title: Why does exist WeakHashMap, but absent WeakSet?
body: <p>From <a href="http://www.informit.com/articles/article.aspx?p=1216151&amp;seqNum=6" rel="noreferrer">J. Bloch</a></p>
  
  <blockquote>
    <p>A ... source of memory leaks is
    listeners
    ... The best way to ensure that
    callbacks are garbage collected
    promptly is to store only weak
    references to them, for instance, by
    storing them only as keys in a
    <a href="http://docs.oracle.com/javase/8/docs/api/java/util/WeakHashMap.html" rel="noreferrer">WeakHashMap</a>.</p>
  </blockquote>
  
  <p>So, why there isn't any WeakSet in the Java <a href="https://en.wikipedia.org/wiki/Java_collections_framework" rel="noreferrer">Collections framework</a>?</p>