Observable that stores last value, but cannot push new ones?

Question

Let's say I have an Person interface, that have an name observable:

interface Person {
  Observable<String> name;
}

And it's implementation:

class David implements Person {
  Observable<String> name = BehaviorSubject<>.createDefault("David");
}

And now there is a problem, because I can't get current value of name observable:

class Main {
  public static void main(String[] args) {
    Person person = PersonFactory.create();
    System.out.println(??person.name.currentValue??);
  }
}

I don't want to expose the name as BehaviorSubject in my interface, because then everyone would be able to change the name, which isn't what I want.

I kind of understand a logic of Observables, so I know they aren't designed to store any value. So what is "an immutable observable with current value" in RxJava?

In Android there is a LiveData and MutableLiveData. To my understanding MutableLiveData is the equivalent of BehaviorSubject, so what is the equivalent of LiveData in RxJava?

Answer 1

I've decided to create my own LiveData, using a field delegation

interface LiveData<out T> {
    val observable: Observable<out T>
    val value: T

    fun subscribe(onNext: Consumer<in T>): Disposable = observable.subscribe(onNext)
}

class MutableLiveData<T>(initial: T) : LiveData<T> {
    private val subject: BehaviorSubject<T> = BehaviorSubject.createDefault(initial)

    override val observable: Observable<out T> = subject

    override var value: T
        get() = subject.value
        set(value) = subject.onNext(value)

    fun mutate(mutationFunction: T.() -> Unit) {
        value.mutationFunction()
        value = value
    }
}

Answer 2

Define the type of varaible name as a subject internally to David and use the hide() method to expose as an observable. The use of hide will ensure the outside world can't invoke onNext i.e. mutate the name. See the docs for hide() here

class David implements Person {
  
    private PublishSubject<String> nameSubject = BehaviorSubject<>.createDefault("David");

    public Observable<String> observeName() {
        return nameSubject.hide();
    }

    // If you want the outside world to update the name
    public void setName(String name) {
        nameSubject.onNext(name)
    }
}