Java Can't Traverse a Doubly Linked List in Java

  • Thread starter zak100
  • Start date
375
8
Hi,
I can't traverse the doubly linked list.
My code is:


Java:
class Student{
   int id;
   String name;
   Student next, prev;
   Student(int id, String name){
      this.id = id;
      this.name = name;
      next = null;
      prev = null;
   }
}

public class DLLAddtoTail {
   Student head =null;
   Student tail = null;
   /**
   * @param args the command line arguments
  */

   public static void main(String[] args) {
      // TODO code application logic here
      DLLAddtoTail obj = new DLLAddtoTail();
      obj.create4ElementList();
      obj.displayFromHead();
   }

   void create4ElementList(){
      Student head=null;
      Student tail = null;
      Student newNode;
      int id =0;
      String name = "CE" +id;
      newNode = new Student(id, name);
      head = newNode;
      tail = newNode;
      for(int i=1; i<4; ++i){
         id =i;
         name = "CE" +id;
         newNode = new Student(id, name);
         tail.next = newNode;
         newNode.prev=tail;
         tail = newNode;
      }
   }

   void displayFromHead(){
      JOptionPane.showMessageDialog(null, "Testing1");
      tail = head;
      while(tail!=null){
         JOptionPane.showMessageDialog(null, "Testing2");
         JOptionPane.showMessageDialog(null, "id = "+tail.id+"name="+tail.name);
         tail = tail.next;
      }
   }
}


Some body please guide me what's the problem. I am using Netbeans 8.1 and I am working on Windows 10.

It is displaying :

Testing1
in JOptionPaneDialog(...)

Zulfi.
 
Last edited by a moderator:
32,344
4,130
It is displaying :

Testing1
in JOptionPaneDialog(...)
The string Testing1 is displayed by the first call to showMessagedialog() in your displayFromHead() method. Since your code isn't printing Testing2, it must mean that tail == head, so presumably your code is not setting tail correctly. I would look at your code in create4ElementList(), particularly these three lines:
Java:
newNode = new Student(id, name);
 head = newNode;
 tail = newNode;
This code creates a Student object named newNode, and then sets both head and tail to point to it. Also, I'm pretty sure you have a problem in the for loop. It would be very helpful for you to draw pictures of your linked list, and what happens at each step.
 

Filip Larsen

Gold Member
1,183
117
You are not accessing the head and tail variables you think in the create4ElementList method.
 
375
8
Hi,
Thanks for your response. Maybe "head = null" so I can't traverse. But I can't figure out why? I have initialized head in line#34. Why this initialization not working?
dll1.jpg
I am uploading the image
 
375
8
Hi,
I have solved this problem.

Zulfi.
 

rcgldr

Homework Helper
8,546
461
Example java code for doubly linked list, with append (at end), insert (before beginning), showf (show list in forward order), and showr (show list in reverse order) functions.

Code:
class DLNode<E>{
    DLNode next;
    DLNode prev;
    E element;
    DLNode(){
        element = null;
    }
    DLNode(E e){
        element = e;
    }
}
class DLList<E>{
    DLNode head;
    DLNode tail;
    int size;
    DLList(){
        head = null;
        tail = null;
        size = 0;
    }
    public void append(E element) {
        DLNode newNode = new DLNode (element);
        size++;
        if(head == null){
            head = newNode;
            tail = newNode;
            return;
        }
        newNode.prev = tail;
        tail.next = newNode;
        tail = newNode;
    }
    public void insert(E element) {
        DLNode newNode = new DLNode (element);
        size++;
        if(head == null){
            head = newNode;
            tail = newNode;
            return;
        }
        head.prev = newNode;
        newNode.next = head;
        head = newNode;
    }
    public void showf(){
        if(head == null || tail == null)
            return;
        DLNode node = head;
        while(node != null){
            System.out.println(node.element);
            node = node.next;
        }
        System.out.println();
    }
    public void showr(){
        if(head == null || tail == null)
            return;
        DLNode node = tail;
        while(node != null){
            System.out.println(node.element);
            node = node.prev;
        }
        System.out.println();
    }
}
 
1,493
601
Find a good Youtube video on how to use breakpoints and this little guy called the debugger. It'll let you step through your program line by line and see exactly what your variables are doing.
242301
 
375
8
Hi,
I tried this debugger several times but its always a head ache. I like to learn it but break points setting and reading variable's values becomes a problem for me. Also I am working with Netbeans and not with Eclipse.

Zulfi.
 
32,344
4,130
I like to learn it but break points setting and reading variable's values becomes a problem for me. Also I am working with Netbeans and not with Eclipse.
It's really worth the time you spend to learn at least these two skills. I haven't written any Java code for more than 20 years, but I would bet that in either Netbeans or Eclipse, setting a breakpoint is just a matter of clicking the mouse on the line you want the debugger to stop on. And being able to watch how your variables change as you step through your program one line at a time takes a lot of the guesswork out of programming.

If you're going to write code, one of the absolute best uses of your time is to learn how to use a debugger, even if only for setting breakpoints, single stepping through code, and inspecting variables.
 

rcgldr

Homework Helper
8,546
461
For Netbeans, you click on the line and press ctrl+F8 to toggle line break on or off (or right click and use the pop-up menu). Then press ctrl+F5 to run the program in debug mode. The variables show up at the bottom of the screen when it stops at a break point.
 

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