Addition of generics to some Swing classes breaks existing code

FULL PRODUCT VERSION :
openjdk version "9-Ubuntu"
OpenJDK Runtime Environment (build 9-Ubuntu+0-9b181-2)
OpenJDK 64-Bit Server VM (build 9-Ubuntu+0-9b181-2, mixed mode)


ADDITIONAL OS VERSION INFORMATION :
Linux astoria 4.10.0-32-generic #36-Ubuntu SMP Tue Aug 8 12:10:06 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

A DESCRIPTION OF THE PROBLEM :
The class "javax.swing.tree.DefaultMutableTreeNode" has gained generics on some protected fields which are for client usage. It contained raw Vector previously, now contains a Vector<TreeNode>.

This breaks existing code horribly, as clients must use their own TreeNode extension, e.g. in the Electric VLSI tool here:
https://sources.debian.net/src/electric/9.07%2Bdfsg-2/com/sun/electric/tool/user/ui/JobTree.java/#L50

children = [..] new Vector<JobTreeNode>();

Also the addition of generics to the "Enumeration children()" requires code changes, but not as horrible. Also left broken in the code example below.

Perhaps this field should be <? extends TreeNode>? Or the class should be generic?

REGRESSION. Last worked in version 8u144


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.util.Enumeration;
import java.util.Vector;
import javax.swing.tree.DefaultMutableTreeNode;
import javax.swing.tree.TreeNode;

class MyNode implements TreeNode {
  public boolean isLeaf() { return false; }
  public TreeNode getParent() { return null; }
  public int getIndex(TreeNode node) { return 0; }
  public int getChildCount() { return 0; }
  public TreeNode getChildAt(int childIndex) { return null; }
  public boolean getAllowsChildren() { return false; }
  public Enumeration<?> children() { return null; }
}

class A extends DefaultMutableTreeNode {
  A() {
    children = new Vector<MyNode>();
  }
}

---------- END SOURCE ----------