I want convert the code from  singly-linked list to doubly-linked list        package Problem; class Node{        private Object value;     // Holds the data of the node        private Node next;        // Holds the follwoing node's address          Node(Object v, Node n){              this.value =v;              this.next = n;        }          public Node getNext(){              return this.next;        }          public Object getValue(){              return this.value;        }          public void setNext(Node n){              this.next=n;        }          public void setValue(Object v)        {              this.value=v;        } }   class SLL{        public Node head;        SLL(){              this.head=null;        }          public  void display(){              Node curr = this.head;              while(curr != null)              {                     System.out.print(curr.getValue());                     if(curr.getNext()!=null)                            System.out.print("-->");                     curr=curr.getNext();              }              System.out.println();        }          public void insertAtFront(Object v){              Node new_node = new Node(v, this.head);              this.head = new_node;        }          public void insertAtEnd(Object v){              Node curr = this.head;              while(curr .getNext()!= null)                     curr = curr.getNext();                Node new_node = new Node(v, null);              curr.setNext(new_node);        }          public void insertAfter(Object v, Object key){              Node curr = this.head;              while(curr.getValue() != key)                     curr=curr.getNext();                Node new_node = new Node(v, curr.getNext());              curr.setNext(new_node);          }          public void Delete(Object v)        {              if(v == this.head.getValue())                     this.head = head.getNext();              else              {                     Node curr = this.head;                     Node previous = this .head;                     while(curr.getValue()!=v)                     {                            previous = curr;                            curr = curr.getNext();                     }                     previous.setNext(curr.getNext());              }        }          public Node Search(Object v){              Node curr = this.head;              while(curr != null)              {                     if(curr.getValue() == v)                            return curr;                     curr = curr.getNext();              }              return null;        }          //The following function gets the last node in the list        public Node Last(){              Node curr = this.head;              while(curr.getNext()!= null)                     curr = curr.getNext();              return curr;        }          // Count the elements of the lsit        public int size(){              Node curr = this.head;              int c=0;              while(curr != null)              {                     curr = curr.getNext();                     c++;              }              return c;        }   } public class f {          public static void main(String[] args) {                SLL ls = new SLL();              ls.insertAtFront(1);              ls.insertAtFront(21);              ls.insertAtFront(22);              ls.insertAtFront(3);              ls.display();                //ls.insertAtEnd(33);              //ls.insertAtFront(33);                //ls.insertAfter(33, 22);              //ls.Delete(22);              //Node g = ls.Search(22);              //System.out.println(g.getNext().getValue());                System.out.println(ls.size());              System.out.println(ls.Last().getValue());              //ls.display(); } }