title: A data structure for many-to-many symmetric relationship (person friends problem)
body: <p>I'm kinda of a newbie. I have a question regarding which Java collection (no Hibernate, databases, etc.) is best to use to implement many-to-many symmetric relationship. HashMap with values V=sets of keys? But then delete, add operations become very slow... Any suggestions?</p>
  
  <p>Thanks.</p>