To improve source compatibility, after the initial generification of swing (), the return type of TreeNode.children was refined to allow covariant overrides (JDK-8054360). However, DefaultMutableTreeNode.children still returns Enumeration<TreeNode> rather than Enumeration<? extends TreeNode>. The latter would allow subclasses to return more precise information.

See the discussion thread:
http://mail.openjdk.java.net/pipermail/jdk9-dev/2014-September/001295.html