为什么在 Java8 中 HashMap 中的 TreeNode 子类扩展 LinkedHashMap.Entry 而不是直接扩展 HashMap 的 Node 子类？

Question

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.

Is there anyone who knows the reason?

Answer 1

Maybe because the gain from having separate TreeNode in HashMap and LinkedTreeNode in LinkedHashMap is marginal?

Or because LinkedHashMap needs a LinkedTreeNode that would need to extend from to separate classes?

Let's assume that HashMap had a HashMap.TreeNode class that extends solely from HashMap.Node .

LinkedHashMap needs a similar class LinkedHashMap.LinkedTreeNode that needs to extend from LinkedHashMap.Entry (which in turn extends from HashMap.Node ), but would also need to extend from HashMap.TreeNode because of the way the two classes are interwoven.

That would require extending from two classes, which Java explicitly prohibits.