title: Concatenate arraylists
body: <p>Can you somehow concatenate two arraylists to a third?
  I know you can do it when you have  (as below):</p>
  
  <pre><code>ArrayList&lt;String&gt;arraylist1 = new ArrayList&lt;&gt;();
      ArrayList&lt;String&gt;arraylist2 = new ArrayList&lt;&gt;();
  
      ArrayList&lt;String&gt;al = new ArrayList&lt;&gt;();
      al.addAll(arraylist1);
      al.addAll(arraylist2);
  </code></pre>
  
  <p>But how do you do it if you have arraylists in different constructors like?:</p>
  
  <pre><code>ArrayList&lt;People&gt;arraylist1 = new ArrayList&lt;&gt;();
  ArrayList&lt;Billing&gt;arraylist2 =new ArrayList&lt;&gt;();
  </code></pre>