Android JUnit testing api

Question

I have a function here which gets the list of Posts from JSONplaceholder api. If want to implement testing on this function how do I do it and what will I need, Mockito, JUnit

 override fun getAllPosts(): Flowable<List<PostEntity>> {
        return postsService.getPosts()
            .flatMap { posts ->
                Flowable.fromIterable(posts)
                    .take(10)
                    .toList().toFlowable()
            }
    }

    @GET("/posts")
    fun getPosts(): Flowable<List<PostEntity>>

Please give me some pointers on how do I go about doing testing it in Android studio

Thanks R