: modifiers ("class" | "interface") SimpleName
 typeParameters?
 primaryConstructor?
 (":" annotations delegationSpecifier{","})?
 typeConstraints
 (classBody? | enumClassBody)

class Empty

classModifier: 类属性修饰符，标示类本身特性。
 abstract //抽象类标示 
 final //标示类不可继承，默认属性
 enum //标示类为枚举
 open //类可继承，类默认是final的
 annotation //注解类

 private //仅在同一个文件中可见
 protected //同一个文件中或子类可见
 public //所有调用的地方都可见
 internal //同一个模块中可见

class User constructor(name:String)

class User(name:String)

class User

class User(name:String){
  init{
    //初始化..  
  }
}

class User(name:String){
  var customName = name.toUpperCase() //初始化属性
}

class User(name:String){

  /**secondary constructor**/
  constructor(name:String,age:Int):this(name){
    //初始化...
  }
}

class User(name:String = “”)

var user = User()
var anotherUser = User("jason")

class User(name:String = "",age = Int)
var user = User() //将无法编译通过 
var anotherUser = User("jason",18) //合法声明

var user = User()

equals()
hashCode()
toString()

class Student(name:String,age:Int):User(name,age){

}

/**用户基类**/
open class User(name:String){

  /**secondary constructor**/
  constructor(name:String,age:Int):this(name){
    //初始化
  }
}

/**子类继承User类**/
class Student:User{

  /**构造函数**/
  constructor(name:String):super(name){

  }

  /**另外一个构造函数**/
  constructor(name:String,age:Int):super(name,age){

  }
}

open class User{

  open fun study(){}

  fun run(){}
}

class Student:User{
  override fun study(){}
}

final override fun study(){}

open class User{
  open fun study(){}
}

interface Reading{
  fun study(){}
}

class Student:User(),Reading{
  override fun study(){
    super<User>.study() 
    super<Reading>.study() //与上者至少二选其一
  }
}

interface Reading{
  fun study()
}

class Student:User(),Reading{
  override fun study(){
    //do nothing
  }
}

abstract class User{
  abstract fun study()
}

class Person{
  abstract fun study() // 编译错误
}

interface Reading{
  abstract fun reading()
}