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Linked List Delete Method

Could anyone explain why the following delete method does not work? It appears to create an infinite loop at the value I am trying to remove. It should loop through a linked list, delete all instances of the value passed to the method, and return the total number of nodes deleted (return num;).
public int delete(T value)
{
int num = 0;
ListNode<T> trav = head;
ListNode<T> next = head.getNext();
while(trav != null) {
if(trav.getValue().compareTo(value) == 0) {
trav = next;
num++;
}
if(next.getValue().compareTo(value) == 0) {
trav = next.getNext();
num++;
}
trav = trav.getNext();
}
return num;
}

You never change the value of next.
if(trav.getValue().compareTo(value) == 0) {
trav = next;
num++;
}
if(next.getValue().compareTo(value) == 0) {
trav = next.getNext();
num++;
}
Since next never changes, you're comparing the same value in the second if every time.

Find least common ancestor of two nodes in java

I have looked at a lot of other answers on stackoverflow and can't find anything that works, I either get the root, or node1 itself returned, I'm not sure how to do this recursively and have tried it many times all ending the same way. Any help would be greatly appreciated!
Here's my code:
private static Node findLCA(Node node1, Node node2) {
Node temp1 = node1, temp2 = node2, currentLargest = null;
int largestDepth = 0;
boolean found = false;
if(node1 == null || node2 == null){
return null;
} else{
while(found == false){
if(temp1.getParent() != null && temp2.getParent() != null && temp1.getParent() == temp2.getParent() && nodeDepth(temp1.getParent()) > largestDepth){
largestDepth = nodeDepth(temp1.getParent());
currentLargest = temp1;
temp1 = temp1.getParent();
temp2 = temp2.getParent();
} else if(temp1.getParent() != null){
temp1 = temp1.getParent();
} else if(temp2.getParent() != null){
temp2 = temp2.getParent();
}
if(temp1.getParent() == null && temp2.getParent() == null){
found = true;
}
}
if(nodeDepth(temp1) >= largestDepth){
return temp1;
} else{
return currentLargest;
}
}
}
I edited it to make a list of ancestors of each node, but I'm not sure how to go around checking each one to see if the elements in the list's match up since they are usually different sizes.
Heres the new code:
ArrayList<PhyloTreeNode> list1 = new ArrayList<PhyloTreeNode>();
ArrayList<PhyloTreeNode> list2 = new ArrayList<PhyloTreeNode>();
if(node1 == null || node2 == null){
return null;
} else{
if(node1.getParent() != null){
list1.add(node1.getParent());
findLeastCommonAncestor(node1.getParent(), node2);
}
if(node2.getParent() != null){
list2.add(node2.getParent());
findLeastCommonAncestor(node1, node2.getParent());
}
}

We can use recursive post order traversal for computing lowest common ancestor,
Here is my Java implementation
Here a & b are given input data for which i have to find lowest common ancestors.
public static int lowestcommanancestors(Node root,int a,int b){
if(root==null)
return 0;
int x=lowestcommanancestors(root.left,a,b);
int y=lowestcommanancestors(root.right,a,b);
if(x+y==2){
System.out.println(root.getData());
return 0;
}
if(root.getData()==a || root.getData()==b){
return x+y+1;
}
else{
return x+y;
}
}
First i am checking whether given input node presenting in left subtree or not,if yes just return 1 else 0,Similarly for right sub tree.When sum becomes 2 first time that node will be lowest common ancestors.
Tell me if i am wrong or you are getting difficulties to understanding the code

implementing binary search tree insert

I'm trying to write code for a binary search tree, the first method I'm working on is the add (insert) method. The root seems to insert properly, but I'm getting null pointer exception when adding the second node. I'll indicate the exact problem spot in my code with comments.
If you can see how to fix the bugs, or let me know if my overall logic is flawed it would be incredibly helpful.-- I will mention that this is for school, so I'm not looking to make a really impressive model...most of my layout choices simply reflect the way we've been working in class. Also, method names were selected by the teacher and should stay the same. Feel free to edit the formatting, had a little trouble.
BINARY TREE CLASS
public class BinarySearchTree
{
private static Node root;
public BinarySearchTree()
{
root = null;
}
public static void Add (Node newNode)
{
Node k = root;
if (root == null)//-----------------IF TREE IS EMPTY -----------------
{
root = newNode;
}
else // -------TREE IS NOT EMPTY --------
{
if (newNode.value > k.value) //-------NEW NODE IS LARGER THAN ROOT---------
{
boolean searching = true;
while(searching) // SEARCH UNTIL K HAS A LARGER VALUE
{ //***CODE FAILS HERE****
if(k.value > newNode.value || k == null)
{
searching = false;
}
else {k = k.rightChild; }
}
if ( k == null) { k = newNode;}
else if (k.leftChild == null){ k.leftChild = newNode;}
else
{
Node temp = k.leftChild;
k.leftChild = newNode;
newNode = k.leftChild;
if(temp.value > newNode.value )
{
newNode.rightChild = temp;
}
else
{
newNode.leftChild = temp;
}
}
}
if (newNode.value < k.value) //-----IF NEW NODE IS SMALLER THAN ROOT---
{
boolean searching = true;
while(searching) // ----SEARCH UNTIL K HAS SMALLER VALUE
{// **** CODE WILL PROBABLY FAIL HERE TOO ***
if(k.value < newNode.value || k == null) {searching = false;}
else {k = k.leftChild;}
}
if ( k == null) { k = newNode;}
else if (k.rightChild == null){ k.rightChild = newNode;}
else
{
Node temp = k.rightChild;
k.rightChild = newNode;
newNode = k.rightChild;
if(temp.value > newNode.value )
{
newNode.rightChild = temp;
}
else
{
newNode.leftChild = temp;
}
}
}
}} // sorry having formatting issues
}
NODE CLASS
public class Node
{
int value;
Node leftChild;
Node rightChild;
public Node (int VALUE)
{
value = VALUE;
}
}
TEST APPLICATION
public class TestIT
{
public static void main(String[] args)
{
BinarySearchTree tree1 = new BinarySearchTree();
Node five = new Node(5);
Node six = new Node(6);
tree1.Add(five);
tree1.Add(six);
System.out.println("five value: " + five.value);
System.out.println("five right: " + five.rightChild.value);
}
}

The conditional statement is checked from left to right, so you need to check whether k is null before you check whether k.value > newNode.value because if k is null, then it doesn't have a value.

NullPointerException with while loop using compareTo

For our homework, I have to take in Chair objects and add them to the DoublyLinkedList that we made; it has to be sorted by alphabetical order, if style is the same alphabetically, we sort by color
When I try to go through the loop, I keep getting a NullPointerException.
public void add(Chair element){
if(isEmpty() || first.object.style.compareTo(element.style) > 0 || (first.object.style.compareTo(element.style) == 0 && first.object.color.compareTo(element.color) >= 0){
addFirst(element);
}else if(first.object.style.compareTo(element.style) <= 0){
Node temp = first;
Node insert = new Node(); insert.object = element;
while(temp.object.style.compareTo(element.style) <= 0) //This is where the nullPointerException occurs
if(temp.hasNext())
temp = temp.next;
while(temp.object.style.compareTo(element.style) == 0 && temp.object.color.compareTo(element.color) <= 0)
if(temp.hasNext())
temp = temp.next;
insert.prev = temp.prev;
insert.next = temp;
temp.prev.next = insert;
temp.prev = insert;
}
}
This is the code where I put the information into the DoublyLinkedList
try{
FileReader fr = new FileReader(filename);
Scanner sc = new Scanner(fr);
String[] temp;
while(sc.hasNext()){
temp = sc.nextLine().split(" ");
if(temp[0].equals("Bed")){}
else if(temp[0].equals("Table")){
// tables.add(new Table(Integer.parseInt(temp[1]), Integer.parseInt(temp[2]), Integer.parseInt(temp[3]), temp[4]));
}else if(temp[0].equals("Desk")){}
else if(temp[0].equals("Chair")){
chairs.add(new Chair(temp[1], temp[2]));
}else if(temp[0].equals("Bookshelves")){}
else{
color = temp[0];
}
}
while(!chairs.isEmpty())
System.out.println(chairs.removeFirst().info());
System.out.println();
//while(!tables.isEmpty())
// System.out.println(tables.removeFirst().info());
}catch(Exception e){e.printStackTrace();}
This is the DoublyLinkedList class that I've made:
class CDoublyLinkedList{
Node first, last;
public CDoublyLinkedList(){
first = new Node(); last = new Node();
first.prev = last.next = null;
first.object = last.object = null;
first.next = last;
last.prev = first;
}
public boolean isEmpty(){
return first.object == null;
}
public void addFirst(Chair element){
Node insert = new Node();
insert.object = element;
insert.prev = null;
insert.next = first;
first.prev = insert;
first = insert;
}
public void add(Chair element){
if(isEmpty() || first.object.style.compareTo(element.style) > 0 || (first.object.style.compareTo(element.style) == 0 && first.object.color.compareTo(element.color) >= 0){
addFirst(element);
}else if(first.object.style.compareTo(element.style) <= 0){
Node temp = first;
Node insert = new Node(); insert.object = element;
while(first.object.style.compareTo(element.style) <= 0)
if(temp.hasNext())
temp = temp.next;
while(first.object.style.compareTo(element.style) == 0 && first.object.color.compareTo(element.color) <= 0)
if(temp.hasNext())
temp = temp.next;
insert.prev = temp.prev;
insert.next = temp;
temp.prev.next = insert;
temp.prev = insert;
}
}
public Chair removeFirst(){
Chair tobedeleted = first.object;
Node temp = first.next;
first = temp;
first.prev = null;
return tobedeleted;
}
private class Node{
Node next, prev;
Chair object;
public boolean hasNext(){
return next != null;
}
}
}
The Chair class:
class Chair extends Furniture{
public String style, color;
public Chair(String s, String c){
style = s; color = c;
}
public String toString(){
return color;
}
public String getType(){
return "Chair";
}
public String info(){
return (color+", "+style);
}
}
Can someone please explain to me why I keep getting this error? Thank you!
EDIT:
while(temp.object.style.compareTo(element.style) <= 0) //This is where the nullPointerException occurs
chairs.add(new Chair(temp[1], temp[2]));
java.lang.NullPointerException
at CDoublyLinkedList.add(Furnish2SS.java:119)
at Furnish2SS.main(Furnish2SS.java:23)
java.lang.NullPointerException
at CDoublyLinkedList.add(Furnish2SS.java:119)
at Furnish2SS.main(Furnish2SS.java:23)
EDIT2: SOLVED!
I changed my while loop to:
while(temp.object != null && element != null && (temp.object.compareTo(element) == 0 || temp.object.compareTo(element) == -1))
The reason I got the error was because I wasn't checking for null every iteration.

You say this is the line of code causing the exception:
while(temp.object.style.compareTo(element.style) <= 0)
You probably should set a debugger breakpoint on that line and use a debugger to determine which of the values is null. But it's hard for me to explain here full instructions on how to setup and use a debugger (that doesn't mean you shouldn't learn! You should. There are lots of tutorials. Google it.) So instead of writing a tutorial on debuggers, I'll just post code that will tell you which variable is null:
if (temp == null) {
System.out.println("temp is null");
} else if (temp.object == null) {
System.out.println("temp.object is null");
} else if (temp.object.style == null) {
System.out.println("temp.object.style is null");
}
if (element == null) {
System.out.println("element is null");
} else if (element.style == null) {
System.out.println("element.style is null");
}
while(temp.object.style.compareTo(element.style) <= 0) //This is where the nullPointerException occurs
{
if(temp.hasNext())
temp = temp.next;
if (temp == null) {
System.out.println("loop: temp is null");
} else if (temp.object == null) {
System.out.println("loop: temp.object is null");
} else if (temp.object.style == null) {
System.out.println("loop: temp.object.style is null");
}
if (element == null) {
System.out.println("loop: element is null");
} else if (element.style == null) {
System.out.println("loop: element.style is null");
}
}
If you use the above code statements to replace these three lines of your code:
while(temp.object.style.compareTo(element.style) <= 0) //This is where the nullPointerException occurs
if(temp.hasNext())
temp = temp.next;
you will know which variable is null based on which statement is printed. Hopefully you can take it from there. (The usual way to fix a NullPointerException is to take the steps necessary to ensure the offending null variable actually has a valid, non-null value by the time the program reaches the line of the NullPointerException).

Take a look at addFirst(Chair element). That method is really screwed up. It creates a new Node which contains the correct Chair. It then sets its prev to null. Then it sets next to first. And this is what's causing all of your troubles. Because first points to an empty Node. You end up with this:
first points to your new Node. That one points to a Node which holds no Chair. That one again points to last.
e:
Your whole code looks like you had at least two different approaches on implementing your list and threw them toghether. There are some more errors but since this is homework I guess it's not that bad if you try fixing it first.
If you can't figure out how to correct that, ask here.
PS: Sorry for all of the editing and (un)deleting my answeres (if you noticed). I'm a bit tired and kept causing new errors by fixing old ones until I finally figured out what was the true cause of all this.

Java: NPE in Circular Linked List :(

import javax.swing.JOptionPane;
public class RotateArrayCircularLL
{
private Node head=null;
public void init()
{
int choice = 0;
while (choice != -1){
choice = Integer.parseInt(JOptionPane.showInputDialog("Enter -1 to stop loop, 1 to continue"));
if(choice == -1)
break;
inputNum();
}
printList();
}
public void inputNum()
{
Node n;
Node temp;
int k;
k = Integer.parseInt(JOptionPane.showInputDialog(null,"Enter a number:"));
n = new Node(k);
if (head == null) {
head = n;
} else {
temp = head;
while (temp.getNext() != null)
temp = temp.getNext();
temp.setNext(n);
}
}
public void printList()
{
Node temp = head;
int count = Integer.parseInt(JOptionPane.showInputDialog("Enter the value to shift to the right"));
for (int i = 1; i <= count; i++) // Rotates the head
temp = temp.getNext();
for (Node c = temp; c != null && c.getNext() != head; c= c.getNext()){ // Prints the new LL
System.out.print(c.getInfo());
}
}
}
I get an NPE during the second for loop. I understand that it is giving me a NPE because I reach the end of the list, but how can I stop it from doing this?

It appears from the behavior you are seeing that one of the nodes in your linked list is returning null instead of the next element of the list. At a guess, I'd suggest that the last node of your list is probably not pointing to the first node of your list. Hence as Hovercraft Full Of Eels suggests, you don't really have a properly circular linked list. If you can post the code showing how temp is populated, it may be possible to give a more concrete solution to your issue. Otherwise, you need to treat the case where getNext() returns null as a special case and ensure that you instead get the first element from the initial list.