title: Is there an STL-Multiset equivalent container in Java?
body: <p>I'm still seeking an ideal solution to <a href="https://stackoverflow.com/questions/6254537/is-there-a-directed-acyclic-graph-dag-data-type-in-java-and-should-i-use-it">this question</a>. To summarize, I am modeling a power subsystem in Java and I need a Directed-Acyclic-Graph (DAG)-type container for my data. </p>
  
  <p>I found exactly what I need in C++'s Standard Template Library (STL). It is the <a href="http://www.sgi.com/tech/stl/multiset.html" rel="nofollow noreferrer">multiset</a>, which supports storing multiple data values for the same key. I can clearly see how storing power nodes and keys, and their upstream/downstream connections as values, could be pulled off with this data structure.</p>
  
  <p>My customer has a hard-requirement that I write the power subsystem model in Java, so I need a data structure identical to the STL multiset. I could potentially roll my own, but it's late in the game and I can't afford the risk of making a mistake. </p>
  
  <p>I'm supremely disappointed that Java is so light on Tree / Graph collections.</p>
  
  <p>Has anyone found an multiset-type structure in Java?</p>