adding single element to a List in a NavigableMap
<p>I want to be able to sort files based on what day they were entered into a Map.  The key is the day, and the value is a List of n Files added on the respective day.  However, I need to be able to add the files to the List one at a time - but I'm stuck on the syntax.  How do I call List.add() from within Map.put()?  Here's my code:</p>

<pre><code>public static NavigableMap&lt;String, List&lt;File&gt;&gt; myFiles = new TreeMap&lt;&gt;();
String today = new Date().toString();
File currentFile;
myFiles.put(today, currentFile);  //problem here adding currentFile
</code></pre>
