public class SinglyLinkedList {

      …



      public void addLast(SinglyLinkedListNode newNode) {

            if (newNode == null)

                  return;

            else {

                  newNode.next = null;

                  if (head == null) {

                        head = newNode;

                        tail = newNode;

                  } else {

                        tail.next = newNode;

                        tail = newNode;

                  }

            }

      }



      public void addFirst(SinglyLinkedListNode newNode) {

            if (newNode == null)

                  return;

            else {

                  if (head == null) {

                        newNode.next = null;

                        head = newNode;

                        tail = newNode;

                  } else {

                        newNode.next = head;

                        head = newNode;

                  }

            }

      }



      public void insertAfter(SinglyLinkedListNode previous,

                  SinglyLinkedListNode newNode) {

            if (newNode == null)

                  return;

            else {

                  if (previous == null)

                        addFirst(newNode);

                  else if (previous == tail)

                        addLast(newNode);

                  else {

                        SinglyLinkedListNode next = previous.next;

                        previous.next = newNode;

                        newNode.next = next;

                  }

            }

      }

}