title: List containing another list
body: <p>I have an object in my application, Customer, that has a list of customers.   </p>
  
  <pre><code>public class CustomerList
  {
  private List&lt;Customer&gt;
  }
  </code></pre>
  
  <p>The customer class in turn has a list of all the items they have shopped at a store on a given day of the week.</p>
  
  <pre><code> public class Customer
  {
  private List&lt;String&gt; itemsOnMonday;
  private List&lt;String&gt; itemsOnTuesday;
  private List&lt;String&gt; itemsOnWednesday;
  private List&lt;String&gt; itemsOnThursday;
  private List&lt;String&gt; itemsOnFriday;
  }
  </code></pre>
  
  <p>Now, I want to get the list of all the items the customer has shopped in a given week. What is the best way to do this? My colleague suggests I create another list, and add items to this list. I am not convinced this is a good approach. I have over 1000 customers, and each customer shops over 500 items/week. He suggests something like this -</p>
  
  <pre><code>for(Customer customer:customerList)
  {
  List&lt;String&gt; items = new ArrayList&lt;String&gt;();
  items.addAll(itemsOnMonday);
  //So on until Friday.
  }
  </code></pre>
  
  <p>This is crazy, because I would end up creating over 1000 objects inside the for loop. Any thoughts on a better way to do this? We have been brain storming for a while now, and can't come up with an efficient implementation to achieve this. Any help will be much appreciated.</p>