如何让数据类在Kotlin中实现接口扩展超类属性

发布于07月18日

I have several data classes which include a var id: Int? field. I want to express this in an interface or superclass, and have data classes extending that and setting this id when they are constructed. However, if I try this:

interface B {
  var id: Int?
}

data class A(var id: Int) : B(id)

它抱怨我忽略了id字段，我就是哈哈..

Q: How can I let the data class A in this case to take an id when it's constructed, and set that id declared in the interface or superclass?

推荐答案

Indeed, you needn't an abstract class yet. you can just override the interface properties, for example:

interface B {
    val id: Int?
}

//           v--- override the interface property by `override` keyword
data class A(override var id: Int) : B

interface没有构造函数，所以不能通过super(..)关键字调用构造函数，但可以使用abstract class.然而，a data class不能在其primary constructor上声明任何参数，因此它将overwritten超类的field，例如:

//               v--- makes it can be override with `open` keyword
abstract class B(open val id: Int?)

//           v--- override the super property by `override` keyword
data class A(override var id: Int) : B(id) 
//                                   ^
// the field `id` in the class B is never used by A

// pass the parameter `id` to the super constructor
//                            v
class NormalClass(id: Int): B(id)