Question: SWAPPING NODES IN A SINGULARLY LINKED LIST: I am attempting to create a program that swaps 2 nodes (no matter where they are in the list) and am having some difficulty. I can't seem to figure out why this swap method is throwing an error.

Code: (SWAPPING METHOD AND TEST LINE BOLDED)

package linkedlists;

public class SinglyLinkedList implements Cloneable { //---------------- nested Node class ---------------- /** * Node of a singly linked list, which stores a reference to its * element and to the subsequent node in the list (or null if this * is the last node). */ private static class Node {

/** The element stored at this node */ private E element; // reference to the element stored at this node

/** A reference to the subsequent node in the list */ private Node next; // reference to the subsequent node in the list

/** * Creates a node with the given element and next node. * * @param e the element to be stored * @param n reference to a node that should follow the new node */ public Node(E e, Node n) { element = e; next = n; }

// Accessor methods /** * Returns the element stored at the node. * @return the element stored at the node */ public E getElement() { return element; }

/** * Returns the node that follows this one (or null if no such node). * @return the following node */ public Node getNext() { return next; }

// Modifier methods /** * Sets the node's next reference to point to Node n. * @param n the node that should follow this one */ public void setNext(Node n) { next = n; } } //----------- end of nested Node class -----------

// instance variables of the SinglyLinkedList /** The head node of the list */ private Node head = null; // head node of the list (or null if empty)

/** The last node of the list */ private Node tail = null; // last node of the list (or null if empty)

/** Number of nodes in the list */ private int size = 0; // number of nodes in the list

/** Constructs an initially empty list. */ public SinglyLinkedList() { } // constructs an initially empty list

// access methods /** * Returns the number of elements in the linked list. * @return number of elements in the linked list */ public int size() { return size; }

/** * Tests whether the linked list is empty. * @return true if the linked list is empty, false otherwise */ public boolean isEmpty() { return size == 0; }

/** * Returns (but does not remove) the first element of the list * @return element at the front of the list (or null if empty) */ public E first() { // returns (but does not remove) the first element if (isEmpty()) return null; return head.getElement(); }

/** * Returns (but does not remove) the last element of the list. * @return element at the end of the list (or null if empty) */ public E last() { // returns (but does not remove) the last element if (isEmpty()) return null; return tail.getElement(); }

// update methods /**swapNodes * @param positionOfNode1 * @param positionOfNode2 */

public void swapNodes(int positionOfNode1, int positionOfNode2){ if (positionOfNode1 >= size() || positionOfNode2 >= size()) { throw new IndexOutOfBoundsException(); } if(positionOfNode1==positionOfNode2)return;//Don't do anything if they're the same Node previousOfNode1 = null; Node previousOfNode2 = null; Node node1=null; Node node2=null; Node currentNode = head;

//I have made a slight change here . i searched for the nodes in the list individually.The reason is that i could figure

//out that any of them is head or not. int position = 1; while(currentNode!=null && position!=positionOfNode1){ previousOfNode1=currentNode; currentNode=currentNode.getNext(); }

node1=currentNode; //Till this point i know which node is node1 and which node is previousOfNode1.

currentNode=null;

position=1;

while(currentNode!=null && position!=positionOfNode2){ previousOfNode2=currentNode; currentNode=currentNode.getNext(); } node2=currentNode; //The next two if will have there else part run only in case when one of them is head.

//If node 1 is head ,set the node 2 as head (done in else part),else follow the normal procedure of switiching the links of previous node of 1 . if(previousOfNode1!=null) { previousOfNode1.setNext(node2); } else { head=node2; }

//If node 1 is head ,set the node 2 as head (done in else part),else follow the normal procedure of switiching the links of previous node of 1 . if(previousOfNode2!=null) { previousOfNode2.setNext(node1); } else { head=node1; } Node temp =node1.getNext(); node1.setNext(node2.getNext()); node2.setNext(temp); Node curr=head; ///to set the tail pointer to the actual end of the list while(curr!=null && curr.getNext()!=null) { curr=curr.getNext(); } tail=curr; }

/** * Adds an element to the front of the list. * @param e the new element to add */ public void addFirst(E e) { // adds element e to the front of the list head = new Node<>(e, head); // create and link a new node if (size == 0) tail = head; // special case: new node becomes tail also size++; }

/** * Adds an element to the end of the list. * @param e the new element to add */ public void addLast(E e) { // adds element e to the end of the list Node newest = new Node<>(e, null); // node will eventually be the tail if (isEmpty()) head = newest; // special case: previously empty list else tail.setNext(newest); // new node after existing tail tail = newest; // new node becomes the tail size++; }

/** * Removes and returns the first element of the list. * @return the removed element (or null if empty) */ public E removeFirst() { // removes and returns the first element if (isEmpty()) return null; // nothing to remove E answer = head.getElement(); head = head.getNext(); // will become null if list had only one node size--; if (size == 0) tail = null; // special case as list is now empty return answer; }

@SuppressWarnings({"unchecked"}) public boolean equals(Object o) { if (o == null) return false; if (getClass() != o.getClass()) return false; SinglyLinkedList other = (SinglyLinkedList) o; // use nonparameterized type if (size != other.size) return false; Node walkA = head; // traverse the primary list Node walkB = other.head; // traverse the secondary list while (walkA != null) { if (!walkA.getElement().equals(walkB.getElement())) return false; //mismatch walkA = walkA.getNext(); walkB = walkB.getNext(); } return true; // if we reach this, everything matched successfully }

@SuppressWarnings({"unchecked"}) public SinglyLinkedList clone() throws CloneNotSupportedException { // always use inherited Object.clone() to create the initial copy SinglyLinkedList other = (SinglyLinkedList) super.clone(); // safe cast if (size > 0) { // we need independent chain of nodes other.head = new Node<>(head.getElement(), null); Node walk = head.getNext(); // walk through remainder of original list Node otherTail = other.head; // remember most recently created node while (walk != null) { // make a new node storing same element Node newest = new Node<>(walk.getElement(), null); otherTail.setNext(newest); // link previous node to this one otherTail = newest; walk = walk.getNext(); } } return other; }

public int hashCode() { int h = 0; for (Node walk=head; walk != null; walk = walk.getNext()) { h ^= walk.getElement().hashCode(); // bitwise exclusive-or with element's code h = (h << 5) | (h >>> 27); // 5-bit cyclic shift of composite code } return h; }

/** * Produces a string representation of the contents of the list. * This exists for debugging purposes only. */ public String toString() { StringBuilder sb = new StringBuilder("("); Node walk = head; while (walk != null) { sb.append(walk.getElement()); if (walk != tail) sb.append(", "); walk = walk.getNext(); } sb.append(")"); return sb.toString(); } //main method public static void main(String[] args) { SinglyLinkedList list = new SinglyLinkedList(); list.addFirst("MSP"); list.addLast("ATL"); list.addLast("BOS"); list.addLast("BOT");

// list.addFirst("LAX"); System.out.println(list); //swap nodes & print them list.swapNodes(2,4); System.out.println(list); } }