title: why in Java8 the TreeNode subclass in HashMap extends LinkedHashMap.Entry instead of directly extending HashMap's Node subclass?
body: <p>Actually a treenode is a node of a red-black tree, and needs a parent field, a left field and a right field to form a tree. but I don't see any necessity for it to have some attributes of an LinkedHashMap.Entry.</p>
  
  <p>Is there anyone who knows the reason?</p>