Question

Here’s the method

public void sortStudentsAlphabeticallyByFirstName()
{
    StudentNode unsorted = tail;
    StudentNode current = header;
    while(current != null)
    {
        while(current != unsorted)
        {
            int result = (current.nextNode().getFirstName()).compareToIgnoreCase(current.getFirstName());
            if(result < 0)
            {
                StudentNode temp = current;
                current = current.nextNode();
                current.setNext(temp);
            }
        }
        current = current.nextNode();
        unsorted = unsorted.prevNode();
    }
}

The problem is that when executed it just keeps running and doesn’t stop and I’m not sure where the problem is.

Answer 1

Consider our Link List has A, C, B and D nodes. say as you enter in your second while loop

current = C;

so using this code :

temp = current; // i.e. temp = C as current = C
current = current.next(); // say current = B now and temp = C
current.setNext(temp); // here B's next is set to C
                       // but you forgot A's next is C in the example, now since B 
                       // is taking it's place so A's next must point to B
                       // B's next must point to C and C's next must point to D.

So seems like you forgot these steps,

When you are moving current to the next node after that, temp and current will swap. But the one previous to temp i.e. A in the example must point to B, which is being swapped with C. Since B was pointing to D before, now after swapping C must point to D (this part you missed) and B must point to C (that’s what you did on the third line.)

EDIT
Whole working code has been added for more information.

import java.io.*;

class Node
{
public Node previous;
public String value;
public Node next;
}

public class LinkedList
{
private BufferedReader br ;
private String str; 
private int totalNodes;

private  Node current, previous, temp, head, tail; 

public LinkedList()
{
    br = new BufferedReader(new InputStreamReader(System.in));
    current = previous = temp = head = tail = null;
    totalNodes = 0;
}

public static void main(String[] args)
{
    LinkedList ll = new LinkedList();
    ll.menu();
}

private void menu()
{
    boolean flag = true;
    int choice = 0;
    while(flag)
    {
        System.out.println("--------------------------------------------------");
        System.out.println("---------------------MENU-----------------------");
        System.out.println("Press 1 : To ADD Node at the END.");
        System.out.println("Press 2 : To ADD Node at the BEGINNING.");
        System.out.println("Press 3 : To Add Node in BETWEEN    the List.");
        System.out.println("Press 4 : To  SORT the List");
        System.out.println("Press 5 : To DISPLAY the List.");
        System.out.println("Press 6 : To EXIT the Program.");
        System.out.println("--------------------------------------------------");
        System.out.print("Please Enter your choice here : ");
        try
        {
            str = br.readLine();
            choice = Integer.parseInt(str);
            if (choice == 6)
            {
                flag = false;
            }
            accept(choice);
        }
        catch(NumberFormatException nfe)
        {
            System.out.println("OUCH!, Number Format Exception, entotalNodesered.");
            nfe.printStackTrace();
        }
        catch(IOException ioe)
        {
            System.out.println("OUCH!, IOException, entotalNodesered.");
            ioe.printStackTrace();

        }
    }
}

private void accept(int choice)
{
    switch(choice)
    {
        case 1:
            addNodeToListAtStart();
            break;
        case 4:
            sortListBubble();
            break;
        case 5: 
            displayList();
            break;
        case 6:
            System.out.println("Program is Exiting.");
            break;
        default:
            System.out.println("Invalid Choice.\nPlease Refer Menu for further Assistance.");
    }
}   

private void addNodeToListAtStart()
{
    if (head != null)
    {
        current = new Node();
        System.out.print("Enter value for the New Node : ");
        try
        {
            str = br.readLine();
        }
        catch(NumberFormatException nfe)
        {
            System.out.println("OUCH!, Number Format Exception, entotalNodesered.");
            nfe.printStackTrace();
        }
        catch(IOException ioe)
        {
            System.out.println("OUCH!, IOException, entotalNodesered.");
            ioe.printStackTrace();              
        }
        current.previous = tail;
        current.value = str;
        current.next = null;
        tail.next = current;
        tail = current;
    }
    else if (head == null)
    {
        current = new Node();
        System.out.print("Enter value for the New Node : ");
        try
        {
            str = br.readLine();
        }
        catch(NumberFormatException nfe)
        {
            System.out.println("OUCH!, Number Format Exception, entotalNodesered.");
            nfe.printStackTrace();
        }
        catch(IOException ioe)
        {
            System.out.println("OUCH!, IOException, entotalNodesered.");
            ioe.printStackTrace();              
        }
        current.previous = null;
        current.value = str;
        current.next = null;            
        head = current;
        tail = current;
    }
    totalNodes++;
}

private void displayList()
{
    current = head;
    System.out.println("----------DISPLAYING THE CONTENTS OF THE LINKED LIST---------");
    while (current != null)
    {
        System.out.println("******************************************");
        System.out.println("Node ADDRESS is : " + current);
        System.out.println("PREVIOUS Node is at : " + current.previous);
        System.out.println("VALUE in the Node is : " + current.value);
        System.out.println("NEXT Node is at : " + current.next);
        System.out.println("******************************************");
        current = current.next;
    }
}

private boolean sortListBubble()
{
    // For Example Say our List is 5, 3, 1, 2, 4
    Node node1 = null, node2 = null; // These will act as reference. for the loop to continue
    temp = head;    // temp is set to the first node.   

    if (temp == tail || temp == null)
        return false;

    current = temp.next; // current has been set to second node.

    for (int i = 0; i < totalNodes; i++) // this loop will  run till whole list is not sorted.
    {
        temp = head; // temp will point to the first element of the list.
        while (temp != tail) // till temp won't reach the second last, as it reaches the last element loop will stop.
        {
            if (temp != null)
                current = temp.next;
            while (current != null) // till current is not null.
            {
                int result = (temp.value).compareToIgnoreCase(current.value); 
                if (result > 0) // if elment on right side is higher in value then swap.
                {
                    if (temp != head && current != tail) // if nodes are between the list.
                    {
                        current.previous = temp.previous;
                        (temp.previous).next = current;
                        temp.next = current.next;
                        (current.next).previous = temp;                     
                        current.next = temp;
                        temp.previous = current;
                    }
                    else if (current == tail) // if nodes to be swapped are second last and last(current)
                    {
                        temp.next = current.next;
                        current.previous = temp.previous;
                        if (temp.previous != null)
                            (temp.previous).next = current;
                        else
                            head = current;
                        temp.previous = current;
                        current.next = temp;
                        tail = temp;
                    }
                    else if (temp == head) // if the first two nodes are being swapped.
                    {
                        temp.next = current.next;                       
                        (current.next).previous = temp;
                        current.previous = temp.previous;
                        temp.previous = current;
                        current.next = temp;
                        head = current;
                    }   
                    current = temp.next; // since swapping took place, current went to the left of temp, that's why
                                                   // again to bring it on the right side of temp.
                }
                else if (result <= 0) // if no swapping is to take place, then this thing
                {
                    temp = current;  // temp will move one place forward
                    current = current.next; // current will move one place forward
                }                                       
            }
            if (temp != null)
                temp = temp.next;
            else // if temp reaches the tail, so it will be null, hence changing it manually to tail to break the loop.
                temp = tail;
        }
    }
    return true;
}
}

Hopefully that might help.

Regards