fun main(args:Array<String>){
        val array = arrayOf(1,2,3)
        println(array)
        println(array is Array)
        println(array::class)
        println(array::class.java)
    }
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    [Ljava.lang.Integer;@5305068a
    true
    class kotlin.Array
    class [Ljava.lang.Integer;

流程控制语句

  • 分支语句 if,when

  • 循环语句 for,while

  • 跳转语句 return,break,continue,throw

if 语句

if作为代码块

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fun main(args: Array<String>) {
 	max(1,9)
}

fun max(a:Int, b: Int):Int{
    val max = if(a>b){
        print("Max is a")
        a//最后一行是该if代码块的返回值
    }else{
        print("Max is b")
        b//最后一行是该if代码块的返回值
    }
    return max
}
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Max is b
  • if表达式必须有一个else分支
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    fun main(args: Array<String>) {
        //val x = if(1==1) true 
        //println(x)
        /*
        'if' must have both main and 'else' branches if used as an expression
        */
        val y = if(1==1) true else false
        println(y)
    }
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    true

when表达式

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fun main(args: Array<String>) {
   val x=1
    val validNumbers = arrayOf(1,2,3)
    when(x){
    	in 1..10 -> print("x is in the range")
        in validNumbers ->print("x is valid")
        !in 10..20 -> print("x is outside the range")
        else -> print("none of the above")
    }
}
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x is in the range

由此可见 when语句只会执行第一个匹配的条目

for循环

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fun main(args: Array<String>) {
    for(arg in args){
        println("No.1:$arg")
    }
   for(i in args.indices){
       println("No.2:${args[i]}")
   }
	println("args.indices:"+args.indices)
}
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No.1:A
No.1:B
No.1:C
No.1:A
No.2:A
No.2:B
No.2:C
No.2:A
args.indices:0..3

return

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fun main(args: Array<String>) {
    returnDemo_1()
    returnDemo_2()
}

fun returnDemo_1(){
    println("Start:"+::returnDemo_1.name)
    val intArray = intArrayOf(1,2,3,4,5)
    intArray.forEach{
        if(it==3)return//遇到a==3的时候直接跳出了函数
        println(it)
    }
    println("End"+::returnDemo_2.name)
}

fun returnDemo_2(){
    println("Start:"+::returnDemo_2.name)
    val intArray = intArrayOf(1,2,3,4,5)
    intArray.forEach(fun(a:Int){
        if(a==3)return//遇到a==3只是跳出了这次循环
        println(a)
    })
    println("End:"+::returnDemo_2.name)
}
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Start:returnDemo_1
1
2
Start:returnDemo_2
1
2
4
5
End:returnDemo_2

参考资料

在线Kotlin IDE

To Be Continue