Question

I know the problem with my code is (should be) stupid. But would appreciate all help.

public void transferFrom(LinkedIntList list2) {

   // Point to first node of list1
   ListNode current = front;

   // Move to the last node of list1
   while(current != null) {
      current = current.next;
   }

   // Last node of list1 -> firs node of list2
   current.next = list2;

   list2 = null;

}

Problem line is current.next = list2;. Data type mismatch because current.next is ListNode and list2 is LinkedIntList.

If I rather use current.next = list2;, I get NullPointerException for this line.

What should I be doing?

EDIT: Fixed!

public void transferFrom(LinkedIntList list2) {

   // Point to first node of list1
   ListNode current = front;

   // Move to the last node of list1
   while(current != null && current.next != null) {
      current = current.next;
   }

   // Last node of list1 -> first node of list2
    if(front == null) {
        front = list2.front;
    } else {
        current.next = list2.front;
   }

   list2.front = null;

}

Answer 1

It would help if you posted the class definitions for LinkedIntList and ListNode, and also told us what this method is actually supposed to be doing. But I’m assuming that a LilnkedIntList contains a ListNode front member and that you are trying to append the contents of list2 to this (which is another LinkedIntList). Your troublesome line should probably be:

current.next = list2.front;

However, you have another problem: your while loop is guaranteed to exit with current == null, which is not what you want at all. The loop condition should be (current.next != null), not (current != null).

Finally, if you want to empty out list2, the way to do it is list2.front = null;. Assigning to list2 inside the method does nothing.

Here’s a version of your code incorporating all of my suggestions:

public void transferFrom(LinkedIntList list2) {

   if (front == null) {
       front = list2.front;
   } else {
       // Point to first node of list1
       ListNode current = front;

       // Move to the last node of list1
       while(current.next != null) {
          current = current.next;
       }

       // Last node of list1 -> firs node of list2
       current.next = list2;
   }

   // empty out list2
   list2.front = null;

}