双链表中的逻辑问题

Question

I seem to have some logic problems happening with my code, I'm new to programming and especially to linked lists. How do my methods in DLL look? Ihey don't seem to work when called from my application class (add after only works for the first word).

public void addAfter(E elem, E prev) {   //use search method then insert
      if (search(prev) == null){
        insert(elem, last);
      }
      else{
        insert(elem, prev); //prev needs to be of type Node<E> rather than just <E> (????)
      }
    }

private void insert(E elem, Node<E> prev) { //HELP
      Node<E> newNode = new Node<E>(elem, prev, null);
         //if list is empty, add to front
      if (first == null) {
         first= newNode;
         last = newNode;
      } else if (prev == null){
         //addFirst(newNode);
         first.prev = newNode;
         newNode.next = first;
         first = newNode;
      }
      //if at end of list
      else if (prev.next == null){
         prev.next = newNode;
         last = newNode;
      }
      else{
         newNode.next = prev.next;
         newNode.next.prev = newNode;
         prev.next = newNode;
      }
   }

    * @param elem  The element to search for.
    * @return The first Node containing the given element, or null if no Node
    *         contains the element.
    */
   private Node<E> search(E elem) { //HELP
      for (Node<E> curr = first; curr != null; curr = curr.next){
         if (curr.element.equals(elem)){
            return curr;
         }
      }
      return null;
   }





public static void handleLine(String values, DLL<String> list){
      Scanner tokens = new Scanner(values);
      if (tokens.hasNext("[aesdpr]")){
         char command = tokens.next().charAt(0);
         System.out.println("test before switch");
         switch (command){
         case 'a':
            while (tokens.hasNext()){
              String nextToke = tokens.next();
               if (tokens.hasNext()){
                  list.addAfter(nextToke, tokens.next());

Answer 1

The problem is here:

public void addAfter(E elem, E prev) { //use search method then insert if (search(prev) == null){ insert(elem, last); } else{ insert(elem, prev); //prev needs to be of type Node<E> rather than just <E> (????) } }

At first, as you have noticed, you need a node instead of an E . Besides, you find the Node which contains the prev element. So you should add Node<E> prevNode = search(prev); before the if statement, and replace the prev with prevNode .