title: A hash Map in which on of the value object's fields is the key?
body: <p>I was wondering if there was an class like HashMap in which the key is an identifying field in the object.</p>
  
  <p>If there is not, I was thinking about having the class I use as the value implement Map.Entry.  Does this seem like a reasonable approach or does that seem dangerous?  (I'll make my key field immutable.)</p>