Kotlin:sealed Class detailed explanation of sealed class

  One ： Definition of sealed class

  Use before the class name sealed Keyword declares a class as a sealed class .
  The importance of sealing classes to restrict the set of types only at compile time to ensure type safety .
  A sealed class is implicitly an abstract class that cannot be instantiated .

Two : The characteristics of sealed classes and their differences from other classes

 ①Sealed class（ Sealing class ） Is a class with a specific number of subclasses , It looks a little similar to enumeration , The difference is , In enumeration , We have only one object per type （ example ）; In the sealed class , The same class can have several objects .

②Sealed class（ Sealing class ） All subclasses of must be in the same file as the sealed class

③Sealed class（ Sealing class ） Subclasses of subclasses of can be defined anywhere , It does not need to be defined in the same file as the sealed class

④Sealed class（ Sealing class ） There is no constructor , You can't instantiate it directly , Only inner subclasses can be instantiated

Two : How to get function methods in sealed classes

  Only sealed subclass objects can be created Call the function method in the sealed class through the subclass object of the sealed class

 sealed class SealedClass{
    class SealedClass1():SealedClass()
    class SealedClass2():SealedClass()
    fun hello(){
        println("Hello World ... ")
    }
}

Objects can only be instantiated through subclasses inside sealed classes , Then you can execute the methods inside

fun main(args:Array<String>){
    var sc:SealedClass = SealedClass.SealedClass1()
    sc.hello()
} 

3、 ... and :

 Sealed classes are usually associated with expressions when When used together .  Because subclasses of sealed classes regard their own types as a case .
  therefore , In the sealed class when The expression covers all cases , So as to avoid using else Clause .

sealed class MyColor {
    class Yellow : MyColor()

    class Red : MyColor()

    class Black : MyColor()
}

fun evals(color: MyColor) =
   when (color) {
    is MyColor.Yellow -> println("yellow")
    is MyColor.Red -> println("red")
    is MyColor.Black -> println("black")
}//todo  No need to use else 了 

fun main(args: Array<String>) {
    evals(MyColor.Black())
}