Котлин - стратегический!

Он настолько косвенно монетизируется, что этот вопрос не стоит

Ну и явно инвестиция на долгие годы)

шаришь!

:((((

Could not locate call adapter for io.reactivex.rxjava3.core

Can anyone help me, i using RxJava 3.0.2

I can not see more option when i try to call CallAdapterFactory :(((

I want to be Android application developer

My condolences, but it does not change what I've said. Idiomatic kotlin code is different from idiomatic Java code. Android has a lot of Java legacy, but it is going Kotlin-first.

Give me one of them for the advice

I believe it would be hard to understand specific examples without prior knowledge of both.
What's your background?

One of which? You asked what to learn first. You have your answer, so start learning. You will have to learn a bit of Java on Android to read the documentation. But you will see it whent you get there.

I'm learning Java

I have IntelliJ Idea

Any languages you have some prior knowledge of?

Not yet

I recently began to learn

I see. I'm not really sure it's reasonable to compare examples of idiomatic Java to idiomatic Kotlin right now.

Some points to consider picking the language:

Java is mature, with tons of tutorials, including ones for Android; Kotlin is official language for quite some time and used far longer than that, but still it's young. Though community is big enough to be sure someone can help, and there are Kotlin-based courses for Android out there.

Kotlin is making it's way to Android vacancies. Right now most of them are not required Kotlin knowledge, but favouring it. Java is still present, there are lots of legacy already written in Java, and some projects still use Java. So depending on when you are going to start searching for the job you could prioritize one of them and learn another later, e.g. in several years it would be much more meaningful to start with Kotlin, right now both options are valid.

Kotlin on Android is of course based on Java standard library, there are lots of 3rd party Java libraries out there, so you'll have to be able to at least read Java. On the other hand, community works on Kotlin libraries which are sometimes better and nearly always nicer to use from Kotlin. Also Kotlin multiplatform is kicking off, and there will be much more libraries in future, which would work both for both JVM/Android and for iOS or browser, for example. There would definitely be less need for Java knowledge in future.

And it should be noted that although Kotlin seems to be a bit more complex than Java, it should be a bit harder to grasp all it's features from the start, but learning Java after should be a breeze.

So take your time to consider this, knowing if you have one month, one year or five years until getting your job, and choose your destiny :)

Mandatory disclaimer: this is of course subjective opinion, take it with a grain of salt, as you should do for most of the text on the Internet :) Thought I've tried to minimize my judgements and provide more facts.