Can't Traverse a Doubly Linked List in Java

  • Java
  • Thread starter zak100
  • Start date
  • Tags
    Java List
In summary, the conversation discusses the use of a debugger and breakpoints in programming. The person is having trouble using it in Netbeans and prefers learning through videos. They also share an example code for a doubly linked list with various functions.
  • #1
zak100
462
11
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:
Technology news on Phys.org
  • #2
zak100 said:
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.
 
  • #3
You are not accessing the head and tail variables you think in the create4ElementList method.
 
  • #4
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
 
  • #5
Hi,
I have solved this problem.

Zulfi.
 
  • #6
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();
    }
}
 
  • #7
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
 
  • Like
Likes Mark44
  • #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.
 
  • #9
zak100 said:
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.
 
  • Like
Likes rbelli1
  • #10
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.
 

1. How do I traverse a doubly linked list in Java?

In order to traverse a doubly linked list in Java, you will need to use a loop to iterate through each node in the list. First, set a temporary variable equal to the head of the list. Then, use a while loop to check if the temporary variable is not equal to null. Inside the loop, you can access the data of the current node and move to the next node by setting the temporary variable to the next node in the list.

2. Why can't I traverse a doubly linked list in Java?

If you are having trouble traversing a doubly linked list in Java, it could be due to a few possible reasons. One common issue is not properly setting the previous and next pointers when adding or removing nodes from the list. Another possible issue is not properly handling edge cases, such as an empty list or a list with only one node.

3. Can I use a for loop to traverse a doubly linked list in Java?

Technically, it is possible to use a for loop to traverse a doubly linked list in Java. However, it is not recommended as the size of the list could change during iteration, causing unexpected behavior. It is best to use a while loop instead to ensure proper traversal.

4. How is traversing a doubly linked list different from a singly linked list?

In a singly linked list, you can only traverse the list in one direction (forward), while in a doubly linked list, you can traverse in both directions (forward and backward). This is because each node in a doubly linked list contains a reference to both the previous and next nodes, while in a singly linked list, each node only contains a reference to the next node.

5. Are there any built-in functions to traverse a doubly linked list in Java?

No, there are no built-in functions specifically for traversing a doubly linked list in Java. You will need to write your own code to properly traverse the list. However, there are built-in functions for manipulating and accessing data in a linked list, such as add(), remove(), and get(). These functions can be used to assist in traversal.

Similar threads

  • Programming and Computer Science
Replies
1
Views
1K
  • Programming and Computer Science
Replies
4
Views
790
  • Programming and Computer Science
Replies
8
Views
1K
  • Programming and Computer Science
Replies
8
Views
1K
  • Programming and Computer Science
Replies
28
Views
2K
  • Programming and Computer Science
Replies
3
Views
3K
  • Programming and Computer Science
Replies
2
Views
1K
  • Programming and Computer Science
Replies
3
Views
1K
  • Programming and Computer Science
Replies
1
Views
3K
  • Programming and Computer Science
Replies
3
Views
2K
Back
Top