I thought I had understanding of this in my previous question about linked lists, but I was terribly mistaken, I'm just as lost as I was when I initially posted.
I realize that I'm technically asking two questions, but hopefully getting at least one should make the other easy (assuming they are just reverse of each other).
I have 3 classes already given to me, they are:
SLinkedList.java
package chapter3.linkedList;
public class SLinkedList<V> {
// instance variables. Add the tail reference.
protected Node<V> head, tail;
protected long size;
// methods, empty list constructor first
public SLinkedList () {
head = null;
tail = null;
size = 0;
} // end constructor of a SLinkedList
// method to add nodes to the list. Storage space for the node
// is already allocated in the calling method
public void addFirst (Node<V> node) {
// set the tail only if this is the very first node
if (tail == null)
tail = node;
node.setNext (head); // make next of the new node refer to the head
head = node; // give head a new value
// change our size
size++;
} // end method addFirst
// addAfter - add new node after current node, checking to see if we are at the tail
public void addAfter (Node<V>currentNode, Node<V>newNode) {
if (currentNode == tail)
tail = newNode;
newNode.setNext (currentNode.getNext ());
currentNode.setNext (newNode);
// change our size
size++;
} // end method addAfter
// addLast - add new node after the tail node. Adapted from Code Fragment 3.15, p. 118.
// Mike Qualls
public void addLast (Node<V> node) {
node.setNext (null);
tail.setNext (node);
tail = node;
size++;
} // end method addLast
// methods to remove nodes from the list. (Unfortunately, with a single linked list
// there is no way to remove last. Need a previous reference to do that. (See
// Double Linked Lists and the code below.)
public Node<V> removeFirst () {
if (head == null)
System.err.println("Error: Attempt to remove from an empty list");
// save the one to return
Node<V> temp = head;
// do reference manipulation
head = head.getNext ();
temp.setNext(null);
size--;
return temp;
} // end method removeFirst
// remove the node at the end of the list. tail refers to this node, but
// since the list is single linked, there is no way to refer to the node
// before the tail node. Need to traverse the list.
public Node<V> removeLast () {
// // declare local variables/objects
Node<V> nodeBefore;
Node<V> nodeToRemove;
// make sure we have something to remove
if (size == 0)
System.err.println("Error: Attempt to remove fron an empty list");
// traverse through the list, getting a reference to the node before
// the trailer. Since there is no previous reference.
nodeBefore = getFirst ();
// potential error ?? See an analysis and drawing that indicates the number of iterations
// 9/21/10. size - 2 to account for the head and tail nodes. We want to refer to the one before the
// tail.
for (int count = 0; count < size - 2; count++)
nodeBefore = nodeBefore.getNext ();
// save the last node
nodeToRemove = tail;
// now, do the pointer manipulation
nodeBefore.setNext (null);
tail = nodeBefore;
size--;
return nodeToRemove;
} // end method removeLast
// method remove. Remove a known node from the list. No need to search or return a value. This method
// makes use of a 'before' reference in order to allow list manipulation.
public void remove (Node<V> nodeToRemove) {
// declare local variables/references
Node<V> nodeBefore, currentNode;
// make sure we have something to remove
if (size == 0)
System.err.println("Error: Attempt to remove fron an empty list");
// starting at the beginning check for removal
currentNode = getFirst ();
if (currentNode == nodeToRemove)
removeFirst ();
currentNode = getLast ();
if (currentNode == nodeToRemove)
removeLast ();
// we've already check two nodes, check the rest
if (size - 2 > 0) {
nodeBefore = getFirst ();
currentNode = getFirst ().getNext ();
for (int count = 0; count < size - 2; count++) {
if (currentNode == nodeToRemove) {
// remove current node
nodeBefore.setNext (currentNode.getNext ());
size--;
break;
} // end if node found
// change references
nodeBefore = currentNode;
currentNode = currentNode.getNext ();
} // end loop to process elements
} // end if size - 2 > 0
} // end method remove
// the gets to return the head and/or tail nodes and size of the list
public Node<V> getFirst () { return head; }
public Node<V> getLast () { return tail; }
public long getSize () { return size; }
} // end class SLinkedList
There's also Node.java
package chapter3.linkedList;
public class Node<V>
{
// instance variables
private V element;
private Node<V> next;
// methods, constructor first
public Node ()
{
this (null, null); // call the constructor with two args
} // end no argument constructor
public Node (V element, Node<V> next)
{
this.element = element;
this.next = next;
} // end constructor with arguments
// set/get methods
public V getElement ()
{
return element;
}
public Node<V> getNext ()
{
return next;
}
public void setElement (V element)
{
this.element = element;
}
public void setNext (Node<V> next)
{
this.next = next;
}
} // end class Node
and finally GameEntry.java
package Project_1;
public class GameEntry
{
protected String name; // name of the person earning this score
protected int score; // the score value
/** Constructor to create a game entry */
public GameEntry(String name, int score)
{
this.name = name;
this.score = score;
}
/** Retrieves the name field */
public String getName()
{
return name;
}
/** Retrieves the score field */
public int getScore()
{
return score;
}
/** Returns a string representation of this entry */
public String toString()
{
return name + ", " + score + "\n";
}
}
EDIT POINT
I created a driver called Scores.java, in it so far all I have is **I have added what I THINK i need for the classes, I'm probably wrong though:
package Project_1;
import chapter3.linkedList.*;
import java.util.*;
/** Class for storing high scores in an array in non-decreasing order. */
public class Scores
{
//add function
public SLinkedList<GameEntry> add(GameEntry rank, SLinkedList<GameEntry> scores)
{
Node<GameEntry> currentNode = scores.getFirst();
Node<GameEntry> nextNode = null;
Node<GameEntry> previousNode = null;
Node<GameEntry> newNode = new Node<GameEntry>();
newNode.setElement(rank);
if(scores.getSize() == 0)
{
scores.addFirst(newNode);
}
else
{
while(currentNode != null)
{
nextNode = currentNode.getNext();
if(nextNode == null)
{
scores.addLast(newNode);
}
else
{
scores.addAfter(currentNode, newNode);
break;
}
previousNode = currentNode;
currentNode = currentNode.getNext();
}
}
return scores;
}
//remove function
public void remove(int i)
{
}
//print function
/*gameenter printing;
printing=node.Getelement; //pseudo code for making it work right
print(printing.getscore)
print(print.getname)
*/
public void print(SLinkedList<GameEntry> scores)
{
Node<GameEntry> currentNode = scores.getFirst();
GameEntry currentEntry = currentNode.getElement();
System.out.printf("[");
for(int i = 0; i < scores.getSize(); i++)
{
System.out.printf(", %s", currentEntry.toString());
currentNode = currentNode.getNext();
currentEntry = currentNode.getElement();
}
System.out.println("]");
}
}
I have my test driver called ScoresTest.java, that I have pretty much filled out:
package Project_1;
import chapter3.linkedList.SLinkedList;
public class ScoresTest {
/**
* #param args
*/
public static void main(String[] args)
{
SLinkedList<GameEntry> highScores = new SLinkedList<GameEntry>(); //Linked List for Game Entry
GameEntry entry;
Scores rank = new Scores();
entry = new GameEntry("Flanders", 681);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Krusty", 324);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Otto", 438);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Bart", 875);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Homer", 12);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Lisa", 506);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Maggie", 980);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Apoo", 648);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Smithers", 150);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Burns", 152);
highScores = rank.add(entry, highScores);
System.out.println("The Original High Scores");
rank.print(highScores);
entry = new GameEntry("Moe", 895);
highScores = rank.add(entry, highScores);
System.out.println("Scores after adding Moe");
rank.print(highScores);
//highScores = rank.remove(4);
System.out.println("Scores after removing Apoo");
rank.print(highScores);
}
}
That's entirely finished, pretty sure I have nothing left to add for it.
I'm not looking for someone to answer it for me, but I have no clue where to start or how to make the add or remove function, in any way. This is an intermediate course, the book does nothing for explaining linked lists (go ahead and look for yourself if you don't believe me, text is called Datastructures and Algorithms in Java, 5th edition). It shows how to do such with an array quite easily...which works perfectly for a linked list, but apparently the teacher does not want us doing it this way, so sadly I am now utterly lost on how to do this.
I've tried looking at other peoples answers on here, and google, and so far nothing has clicked or made any sense at all, I just can't grasp how it works, and the teacher's explanation and example was only to draw boxes on the board, I've never seen a sort, add, or remove function coded for a linked list...can't know what I've not been taught or can't locate.
Any help is greatly appreciated, and thank you in advance!
EDIT
I looked at the import java.util.*; and the commands within it for linked lists, they seem painfully easy. to remove I'd just use list.sublist(i, i).clear(); and the value I wish to remove is removed, super easy, it seems to be just trying to utilize the slinkedlist.java and node.java, I just can't seem to follow them in any way shape or form. I believe the teacher did indeed write them, and I've tried asking for his assistance, stayed 2 hours after class trying to get any understanding from him, and as you can see it did not help much at all. Thank you again for the assistance!
EDIT
I also apologize if this seems like it is vague, but I don't have a specific point where my confusion seems linked, I understand linked lists if we're talking about the java.util.linkedList;, but as far as using what I've been given in this circumstance, I can't follow the logic at all, leaving me quite lost and unsure of where to begin.
In pseudo-code (please note I am not including bound checking etc, simply the logic)
To add a node to the front of the list:
newNode->nextNode = startNode
startNode = newNode
To add to a specific index:
index = 0
currentNode = startNode
// find the node in the list. here you will need to do all kinds of bound checking
while index is less than position
currentNode = currentNode.nextNode // move your node pointer to the position
increment index
// so here we basically insert the new node into the list. what needs to happen is
// to NOT break the list by forgetting the node after the current node. this is why
// we first set the new nodes' next one, to the current nodes' (the one already in
// the list) next node. this way, we still have all the information we need. then,
// when we set the current nodes' next node to the new node, we essentially "break"
// the link and "repair" it by adding the new link.
newNode.nextNode = currentNode.nextNode // some more bound checking required
currentNode.nextNode = newNode
To remove from a specific index:
index = 0
delNode = startNode
// find the node in the list. here you will need to do all kinds of bound checking
while index is less than (position - 1)
delNode = delNode.nextNode // move your node pointer to the position
increment index
delNode.nextNode = delNode.nextNode.nextNode
// that's it. by setting the node's (before the one you whish to delete)
// next node to the node AFTER the one you want to delete, you basically
// "skip" over that node. since it is no longer referenced, the garbage
// collector will take care of the rest. if you wish to return that node
// you can do it quite easily by remembering it.
storeNode = delNode.nextNode // some more bound checking required
delNode.nextNode = delNode.nextNode.nextNode // some more bound checking required
// now you still have a reference to the deleted node in storeNode
UPDATE
OK, so if I understand correctly, you need to create a linked list that stores scores in a ascending order. As far as I can see, the entire linked list has been implemented for you, you simply need to use the classes provided, and add the logic in Scores.java to keep the list sorted.
First off, I see your nodes are not comparable. If you are at all allowed to change the source given to you, I would suggest having them implement Comparable<Node> and also override the equals(Object o) so that you have logic to compare them. Two nodes can contain the same element, but that does not mean that they are equal.
Please note the change in the method signatures!
//add function
public void add(Node<GameEntry> score) {
// adding is where you now want to keep everything sorted. so I highly
// recommend that you implement `Comparable` as I mentioned above. if not,
// you have to put the logic in here.
Node<GameEntry> currentNode = highScored.getFirst();
Node<GameEntry> prevNode = null;
// if the list is empty, or the new node must go in before the head,
// simply add it as the head.
if (highScores.size() == 0 || score.compareTo(currentNode) < 0) {
highScores.addFirst(score);
}
// search for the position of the new node. while the node has a higher score
// than the current node, we need to continue on so we can place it in the
// correct place.
while (currentNode != null && currentNode.compareTo(score) > 0) {
prevNode = currentNode;
currentNode = currentNode.getNext();
}
// if the currentNode is null, it means it is the highest score, so
// we can simply add it to the end
if (currentNode == null) {
highScores.addLast(score);
} else {
// otherwise just add it after the correct node
highScores.addAfter(prevNode, score);
}
}
//remove function
public void remove(Node<GameEntry> score) {
// removing an element should be as described above. if you keep
// your list sorted during the ADD method, removing any element
// should not break the order.
// find the element - removal from a linked list is O(n),
// since we need to know what the element BEFORE the one
// is that you want to remove. assuming you have implemented
// the equals method to check equality of nodes:
Node<GameEntry> currentNode = highScores.getFirst();
Node<GameEntry> prevNode = null;
while (currentNode != null && !currentNode.equals(score)) {
prevNode = currentNode;
currentNode = currentNode.getNext();
}
// if currentNode is null, the node we wanted to remove was not
// in the list.
if (currentNode == null) {
System.out.println("Node not found");
return;
}
// now, we need to check if there is a node after the one we want
// to remove.
if (prevNode.getNext().getNext() != null) {
// if there is, we follow the logic from the pseudo code
prevNode.setNext(prev.getNext().getNext());
} else {
// if not, we only need to remove the last entry (since the
// one we want to remove is the last one)
highScores.removeLast();
}
}
IMPORTANT
Please just double check the logic here. I did it really quickly without an IDE as I'm not at my development computer at the moment. If anyone finds any issues, please leave a comment and I'll fix it.
If this is not exactly what you asked (your question is a bit vague), let me know.
UPDATE 2
Read up on Comparators here, here and here.
Related
I have a LinkedList full of Book objects which are stored in Nodes, which contain the year of publication and the title of the book. I am trying to sort the list so that it is arranged from oldest year to most recent year, however, after the first Book, the rest of my books now have the same title and year of publication. I believe it has something to do with when I make the swap using my setBook method.
public void sortByYear(){
Node current = head;
Node next = null;
if(isEmpty()){ //if the head is null
return;
}
while(current != null){
next = current.getNext();
while(next != null){
if(current.getBook().getYear() > next.getBook().getYear()){
Book temp = current.getBook();
current.setBook(next.getBook().getYear(), next.getBook().getTitle());
next.setBook(temp.getYear(), temp.getTitle());
// current.getBook() = next;
// next.getBook() = temp;
}
next = next.getNext();
}
current = current.getNext();
}
}
A few issues:
Your setBook method -- taking a year and title -- seems to mutate the book that is assigned to the node. This is not really the right approach. You should define a method that leaves the books untouched, but assigns a different book (all together) to a node.
Bubble sort will always start the inner loop from the first item in the list. The outer loop would only serve to count the number of times the inner loop has to kick in. This is not how you have implemented it. If the minimum node is not in the first or second place, your algorithm will never move it all the way back to the first position.
I would suggest to take a different approach and only mutate the next references in the list, so that you swap nodes as a whole, without having to touch the data at all. For this to work, you need a prev reference that walks "behind" the current node, so that you can rewire the preceding node to the node that gets swapped with the current node.
Here is an implementation:
public void sortByYear(){
if (head == null || head.getNext() == null) {
return;
}
for (Node iter = head; iter != null; iter = iter.getNext()) {
Node prev = null;
Node current = head;
for (Node next = current.getNext(); next != null; next = current.getNext()) {
if (current.getBook().getYear() > next.getBook().getYear()) {
if (prev == null) {
head = next;
} else {
prev.setNext(next);
}
current.setNext(next.getNext());
next.setNext(current);
prev = next;
} else {
prev = current;
current = next;
}
}
}
}
Something you could still work on
It would become even nicer if you would make your Node class comparable. That way the if statement can become more generic and less tied to your book-related implementation.
Hello i'm trying to delete a node based on a key. I'm learning dictionary implementation and decided to implement it from scratch to fully understand the concept. i successfully was able to add and return the value using 2 node references a head and tail pointer. But i'm having difficulty using the key to delete a node from the list.
Below is my code to delete from the list
public V remove(K key) {
V result = null;
if(!this.isEmpty()&&head==tail){ //if head and tail are only nodes in the list
if(tail.getKey().equals(key)){
result = tail.getValue();
head=null;
tail = null;
count--; //decrements the count in the list
}
}
else {
boolean found = false;
Node current = head;
Node previous = null;
while(current!=null&&!found){
if(current.getKey().equals(key)){
previous = current;
result = current.getValue();
previous.setNextNode(current.getNextNode());
previous.setNextNode(null);
count--;
found = true;
}
current = current.getNextNode();
}
}
return result;
}
when i enter the desired key to be deleted. It deletes all the keys after the desired key to be deleted.
PS it's not a double linked list. I just created a tail node to access the last node in the list
It seems that you are getting bogged down with updating the list. This code simplifies your algorithm:
// this method returns the head of the new list with the item possibly removed
public Node remove (K key) {
V result = null;
if (this.isEmpty()) {
return null; // return null for empty list
} else if (head.getKey().equals(key)) {
return head.getNextNode(); // return head if head matches
} else {
Node current = head;
Node next = head.getNextNode();
while (next != null) { // walk down the list and search
if (next.getKey().equals(key)) {
current.setNextNode(next.getNextNode());
break;
}
current = next; // advance list pointers
next = next.getNextNode();
}
return head; // will return head for list with 1 element
}
}
You got the general idea, except for 2 things: previous = current should be done outside the if block so that it always gets assigned before moving current forward, and previous.setNextNode(null) should be removed as it undoes the preceding line.
Also, you need to make a special case when the first node in the list matches the key so you can reassign the head.
while(current!=null&&!found){
if(current.getKey().equals(key)){
result = current.getValue();
previous.setNextNode(current.getNextNode());
count--;
found = true;
}
previous = current;
current = current.getNextNode();
}
You have two errors:
You set previous = current;, as the first statement, meaning previous and current are always the same.
You should remove this line:
previous.setNextNode(null); As you do it right after assigning the new next node.
I need to implement a circular single-linked list data structure. What I'm having trouble understanding is when and where do I have to state that the final node of the list must point to the first node. I have the following empty constructor to build the list:
public class SList<E> implements IList<E> {
protected SNode<E> firstNode = null;
public SList() {
firstNode = null;
}
So basically each list starts with a null object that points to null again, signifying the end of the list:
public class SNode<E> {
E elem;
public SNode<E> nextNode = null;
...
What I am unaware of however is how to make it so that when the list contains at least one node, that node will point to the first node of the list.
For example, taking a look at this addLast() method that I implemented for my regular linked list:
public void addLast(E newElem) {
SNode<E> newNode= new SNode<E>(newElem);
SNode<E> nodeTraveler = firstNode;
while (nodeTraveler.nextNode != null) {
nodeTraveler= nodeTraveler.nextNode;
}
nodeTraveler.nextNode = newNode;
}
I would have to change it into something like:
public void addLast(E newElem) {
SNode<E> nodeTraveler = firstNode;
SNode<E> newNode = new SNode<E>(newElem);
while (nodeTraveler.nextNode != firstNode){
nodeTraveler = nodeTraveler.nextNode;
}
nodeTraveler.nextNode = newNode;
newNode.nextNode = firstNode;
}
nodeTraveler would stop traversing the list once it's next node matched that of the firstNode (meaning it's at the last position), and then it would change its nextNode to the one we want to add, newNode, and point newNode to the firstNode.
Theoretically, that should work, but since I'm never actually pointing the last element to the first one prior to this method (which is my main question, how to point the last element to the first one by default), I get a nullPointer exception when the code reaches the while iteration.
Any help would be appreciated, thanks.
The only special case is when firstNode == null, which is the initial case.
After that, the first add will give a node whose next element is self:
public void addLast(E newElem) {
SNode<E> newNode = new SNode<E>(newElem);
if(firstNode == null) {
firstNode = newNode;
} else {
SNode<E> traveler = firstNode;
for( ; traveler.nextNode != firstNode ; traveler = traveler.nextNode) {}
traveler.nextNode = newNode;
}
newNode.nextNode = firstNode;
}
"RemoveAll" class is a part of Linked List class. The class I wrote, removes all the keys from a Linked List but it doesn't remove duplicate keys.
Does anybody know why? How I can remove duplicate keys too?
public class LinkedIntList {
private ListNode front;
private String name = "front";
// Constructs an empty list.
public LinkedIntList() {
front = null;
}
public void removeAll(int key){
if(front == null){
throw new RuntimeException();
}else if( front.data == (key)) {
front = front.next;
return;
}
ListNode cur = front;
ListNode prev = null;
while(cur != null && cur.data != (key) ){
prev = cur;
cur = cur.next;
}
if(cur == null)
throw new RuntimeException();
prev.next = cur.next;
}
If you have duplicates, a brute force approach is to keep calling removeAll() until it returns false.
while (myCollection.removeAll(someOthercollection))
; // comment here that the loop does nothing for clarity
ADDED: as #Dukeling correctly points out, you shouldn't have to do this. But if the removeALl() is poorly implemented you may have to.
Well, your method only finds the first matching key, then attempts to remove that item from the list, then returns. Once you find the first match, you're going to have to loop, removing items until you find a key that does not match (or you hit the end of the list).
public void removeAll(T value) {
if (this.front == null) {
return;
}
Node<T> prevNode = null;
Node<T> currNode = this.front;
// If the node we want to remove is front node...
while (currNode.data.equals(value)) {
prevNode = currNode;
currNode = currNode.next;
prevNode.next = null;
}
this.front = currNode;
while (currNode != null) {
if (currNode.data.equals(value)) {
prevNode.next = currNode.next;
currNode.next = null;
} else {
prevNode = currNode;
}
currNode = prevNode.next;
}
}
You need to remove nodes while data == key, so that you make sure to remove all occurrences, not just the first one.
In the lines else if( front.data == (key)) { front = front.next; you are only removing the first occurence. It should be while ( front.data == (key)) { front = front.next;
Here again you are only removing one node:
prev.next = cur.next
You need a while cur.data == key block to adjust the prev and cur nodes to the last occurence of the matching key before calling prev.next = cur.next;. Otherwise you'll be only deleting the first occurrence.
I have a problem with my code, I have made a singly linked list class in which you can add, remove, modify, merge etc... however, I am attempting a simple bubble sort and have come across problems in which the list is not correctly sorted. here are some things to note:
it is a custom implementation of a linked list
the nodes of the singly linked list contain 2 things: a CustomerFile object with all the data for a customer and a 'next' node pointer to the next item in the list
the list is sorted in ascending order (A-Z) by the surname stored in the customer file of each node
the add record function inserts the nodes at the correct position in the list so that the list does not need to be sorted initially - however if the surname is changed, as part of the program, the list needs to be sorted again
I would rather not create a new list and re-use this insert record on that list to create a new list as this is memory intensive and my task is to be as efficient as possible
the very structure of the linked list cannot be changed - it is decided and I am too far to change to something such as an array
the list has a head node, it has next items but does not have a tail node. It has an appointed NULL next pointer to indicate the end pf the list
the code
public static void sortList()
{
if (isEmpty() == true)
{
System.out.println("Cannot sort - the list is empty");
}
else if (getHead().getNext() == null)
{
System.out.println("List sorted");
}
else
{
Node current = getHead().getNext();
CustomerFile tempDat;
boolean swapDone = true;
while (swapDone)
{
current = getHead().getNext();
swapDone = false;
while (current != null)
{
if (current.getNext() != null &&
current.getData().getSurname().compareTo(
current.getNext().getData().getSurname()) >0)
{
tempDat = current.getData();
current.setData(current.getNext().getData());
current.getNext().setData(tempDat);
swapDone = true;
}
current = current.getNext();
}
}
if (getHead().getData().getSurname().compareTo(
getHead().getNext().getData().getSurname()) >0)
{
current = getHead().getNext();
getHead().setNext(current.getNext());
setHead(current);
}
}
}
I would appreciate the feedback
Your code is an odd mixture, mostly swapping data but treating the head specially trying to swap it with the next by switching the links.
As noted in another answer, this last swap is not correctly implemented, but really there's no reason to treat the head specially if you're doing things by swapping the data.
All you have to do is start each traversal of the list at the head in the outer loop.
This yields a working solution:
public void sortList()
{
if (isEmpty())
{
System.out.println("An empty list is already sorted");
}
else if (getHead().getNext() == null)
{
System.out.println("A one-element list is already sorted");
}
else
{
Node current = getHead();
boolean swapDone = true;
while (swapDone)
{
swapDone = false;
while (current != null)
{
if (current.getNext() != null && current.getData().getSurname().compareTo(current.getNext().getData().getSurname()) >0)
{
CustomerFile tempDat = current.getData();
current.setData(current.getNext().getData());
current.getNext().setData(tempDat);
swapDone = true;
}
current = current.getNext();
}
current = getHead();
}
}
}
I've made this non-static because as noted in comments it wasn't clear to me how yours could work as a static. You may be able to make it static in your context.
I also changed a couple other minor things. It's never necessary to check a condition by code like
if (isEmpty() == true)
In Java, this is entirely equivalent to
if (isEmpty())
And tempDat doesn't need to be declared outside of where it's used.
I think the main issue here is that you start with
current = getHead().getNext()
With this code you will leave the head completely out of the sorting process and it could be the case that the data in here needs to go to the other end of the list, but in this case it will always be either the data in the head element, or be the data directly after the head node (the latter is due to your if statement at the end).
Try starting from the head of the list instead and see what that brings up.
You are not including the head in your sorting.
public static void sortList()
{
if (isEmpty() == true)
{
System.out.println("Cannot sort - the list is empty");
}
else if (getHead().getNext() == null)
{
System.out.println("List sorted");
}
else
{
Node current = getHead();
// Node current = getHead().getNext();
CustomerFile tempDat;
boolean swapDone = true;
while (swapDone)
{
current = getHead().getNext();
swapDone = false;
while (current != null)
{
if (current.getNext() != null && current.getData().getSurname().compareTo(current.getNext().getData().getSurname()) >0)
{
tempDat = current.getData(); // td -> objectA, cd -> objectA
current.setData(current.getNext().getData()); // cd -> objectB
current.getNext().setData(tempDat); // nd -> td -> objectA
swapDone = true;
}
current = current.getNext();
}
}
//if (getHead().getData().getSurname().compareTo(getHead().getNext().getData().getSurname()) >0)
//{
// current = getHead().getNext(); // current -> head+1
// getHead().setNext(current.getNext()); //head+1 -> head+2
// setHead(current); // head -> head+1
//}
}
}
Also there is a error in the commented out code above. Applying that code drops a node.
// list: a -> b -> c -> d
current = getHead().getNext(); // current = b
getHead().setNext(current.getNext()); // a -> c
setHead(current); // head = b
// list: b -> c -> d
// and nothing points to 'a' anymore
Rather then swapping data you should try swapping nodes. If you go about that approach you should find a better solution.