N-ary Tree with K-ary nodes

If I understand correctly what you're trying to do then I think you could use a class like this:

	private static class CheckedList extends ArrayList<Object> {
		private final List<Class> m_T ;

		public CheckedList(List<Class> t) {
			m_T = t ;
		}

		@Override
		public boolean add(Object o) {
			boolean isAllowed = false ;
			for (Class c : m_T) {
				if (c.isInstance(o))
					isAllowed = true ;
			}

			if (!isAllowed)
				throw new IllegalArgumentException("Object " + o + " is not of a valid type for this list") ;

			return super.add(o) ;
		}
	}

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The idea is that this CheckedList takes a list of types that it will accept.  So for the A list you could create a CheckedList with the classes B and C.

Then if you try to add something (like D) to a list that can't accept it (like A) it will throw an exception.  Other than the add method, it will work just like an ordinary list.

You can build up the full tree using these CheckedLsts - initializing each layer with the appropriate types that layer would accept.

Doug