title: How to make LruCache only accept key?
body: <p><code>LruCache</code> is a map, but I want to use <code>LruCache</code> to only cache strings, i.e. I want LruCache to work as a HashSet. </p>
  
  <p>How should I define a <code>LruCache</code> class? </p>
  
  <p><code>LruCache&lt;String, Void&gt;</code> won't work, because when my code called <code>lruCache.put("abc", null)</code>, <code>NullPointerException</code> was thrown.</p>
  
  <p>Thanks.</p>