This issue is rised from the JDK 8 Warnings Cleanup Day 
issue 7116950 Reduce number of warnings in swing

To reduce number of the JDk warning in the swing area it needs to change the TreeNode.children() return type from the Enumeration to Enumeration<TreeNode> and update the children() method usage from  Enumeration children = node.children(); to Enumeration<TreeNode> children = node.children();

It allows to change return type for some methods of the DefaultMutableTreeNode from Enumeration to Enumeration<TreeNode>
The request about API change should be sent.