12.1

public void printList()
{
   Node temp = head;
   if (temp == null)
      System.out.println("The list is empty.");
   while (temp != null)
   {
      System.out.println(temp.info);
      temp = temp.link;
   }
} 

public int sum ()
{
   int result = 0;
   Node temp = head;
   while (temp != null)
   {
      result += temp.info;
      temp = temp.link;
   }
   return result;
} 

public void deleteFirst()
{
   if (head == null)
      System.out.println("The list is empty, nothing to delete.");
   else if (head.link == null)
      head = null;
   else
   {
      Node temp = head.link;
      head.link = null;
      head = temp;
   }
} 

public void deleteLast()
{
   Node temp = head;
   if (head == null)
      System.out.println("The list is empty, nothing to delete.");
   else if (temp.link == null)
      head = null;
   else
   {
      while (temp.link.link != null)
      {
        temp = temp.link;
      }
      temp.link = null;
   }
} 

public String toString()
{
   Node temp = head;
   String result = "";
   if (head != null)
      while(temp != null)
      {
         result += temp.info;
         temp = temp.link;
         if (temp != null)
            result +="//";
      }
   return result;
} 

public void addAtRear (int item)
{
   Node temp = head;
   Node newNode = new Node();
   newNode.info = item;
   newNode.link = null;
   if (temp == null)
      head = newNode;
   else
   {
      while (temp.link != null)
      {
         temp = temp.link;
      }
      temp.link = newNode;
   }
}