1.Kotlin的可空性

fun main() {
    var str:String? = "butterfly" //?问好代表可空类型
    str= null
}

 

安全的管理

1.1 安全操作调用符

fun main() {
    var str:String? = "butterfly" //?问好代表可空类型
    str= null
    println(str?.capitalize())//当String为null时候输出null
}

fun main() {
    var str:String? = "butterfly" //?问好代表可空类型
//    str= ""

   str = str?.let {

        //非空白的字符串
        if (it.isNotBlank()){
        it.capitalize()
        }else{
           "butterfly"
        }
    }

    println(str)


}

 1.2 非空断言操作符

fun main() {

    var str:String ?= "1"
    str = null
    println(str!!.capitalize())
}

1.3 使用if判断

fun main() {

    var str:String? = readLine()
    if (str!=null){
        str=str.capitalize()
    }else{
        println("为null.")
    }
}

1.4 空合并操作符

fun main() {

    var str: String? = "12"
    str = null
    if (str != null) {
        str = str.capitalize()
    } else {
        println("为null.")
    }

    str = "rose "
    str = str?.capitalize()?.plus("is great.")
    println(str ?: "jack") //如果为null就是jack，否则 就按正常输出
    


}

   str = null
    str=str?.let { it.capitalize() }?: "butterfly"
    println(str ?: "jack") //如果为null就是jack，否则 就按正常输出

 2.异常

import java.lang.NullPointerException

fun main() {
    var number:Int ?= null

    try {
        checkOperation(number)
        number!!.plus(1)
    }catch (e:NullPointerException){
        e.printStackTrace()
        println("出现异常")
    }

}

fun checkOperation(number:Int?){
    number?:throw UnskilledException()
}

//自定义异常
class UnskilledException() : IllegalArgumentException("操作不当")

3.先决条件函数

就是判断条件是否满足，例如非空 

import java.lang.NullPointerException

fun main() {
    var number:Int ?= null

    try {
        checkOperation(number)
        number!!.plus(1)
    }catch (e:NullPointerException){
        e.printStackTrace()
        println("出现异常")
    }

}

fun checkOperation(number:Int?){
    checkNotNull(number,{"Something is not good"})
//    number?:throw UnskilledException()
}

//自定义异常
class UnskilledException() : IllegalArgumentException("操作不当")

 4.字符串的操作

4.1 substring

const val NAME="Jimmy's friend"

fun main() {
    val index = NAME.indexOf("'")
//    println(NAME.substring(0, index))
    var str = NAME.substring(0..<index)

}

 4.2 split

import javax.xml.transform.Source

const val NAME="Jimmy's friend"
const val NAMES = "jack,jacky,jason"
fun main() {
    val index = NAME.indexOf("'")
//    println(NAME.substring(0, index))
    var str = NAME.substring(0..<index)
    println(str)

    /**
     * 称之为结构语法
     */
    val (origin,dest,proxy)= NAMES.split(",")
    println("$origin $dest $proxy")


}

4.3 replace

fun main() {
    val str = "The people's Republic of China."
    val str1 = str.replace(Regex("[aeiou]"),{it->
        when(it.value){
            "a" -> "8"
            "e" -> "6"
            "i" -> "9"
            "o" -> "1"
            "u" -> "3"
            else -> it.value
        }
    })
    println(str)
    println(str1)
}

5.字符串的比较

fun main() {
    val str1 ="Jason"
    val str2 = "jason".capitalize()
    println(str1==str2)
    println(str1===str2)
}

 是因为第二种方式是存在堆内存的引用而不是 常量池

6. 字符串的遍历

fun main() {
    "The people's Republic of China.".forEach(::println)
}

7.数字类型的安全转换函数

fun main() {
//    val number1:Int = "8.98".toInt()
    val number1:Double? = "8.98".toDoubleOrNull()
    println(number1)
}

8. Double转Int 与 类型格式化 

    println(8.98656.toInt())//取整
    println(8.98656.roundToInt())//四舍五入取整
    val s = "%.2f".format(8.956756)//四舍五入保留两位小数
    println(s)

9.标准库函数

9.1 apply

import java.io.File

fun main() {
    val file1 = File("H:\\test\\i have a dream_copy.txt")
    file1.setReadable(true)
    file1.setWritable(true)
    file1.setExecutable(false)

    val file2 = File("H:\\test\\i have a dream_copy.txt").apply {
        setReadable(true)
        setWritable(true)
        setExecutable(false)
    }



}

 9.2 let

fun main() {
    val result = listOf(3, 2, 1).first().let {
        it * it
    }
    println(result)
}

fun main() {
    val result = listOf(3, 2, 1).first().let {
        it * it
    }
    println(result)

    println(formatGreeting("jack"))

}


fun formatGreeting(guestNmae: String?): String {
    return guestNmae?.let { "Welcome ,$it" } ?: "What's your name?"
}

  链式调用 流行

9.3 run

import java.io.File
import java.nio.charset.Charset

fun main() {
    var file = File("H:\\test\\test.txt")
    val run = file.run {
        //和apply很像
        readText().contains("great") //返回的类型 就是lambda表达式的最后一行
    }
    println(run)

    println("========================")

   "The people's Republic of China.".run(::isLong).run(::showMessage).run(::println)



}

fun isLong(name: String) = name.length >= 10

fun showMessage(isLong:Boolean):String{
    return if (isLong){
        "Name is too long."
    }else{
        "Please rename."
    }
}

9.4 with

fun main() {

    val run = "The people's Republic of China.".run { length >= 10 }
    val with = with("The people's Republic of China.") {
        length >= 10
    }
    println(with)


}

 9.5 also 

新版本 他是把 作用域传给 里面，然后返回这个对象 

import java.io.File

fun main() {
    var fileContents:List<String>
    val apply:File = File("H:\\test\\test.txt").apply {
        println(name)
    }.apply {
        fileContents = readLines()
    }

    println(fileContents)
}

9.6 takeif

import java.io.File

fun main() {
    val result = File("H:\\tes1t\\test.txt")
        .takeIf { it.exists() && it.canRead() }
        ?.readText()
    println(result)

}

 9.7 takeUnless

import java.io.File

fun main() {
    val result = File("H:\\test\\test.txt")
        .takeUnless { it.isHidden }
        ?.readText()
    println(result)

}