title: Why Java Collection Framework doesn't contain Tree and Graph
body: <p>I am familiar with Java Collection Framework which contains basic interfaces: <code>Collection</code> and <code>Map</code>. I am wondering why the Framework doesn't contain structures as Tree and Graph which are basic collections. Both can be regarded as sub types of <code>Collection</code>. </p>
  
  <p>By the way, I know <code>TreeSet</code> is implemented by Red-Black Tree underlying. However, the <code>TreeSet</code> is not a Tree but a <code>Set</code>, so there's no real Tree in the framework.</p>