if we have a base class with a property like: ```...

# announcements

[GarouDan]

if we have a base class with a property like:

var parent: MyAbstractClass? = null

how can I override this property in some of the concrete class to have something like:

override var parent: MyConcreteClass? = null

I know that is possible to override properties, but it looks like kotlin do not understand that the MyConcreteClass is a supertype coming from the MyAbstractClass.

Is there a way to get around this?

[Icaro Temponi]

abstract var/val or open var/val

[Andreas Sinz]

You cannot override a var with a subclass, the following code would fail at runtime:

abstract class MyAbstractClass

class MyConcreteClass : MyAbstractClass()

class MyConcreteClass2 : MyAbstractClass()

open class BaseClass {
    var parent: MyAbstractClass? = null
}

class SubClass : BaseClass() {
    override var parent: MyConcreteClass? = null
}

val subClass: BaseClass = SubClass()
subClass.parent = MyConcreteClass2() //Is allowed, but fails at runtime

[Icaro Temponi]

abstract class BaseClass {
    open val someProp = 1337
    abstract var someVar: String
}

class DerivedClass: BaseClass() {
    override val someProp = 42
    override var someVar = "Hello, World!"
}

[Andreas Sinz]

@Icaro Temponi he wants to override a property with a subclass of the property

[Icaro Temponi]

oh, sry, i misunderstood the question 😥

[GarouDan]

When using your solution, my case is a little more complicated because it has a generic (<Something>). But copying your code, the strange is I receive the following error:

Var-property type is 'MyConcreteClass?', which is not a type of overridden

public open var parent: MyAbstractClass? defined in …BaseClass

It’s not needed to me use this line:

subClass.parent = MyConcreteClass2()

But this one will be fine:

subClass.parent = MyConcreteClass()

Should I get the error above? I’m using IntelliJ

[Andreas Sinz]

the error is fine. just because you don't need it doesn't mean you wouldn't run into this problem by accident in the future

[Andreas Sinz]

so kotlin prohibits this case altogether

[GarouDan]

The funny thing is that I’ve found a way to put it to work.

I used the abstract and I changed MyAbstractClass? to something like Entity (a generic that I’m already using). And it works 😃

[GarouDan]

@MappedSuperclass
abstract class AbstractTestEntity<Entity : EntityInterface<Entity>> : AbstractEntity<Entity>() {

       ...

       abstract var parent: Entity?

        ...

}

where Entity is a generic,
so in the concret class I’ve put

@Entity
@Table(name = "tests")
class TestEntity : AbstractTestEntity<TestEntity>() {

       ....

        @JsonIgnore
	@ManyToOne(fetch = FetchType.LAZY)
	@JoinColumn(name = "parent_id", insertable = false, updatable = false)
	override var parent: TestEntity? = null

        ...

}

I need the generic for other things, but it helped 😃

[Andreas Sinz]

This version is different, because the class of the property doesn't change, its just generic

[Andreas Sinz]

because TestEntity<MyConcreteEntity> can only be used as a AbstractTestEntity<MyConcreteEntity>, not as AbstractTestEntity<MyAbstractEntity>