Call super class constructor in Kotlin, Super is not an expression

Question

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)
}

Answer 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().

Answer 2

You can also move your primary constructor down into the class like this:

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

Advantage of this is, compiler will not require your secondary constructor to call primary constructor.

Answer 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)
        }
    }
}