Can a kotlin interface cache a value?

Issue

I really like using default implementations for interfaces in Kotlin, especially for common patterns like Observable. Here's my interface,

interface Observable<T>{

    // How do I cache this?
    val observers: MutableList<Observer<T>>
        get() = LinkedList<>()

    fun addObserver(o:Observer<T>){
        observers.add(o)
    }

    fun removeObserver(o:Observer<T>){
        observers.remove(o)
    }

    fun notifyObservers(u:T){
        for (o in observers){
            o.update(u)
        }
    }

}

The interface refers to a list of observers, but the get() call is returning a new LinkedList() each time. How can I cache the value of observers so that it's only created once? I've tried using kotlin-lazy, but either can't get the syntax right, or it's not meant for interfaces. My IDE complains "Delegated properties are not allowed in interfaces."

UPDATE

Based on Yoav's answer, I've change my interface to

interface Observable<T>{
    val observers: MutableList<Observer<T>>
}

And then in the implementing class,

class MyObservable : Observable<String>

   private val _observers = LinkedList<Observer<String>>()

   override val observers: MutableList<Observer<String>>
       get() = _observers

Any tips to make this more succinct?

Solution

According to Kotlin docs:

Interfaces in Kotlin are very similar to Java 8. They can contain declarations of abstract methods, as well as method implementations. What makes them different from abstract classes is that interfaces cannot store state.

Interface can't hold any state as they are fully abstract. Perhaps you should use an abstract class in order to cache the values?

See this question for more information about the reason for interfaces being stateless.

An interface is a contract specifying what its implementer promises to be able to do. It does not need to specify state because state is an implementation detail and only serves to constrain implementers in how this contract is fulfilled. If you want to specify state you might want to rethink you use of interfaces and look at abstract base classes instead.

Answered By - Yoav Sternberg