kotlin密封类

In this tutorial, we’ll be looking into Kotlin Sealed Class. What are they? What’s their use? We’ll be addressing all of these things below.

在本教程中，我们将研究Kotlin Sealed Class。 这些是什么？ 它们有什么用？ 我们将在下面解决所有这些问题。

Kotlin密封级 (Kotlin Sealed Class)

In layman terms, as the name suggests, sealed classes are sealed or closed, hence making them restricted.
Sealed classes are used for representing restricted class hierarchies wherein the object or the value can have value only among one of the types, thus fixing your type hierarchies.
Sealed classes are commonly used in cases, where you know what a given value to be only among a given set of options.

用外行人的名字命名，密封类是密封的或封闭的，因此受到限制。
密封类用于表示受限的类层次结构，其中对象或值只能在一种类型中具有值，从而固定了您的类型层次结构。
当您知道给定值仅在给定的一组选项中具有什么值时，通常使用密封类。

实施Kotlin密封课程 (Implementing Kotlin Sealed Classes)

Sealed classes in Kotlin are implemented in the following manner.

Kotlin中的密封类以以下方式实现。

sealed class A{class B : A()class C : A()
}

To specify a sealed class, you need to add the modifier sealed.
A sealed class cannot be instantiated. Hence, are implicitly abstract.
The following WON’T work.

要指定密封类，您需要添加修饰符sealed 。
密封的类无法实例化。 因此，是隐式的抽象。
以下将不起作用。

fun main(args: Array<String>)
{var a = A() //compiler error. Class A cannot be instantiated.
}

Constructors of a sealed class are private by default.
All subclasses of a sealed class must be declared within the same file.
Sealed classes are important in ensuring type safety by restricting the set of types at compile-time only.

默认情况下，密封类的构造函数是私有的。
密封类的所有子类必须在同一文件中声明。
密封类对于仅通过在编译时限制类型集来确保类型安全非常重要。

sealed class A{class B : A() {class E : A() //this works.}class C : A()init {println("sealed class A")}}class D : A() //this works
{
class F: A() //This won't work. Since sealed class is defined in another scope.
}

Creating a sealed class with constructors.

用构造函数创建一个密封的类。

sealed class A(var name: String){class B : A("B")class C : A("C")
}class D : A("D")

fun main(args: Array<String>) {var b = A.B()var d = D()
}

Adding Data Class and Object in a sealed class.

在密封的类中添加数据类和对象 。

fun main(args: Array<String>) {val e = A.E("Anupam")println(e) //prints E(name=Anupam)var d = A.Dd.name() //prints Object D
}sealed class A{class B : A()class C : A()object D : A(){fun name(){println("Object D")}}data class E(var name: String) : A()}

枚举和密封类之间的区别 (Difference between enum and sealed classes)

In Kotlin, Sealed Classes can be termed as Enum classes on steroids.
Sealed classes allow us to create instances with different types, unlike Enums which restrict us to use the same type for all enum constants.
The following isn’t possible in Enum classes.

在Kotlin中，密封类可以称为类固醇的Enum类。
密封类允许我们创建具有不同类型的实例，这与枚举不同， 枚举将我们限制为所有枚举常量使用相同的类型。
在Enum类中不可能执行以下操作。

enum class Months(string: String){
January("Jan"), February(2),
}

Enum classes allow only a single type for all constants.
Here’s where sealed classes come to our rescue by allowing multiple instances.

枚举类仅允许所有常量使用单一类型。
在这里，密封类通过允许多个实例来拯救我们。

sealed class Months {class January(var shortHand: String) : Months()class February(var number: Int) : Months()class March(var shortHand: String, var number: Int) : Months()
}

How can you use this feature of Sealed classes in your Projects?
In a newsfeed like application, you can create three different class types for Status, Image, Video posts as shown below.

如何在项目中使用Sealed类的此功能？
在类似新闻源的应用程序中，您可以为状态，图像和视频帖子创建三种不同的类类型，如下所示。

sealed class Post
{class Status(var text: String) : Post()class Image(var url: String, var caption: String) : Post()class Video(var url: String, var timeDuration: Int, var encoding: String): Post()
}

This isn’t possible with Enum classes.

使用Enum类是不可能的。

密封班和何时 (Sealed classes and when)

Sealed classes are commonly used with when statements since each of the subclasses and their types act as a case. Moreover, we know that the Sealed class limits the types. Hence, the else part of the when statement can be easily removed.
Following example demonstrates the same.

密封类通常与when语句一起使用，因为每个子类及其类型都作为个案。 此外，我们知道Sealed类会限制类型。 因此， when语句的else部分可以轻松删除。
下面的示例演示了相同的内容。

sealed class Shape{class Circle(var radius: Float): Shape()class Square(var length: Int): Shape()class Rectangle(var length: Int, var breadth: Int): Shape()
}fun eval(e: Shape) =when (e) {is Shape.Circle -> println("Circle area is ${3.14*e.radius*e.radius}")is Shape.Square -> println("Square area is ${e.length*e.length}")is Shape.Rectangle -> println("Rectagle area is ${e.length*e.breadth}")}

Let’s execute the eval function in our main function as shown below.

让我们在main函数中执行eval函数，如下所示。

fun main(args: Array<String>) {var circle = Shape.Circle(4.5f)var square = Shape.Square(4)var rectangle = Shape.Rectangle(4,5)eval(circle)eval(square)eval(rectangle)//eval(x) //compile-time error.}//Following is printed on the console:
//Circle area is 63.585
//Square area is 16
//Rectangle area is 20

Note: The is modifier checks if the class is of the following type.
is modifier is required only for classes. Not with Kotlin objects as shown below:

注意： is修饰符检查类是否为以下类型。
is修饰符仅对于类是必需的。 不适用于Kotlin对象，如下所示：

sealed class Shape{class Circle(var radius: Float): Shape()class Square(var length: Int): Shape()object Rectangle: Shape(){var length: Int = 0var breadth : Int = 0}
}fun eval(e: Shape) =when (e) {is Shape.Circle -> println("Circle area is ${3.14*e.radius*e.radius}")is Shape.Square -> println("Square area is ${e.length*e.length}")Shape.Rectangle -> println("Rectangle area is ${Shape.Rectangle.length*Shape.Rectangle.breadth}")}

This brings an end to kotlin sealed class tutorial.

这样就结束了kotlin密封类教程。

References: Kotlin Docs

参考： Kotlin Docs

翻译自: https://www.journaldev.com/18719/kotlin-sealed-class

kotlin密封类