title: What does List<Set<Integer>> mean?
body: <p>One of the questions I have been given asks:</p>
  
  <blockquote>
    <p>All the lines should be stored in an object of
    type <code>List&lt;Set&lt;Integer&gt;&gt;</code>.</p>
  </blockquote>
  
  <p>How do you write this in Java, as in how do you initialise this list? I've never seen this before.</p>
  
  <p>Please provide a link to an explanation as i'm not sure what this is called in Java so have no idea about how to learn about it. Thank You. </p>