./curric/kotlin/oop/open-vs-sealed/

Open vs Sealed

• Open
  All classes in Kotlin have a common superclass Any, that is the default superclass for a class with no supertypes declared:

    class Example // Implicitly inherits from Any
  

  Any has three methods: equals(), hashCode() and toString(). Thus, they are defined for all Kotlin classes. By default, Kotlin classes are final: they can’t be inherited. To make a class inheritable, mark it with the open keyword.

    open class Base //Class is open for inheritance
  

  To declare an explicit supertype, place the type after a colon in the class header:

    open class Base(p: Int)
      
    class Derived(p: Int) : Base(p)
  

  If the derived class has a primary constructor, the base class can (and must) be initialized right there, using the parameters of the primary constructor.

  If the derived class has no primary constructor, then each secondary constructor has to initialize the base type using the super keyword, or to delegate to another constructor which does that. Note that in this case different secondary constructors can call different constructors of the base type:

    class MyView : View {
        constructor(ctx: Context) : super(ctx)
  
        constructor(ctx: Context, attrs: AttributeSet): super(ctx, attrs)
    }
  

• Sealed
  • Kotlin Docs for: Sealed Classes