The data in linked list won't show on the Text Area in GUI. Java. Why?

Question

I am currently doing a book inventory system in linked list using Java GUI. I have to add book information into a Node in linked list and display it by implementing the iterator.

I have done with my code and it shows no error. However, when I run the GUI and successfully add a book into the linked list then press the display button. It did not show the information I just added to the text area.

Did anywhere of my code have problem?

This is my Node class:

public class Node 
{
    Data data;
    Node next;

    public Node()
    {
        next = null;
    }

    Node(Data data, Node next)
    {
        this.data = data;
        this.next = next;
    }

    public Object getData()
    {
        return data;
    }

    public Node getNext()
    {
        return next;   
    }

    public void setNext(Node next)
    {
        this.next=next;
   }
}

And this is my LinkedList class with the insertion and display method:

public class LinkedList
{
    Node node = new Node();
    static Data data;
    static Node head;

    public LinkedList()
    {
        head=null;
    }

    public Node getHead()
    {
        return head;
    }

    public static void addNode(Data data)
    {
        Node newNode = new Node(data, head);
        Node previous = null;
        Node current = head;

        while(current != null && data.name.compareTo(current.data.name) >= 0){
            previous = current;
            current = current.next;
        }

        if(previous == null){
            head = newNode;
        }else{
            previous.next = newNode;
        }
            newNode.next = null;
            JOptionPane.showMessageDialog(null,"Book Information has been added to the inventory.");
    }
}

    public static String displayNode()
    {
        DisplayIterator i;
        Node current = head;
        String output = "";       
        while(DisplayIterator.hasNext())
        {
            output+= DisplayIterator.next();
            current=current.next;
        }      
        return output+"NULL";
    }

This is my data class which I used to store all the information into one Node:

public class Data {
    String name;
    String author;
    int isbn;
    int number;
    String genre;
    Node head;

    public Data(String name, String author, int isbn, int number, String genre)
    {
        this.name = name;
        this.author = author;
        this.isbn = isbn;
        this.number = number;
        this.genre = genre;
    }

    public String toString(String name, String author, int isbn, int number, String genre)
    {
        return("Book Name: "+name+"\nAuthor: "+author+"\nISBN Number: "+isbn+"\nNumber of Copies: "+number+"\nGenre: "+genre+"\n");
    }
}

And lastly this is my Iterator class:

public class DisplayIterator
{
    Data data;
    static Node current;

    DisplayIterator(Data data)
    {       
        this.data = data;
        current = data.head;    
    }

    public static boolean hasNext()
    {
        if(current != null){            
            return true;
        }       
        return false;       
    }

    public static Object next()
    {
        if(hasNext()){
        current = current.getNext();
        return current.getData().toString();            
    }       
        return null;        
    }

    public void remove()
    {
        throw new UnsupportedOperationException("It is read-only.");        
    }       
}

I think the problem is on the DisplayIterator class but I can't see where. Can anyone help me? Thank you.

Answer 1

Your data.head is always null

DisplayIterator(Data data)
{       
    this.data = data;
    current = data.head;  // this will always make current as null.  
}

In your following class , the constructor does not initialize your head node.

public class Data {
   String name;
   String author;
   int isbn;
   int number;
   String genre;
   Node head; // not set any where?

Also your addNode is not correct.

public static void addNode(Data data)
{
    Node newNode = new Node(data, head);
    Node previous = null;
    Node current = head; //this will be always NULL on first addNode call

    while(current != null && data.name.compareTo(current.data.name) >= 0){
        previous = current;
        current = current.next;
    }

    if(previous == null){
        head = newNode;
    }else{
        previous.next = newNode;
    }
        newNode.next = current; // how can be newNode.next is current?
        JOptionPane.showMessageDialog(null,"Book Information has been added to the inventory.");
}