title: Combine multiple lists in Java
body: <p>If I want to make two lists into one in Java, I can use <code>ListUtils.union(List list1,List list2)</code>. But what if I want to combine multiple lists?</p>
  
  <p>This works:</p>
  
  <pre><code>import org.apache.commons.collections.ListUtils;
  List&lt;Integer&gt;list1=Arrays.asList(1,2,3);
  List&lt;Integer&gt;list2=Arrays.asList(4,5,6);
  List&lt;Integer&gt;list3=Arrays.asList(7,8,9);
  List&lt;Integer&gt;list4=Arrays.asList(10,0,-1);
  System.out.println(ListUtils.union(ListUtils.union(list1, list2),ListUtils.union(list3, list4)));
  </code></pre>
  
  <p>But it doesn't really look like the best solution, neither is it particularly great to read. Sadly <code>ListUtils.union(list1,list2,list3,list4)</code> doesn't work. Using <code>addAll</code> multiple times and creating its own list just for that with duplicates of all the entries also doesn't seem ideal to me. So what can I do instead?</p>