Kotlin-标准函数

什么是标准函数？
是指在Standard.kt文件中定义的函数，在任何的Kotlin代码都可以自由地调用。

1.let函数
这个在?.后面加let函数就可以不用每次都坐非空判断了

study?.let { 
        it.readBooks()
        it.doHomework()
    }

2.with函数,返回值是with函数的最后一行toString的返回值

fun doWith(){
    val list= listOf("Apple","pear","Banana","Orange")
    val result= with(StringBuilder()){
        append("Start eating fruits.\n")
        for (fruit in list){
            append(fruit).append("\n")
        }
        append("Ate all fruits")
        toString()
    }
    println(result)

}

3.run函数，返回值也是函数最后一行的toString（）的返回值

fun doRun(){
    val list= listOf("Apple","pear","Banana","Orange")
    val result=StringBuilder().run {
        append("Start eating fruits.\n")
        for (fruit in list){
            append(fruit).append("\n")
        }
        append("Ate all fruits")
        toString()
    }
    println(result)
}

4.apply函数，返回值是StringBuilder（）对象

fun doApply(){
    val list= listOf("Apple","pear","Banana","Orange")
    val result = StringBuilder().apply {
        append("Start eating fruits.\n")
        for (fruit in list){
            append(fruit).append("\n")
        }
        append("Ate all fruit")
        toString()
    }
    println(result.javaClass)
}