Data Structures, Algorithms, & Applications in Java
Chapter 6, Exercise 47

public class DoubleCircularListWithJoin extends DoubleCircularList
{
   /** append list a to this
     * following the join, a is empty */
   public void join(DoubleCircularListWithJoin a)
   {
      // special cases
      if (a.size == 0)
         // a is empty, nothing to join to this
         return;

      if (size == 0)
      {// this is empty,
         // move all nodes of a to this
         lastNode = a.lastNode;
         size = a.size;

         // make a empty
         a.lastNode = null;
         a.size = 0;

         return;
      }
   
      // neither this nor a is empty
      // link a into end of this
      DoubleNode firstNode = lastNode.next;
      DoubleNode aFirstNode = a.lastNode.next;
      lastNode.next = aFirstNode;
      aFirstNode.previous = lastNode;
      lastNode = a.lastNode;
      firstNode.previous = lastNode;
      lastNode.next = firstNode;
      size += a.size;

      // make a empty
      a.clear();

      return;
   }
}