title: How to zip few observables in Kotlin language with RxAndroid
body: <p>I have some problem. I'm a beginer in RxJava/RxKotlin/RxAndroid, and dont understand some features. For Example:</p>
  
  <pre><code>import rus.pifpaf.client.data.catalog.models.Category
  import rus.pifpaf.client.data.main.MainRepository
  import rus.pifpaf.client.data.main.models.FrontDataModel
  import rus.pifpaf.client.data.product.models.Product
  import rx.Observable
  import rx.Single
  import rx.lang.kotlin.observable
  import java.util.*
  
  
  class MainInteractor {
  
      private var repository: MainRepository = MainRepository()
  
      fun getFrontData() {
  
          val cats = getCategories()
          val day = getDayProduct()
          val top = getTopProducts()
  
          return Observable.zip(cats, day, top, MainInteractor::convert)
      }
  
      private fun getTopProducts(): Observable&lt;List&lt;Product&gt;&gt; {
          return repository.getTop()
                  .toObservable()
                  .onErrorReturn{throwable -&gt; ArrayList() }
  
      }
  
      private fun getDayProduct(): Observable&lt;Product&gt; {
          return repository.getSingleProduct()
                  .toObservable()
                  .onErrorReturn{throwable -&gt; Product()}
  
      }
  
      private fun getCategories(): Observable&lt;List&lt;Category&gt;&gt; {
          return repository.getCategories()
                  .toObservable()
                  .onErrorReturn{throwable -&gt; ArrayList() }
      }
  
      private fun convert(cats: List&lt;Category&gt;, product: Product, top: List&lt;Product&gt;): FrontDataModel {
  
      }
  }
  </code></pre>
  
  <p>Then I'm use <strong>MainInteractor::convert</strong> Android studio tell me next</p>
  
  <p><a href="https://i.stack.imgur.com/Caoua.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Caoua.jpg" alt="enter image description here"></a></p>
  
  <p>I tried a lot of variant and tried to understand what does it want, but no success. Help me please... Best Regards.</p>