Kotlin 中调用超类构造函数，Super 不是表达式 [英] Call super class constructor in Kotlin, Super is not an expression

问题描述

我有两个类 Entity 和 Account 作为

I have two classes Entity and Account as

abstract class Entity(
    var id: String? = null,
    var created: Date? = Date()) {

    constructor(entity: Entity?) : this() {
        fromEntity(entity)
    }

    fun fromEntity(entity: Entity?): Entity {
        id = entity?.id
        created = entity?.created
        return this;
    }
}

和

data class Account( 
    var name: String? = null,
    var accountFlags: Int? = null
) : Entity() {

    constructor(entity: Entity) : this() {
        super(entity)
    }
}

这给了我错误

super 不是一个表达式，它只能用在一个点'.'

Super is not an expression, it can be only used in the left-hand side of a dot '.'

为什么我不能这样做?

以下将通过编译错误，但我不确定是否正确.

The following will pass the compilation error, but I am not sure if it is correct.

 constructor(entity: Entity) : this() {
    super.fromEntity(entity)
}

推荐答案

你的代码有几个问题.

首先，这是从二级构造函数调用超级构造函数的正确语法:

First, this is the correct syntax, to call a super constructor from a secondary constructor:

constructor(entity: Entity) : super(entity)

第二，如果你的类有一个主构造函数(你的类有)，你不能从二级构造函数调用超级构造函数.

Second, you can't call a super constructor from a secondary constructor if your class has a primary constructor (which your class does).

abstract class Entity(
        var id: String,
        var created: Date
)

class Account(
        var name: String,
        var accountFlags: Int,
        id: String,
        created: Date
) : Entity(id, created) {
    constructor(account: Account) : this(account.name, account.accountFlags, account.id, account.created)
}

这里，复制构造函数在子类中，它只是委托给主构造函数.

Here, the copy constructor is in the child class which just delegates to the primary constructor.

abstract class Entity(
        var id: String,
        var created: Date
) {
    constructor(entity: Entity) : this(entity.id, entity.created)
}

class Account : Entity {
    var name: String
    var accountFlags: Int

    constructor(name: String, accountFlags: Int, id: String, created: Date) : super(id, created) {
        this.name = name
        this.accountFlags = accountFlags
    }

    constructor(account: Account) : super(account) {
        this.name = account.name
        this.accountFlags = account.accountFlags
    }
}

这里我只在子类中使用二级构造函数，这让我可以将它们委托给单独的超级构造函数.请注意代码很长.

Here I'm only using secondary constructors in the child class which lets me delegate them to individual super constructors. Notice how the code is pretty long.

abstract class Entity {
    abstract var id: String
    abstract var created: Date
}

data class Account(
        var name: String,
        var accountFlags: Int,
        override var id: String,
        override var created: Date
) : Entity()

这里我省略了复制构造函数并将属性抽象化，以便子类拥有所有属性.我还将子类设为 data class.如果你需要克隆这个类，你可以简单地调用 account.copy().

Here I omitted the copy constructors and made the properties abstract so the child class has all the properties. I also made the child class a data class. If you need to clone the class, you can simply call account.copy().

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