//常量数组int[][][] arrs = new int[3][2][1];
val arrs = Array(3) { Array(2) { IntArray(1) } }
//空安全变量
var str: String = "hello"
//可为空字符串变量
var str1: String? = null

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")
}

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)
  }
})

//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>() {
  //...
}

val intent = Intent(this, MainActivity::class.java)

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

public class User {
  private String name;
  private String age;

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }

  public String getAge() {
    return age;
  }

  public void setAge(String age) {
    this.age = age;
  }
}

val user = User()
//赋值
user.name = "tutu"
user.age = "23"
//取值
val name = user.name
val age = user.age

class User {
  var name: String? = null
  var age: String? = null
}

class User {
  companion object {
    @Volatile var instance: User? = null
      get() {
        if (field == null) {
          synchronized(User::class.java) {
            if (field == null)
              field = User()
          }
        }
        return field
      }
  }

  var name: String? = null
  var age: String? = null
}

val user = User()
//赋值
user.name = "tutu"
user.age = "23"
//取值
val name = user.name
val age = user.age
var userInfo = "name:${user.name}, age:$age"
//输出结果：name:tutu, age:23

//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()