title: How to concat two double-arrays with streams
body: <p>I have two Lists:</p>
  
  <p>Class Matrix: <code>List&lt;Stroke&gt; stroke = new ArrayList&lt;&gt;();</code></p>
  
  <p>Class Stroke: <code>List&lt;Point2D&gt; points = new ArrayList&lt;&gt;();</code></p>
  
  <p>Every entry of <code>points</code> does map to <code>{x, y, z}</code>:</p>
  
  <pre><code>points.stream().map(p -&gt; new double[]{p.getX(), p.getY(), 0.0})
  </code></pre>
  
  <p>Every stroke gives a <code>double[][]</code>.</p>
  
  <p>Now i want to convert the <code>stroke</code> list to <code>double[][]</code>.
  Since every <code>stroke</code> gives a <code>double[][]</code> something is need to concatenate every array.</p>
  
  <p><strong>How to do this with streams?</strong></p>
  
  <pre><code>stroke.stream()....
  </code></pre>