title: Java 8 parallelstream collect return list into one single list instead of list of list
body: <p>I want to collect a list result from a parallel stream into one list. Lets look at this code to explain my issue better. This is what I get:</p>
  
  <pre><code>List&lt;Object&gt; a = new ArrayList&lt;&gt;();
  
  List&lt;List&lt;Object&gt;&gt; result a.parallelStream() // I get a List of List
     .map(a -&gt; { return new ArrayList(); })
     .collect(Collectors.toList());
  </code></pre>
  
  <p>But what I really want to have is more like this:</p>
  
  <pre><code>List&lt;Object&gt; a = new ArrayList&lt;&gt;();
  
  List&lt;Object&gt; result a.parallelStream()   // I want to add all list elements into one List of objects
     .map(a -&gt; { return new ArrayList(); })
     .collect(Collectors.???); 
  </code></pre>