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

Kotlin Programming

Question or issue of Kotlin Programming:

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

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)
}

How to solve this issue?

Solution no. 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().

Solution no. 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.

Solution no. 3:

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.fromEntity(entity)
}

To know more read Overriding Rules

Solution no. 4:

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)
        }
    }
}

Hope this helps!