title: Is there a way like Promise.all() in RxJava?
body: <p>I have a question.</p>
  
  <p>I need to get list of some items by a list of the item ids.
  At first, I tried</p>
  
  <pre><code>Observable.from(itemIds)
    .flatMap(itemId -&gt; requestToServer(itemId))
    .subscribe(item -&gt; { /* do something */ });
  </code></pre>
  
  <p>But the operator <code>flatMap</code> does not guarantee the order of items. 
  I need to get item in order that <code>ItemIds</code> have. </p>
  
  <p>It would be great if there was the api like <code>Promise.all()</code>. 
  Is there a way like <code>Promise.all()</code> in RxJava? or any other ways? </p>