public void insertAtBack(Object data)// add Object at first position
    {
        if(isEmpty()){
            head.next = new Node(data); // second method : head = new Node(data);
            tail = head.next; // second method : tail = new Node(data);
        }
        else{
            current = head;

            while(current.getNext()!=null){
                current = current.getNext();
            }
            current.setNext(new Node(data));
        }
        length++;
    }// end insertAtFront