Data Structures, Algorithms, & Applications in Java
Chapter 6, Exercise 53

Two doubly-linked circular lists with header nodes may be joined in O(1) time as below. 
public class HeadDoubleCircularListWithJoin
             extends HeadDoubleCircularList
{
   /** append list a to this
     * following the join, a is empty */
   public void join(HeadDoubleCircularListWithJoin a)
   {
      // special cases
      if (a.size == 0)
         // a is empty, nothing to join to this
         return;

      // link nodes of list a (other than its header) into end of this
      DoubleNode lastNode = headerNode.previous;
      DoubleNode aFirstNode = a.headerNode.next;
      DoubleNode aLastNode = a.headerNode.previous;
      lastNode.next = aFirstNode;
      aFirstNode.previous = lastNode;
      aLastNode.next = headerNode;
      headerNode.previous = aLastNode;
      size += a.size;

      // make a empty
      a.clear();

      return;
   }
}