Call super class constructor in Kotlin, Super is n

问题:

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;
    }
}

and

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

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

Which gives me the error

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

Why cannot I do that?

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

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

回答1:

You have a couple of problems in your code.

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

constructor(entity: Entity) : super(entity)

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

Solution 1

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)
}

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

Solution 2

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
    }
}

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.

Solution 3 (most idiomatic)

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()

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().

回答2:

Use this super<Entity>.fromEntity(entity) to call super class methods.

As Documentation says:

In Kotlin, implementation inheritance is regulated by the following rule: if a class inherits many implementations of the same member from its immediate superclasses, it must override this member and provide its own implementation (perhaps, using one of the inherited ones). To denote the supertype from which the inherited implementation is taken, we use super qualified by the supertype name in angle brackets, e.g. super.

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

To know more read Overriding Rules

回答3:

Another option is to create companion object and provide factory method e.g.

class Account constructor(
        var name: String? = null,
        var accountFlags: Int? = null,
        id: String?,
        created: Date?
) : Entity(id, created) {

    companion object {
        fun fromEntity(entity: Entity): Account {
            return Account(null, null, entity.id, entity.created)
        }
    }
}