Kotlin之从入门到放弃

Kotlin之从入门到放弃
var：变量
val：常量

声明变量\常量：

    var code: Int = 0
    var isSuccess: Boolean = false
    var message: String? = null
    //空安全变量
    var str: String = "hello"
    //常量数组int[][][] arrs = new int[3][2][1];
    val arrs = Array(3) { Array(2) { IntArray(1) } }

条件：

1. if else 同java一样
2. java的switch ==> kotlin的when ，when子式可以是各种返回Boolean的表达式

val x = 7
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")
}

  override fun onTouchEvent(event: MotionEvent): Boolean {
        when (event.action) {
            MotionEvent.ACTION_DOWN -> {
                val currentTime = System.currentTimeMillis()
                if (currentTime - lastTime < 500) {
                    lastTime = currentTime
                    return true
                } else {
                    lastTime = currentTime
                }
            }
        }
        return super.onTouchEvent(event)
    }

3.循环
while 和 do...while 同Java一样；
for有变化

val list = ArrayList<String>()
//递增for (int i = 0; i < list.size(); i++)
for (i in list.indices) {
   print(list[i])
}
//递增for (int i = 2; i < list.size(); i++)
for (i in 2..list.size-1) {
   print(list[i])
}
//递减for (int i = list.size(); i >= 0; i--)
for (i in list.size downTo 0) {
    print(list[i])
}
//操作列表内的对象
for (item in list) {
    print(item)
}
//加强版
for（（i， item） in list.witnIndex()） {
    print(list[i])
    print(item)
}
//变种版
list.forEach {
    print(it)
}

list.forEach {
    print(it)
}

list.forEachIndexed { i, s ->
    print(list[i])
    print(s)
}

list.forEachIndexed(object :(Int,String) -> Unit{
    override fun invoke(i: Int, s: String) {
        print(list[i])
        print(s)
    }
})

冒号用法

//val表示常量var表示变量声明
val name: String = "tutu" 
//省略类型说明
var age = "23"
//fun表示函数
fun getName(): String{
   return "tutu"
}
//类继承
class UserList<E>(): ArrayList<E>() {
    //...
}

//java类写成  类名::class.java
 val intentGo = Intent(this@MainActivity, RegisterActivity::class.java)

@用法

它告诉你 this 和 return 指的是哪一个

 Toast.makeText(this@CaptchaActivity, resources.getText(R.string.please_input_correct_captcha), Toast.LENGTH_LONG).show()

valintentGo=Intent(this@MainActivity,RegisterActivity::class.java)
startActivity(intentGo)

class User {
    inner class State{
        fun getUser(): User{
            //返回User
            return this@User
        }
        fun getState(): State{
            //返回State
            return this@State
        }
    }
}

bean类的get set方法省略

class BannerBean {
    var code: Int = 0
    var isSuccess: Boolean = false
    var message: String? = null
    var data: List<DataBean>? = null

    class DataBean {
        var id: Int = 0
        var bannerUrl: String? = null
        var forwardUrl: String? = null
        var bannerSource: String? = null
    }
}

//赋值
val bannerBean =BannerBean ()
bannerBean .message="我是message"
bannerBean .code=1

//取值
val message=bannerBean .message
val code=bannerBean .code=1

$赋值

Log.i("wzx",message:${bannerBean .message})
//输出
message:我是message

空判断

//类型后面加?表示可为空
var age: String? = "23" 
//抛出空指针异常
val ageInt = age!!.toInt()
//不做处理跳过
val ages = age?.toInt()

权限

Java中内部类和参数默认为为private，Kotlin内部类和参数默认为public；
类默认为不可继承(final)，想要可被继承要声明为 open 或 abstract；
取消了static关键字，静态方法和参数统一写在 companion object 块；
internal模块内可见，inner内部类。