LinkedList中的空对象设计模式

Question

I am trying to Implement a doubly linked with null objects at the beginning and end of the list using null object design pattern. So an empty list will contain two null objects. So I wrote this code Does this follow null object design pattern? If not how can I achieve that. ANy suggestions will be appreciated.

Updated Code-

// Creating a doubly linked list.
        doubleLinkedList = new DoubleLinkedList();

    class DoubleLinkedList {

        private NewLink firstNode;
        private NewLink lastNode;
        private NewLink rootNode;



        public DoubleLinkedList() {

//So this satisfies my question that I have asked meaning null objects at beginning and last node or something else I have to do.
            firstNode = NewLink.NULL_NODE;
            lastNode  = NewLink.NULL_NODE;

        }

    }



    class NewLink {

        public String  data;
        public NewLink nextPointer;
        public NewLink previousPointer;


public static final NewLink NULL_NODE = new NewLink(); 



        public NewLink(String id) {

            data = id;

        }

public NewLink() {

    }
        // Overriding toString method to return the actual data of the node
        public String toString() {

            return "{" + data + "} ";

        }
    }

Answer 1

No, your code doesn't implement Null Object Design Pattern . Its essence is not to use null but to create an object which will represent the null .

For example:

public static final NewLink NULL_NODE = new NewLink();

And then:

firstNode = NULL_NODE;
lastNode  = NULL_NODE;

Answer 2

We use Null object design pattern if we want to assign some default behaviors (or prevent some behaviors to happen as a default behavior) For example using Null object pattern we can replace this code :

if(myObj!=null) 
myObj.DoSomething();
else
DoSomethingElse();

wtih this one:

myObj.DoSomething() //assuming that myObj can be a Null object (not a null reference) that has implemented DoSomethingElse when you ask it to DoSomething()

Thus a Null Object design pattern actually uses a default object reference (not a null reference )

Answer 3

public static final NewLink NULL_NODE = new NewLink(); 

must be in NewLink class

so

firstNode = NewLink.NULL_NODE;
secondNode = NewLink.NULL_NODE;

also you can make all methods from NewLink - abstract
and make two nested classes: for NULL objects and for not NULL object.
It's can be very helpful in difficult situations