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How to modify the head of a LinkedList without a wrapper class?

When I create a Node object and call "appendToTail" the Node object has a sequence of nodes via the next attribute (as expected). I tried creating a pop, where it takes the head (aka 'this') and reference it with a variable and overwrite it with its next. However, 'this' remains the same as the original head. What am I doing wrong, or is there no way to modify 'this'?
public class Node {
Node next = null;
int data;
public Node(int d) {
data = d;
}
public void appendToTail(int d) {
Node end = new Node(d);
Node n = this;
while (n.next != null) {
n = n.next;
}
n.next = end;
}
public void popHead() {
Node n = this;
n = n.next;
}
}

Basically you need to construct a custom List and add each node at End. Also you need to store first node in order to have the starting point for looping.
public class NodeList
{
Node head=null;
public static void main(String args[])
{
NodeList nl = new NodeList();
nl.addNode(1);
nl.addNode(2);
nl.addNode(3);
nl.listNodes();
}
public void addNode(int data)
{
if(head==null)
{
head = new Node(data);
}
else
{
Node curent = head;
while(curent.next != null)
{
curent = curent.next;
}
curent.next = new Node(data);
}
}
public void listNodes()
{
if(head !=null)
{
Node curent = head;
System.out.println(curent.data);
while(curent.next !=null)
{
curent = curent.next;
System.out.println(curent.data);
}
}
}
class Node
{
Node next = null;
int data;
public Node(int d) {
data = d;
}
}
}
Output
1
2
3

Make an int search(Object o) method for a stack that uses nodes

I'm trying to make a generic stack and queue class that uses the generic node class. It has empty(), pop(), peek(), push(), and a search() method. I know there is a built-in Stack class and stack search method but we have to make it by using the Node class.
I am unsure of how to make the search method. The search method is supposed to return the distance from the top of the stack of the occurrence that is nearest the top of the stack. The topmost item is considered to be at distance 1; the next item is at distance 2; etc.
My classes are below:
import java.io.*;
import java.util.*;
public class MyStack<E> implements StackInterface<E>
{
private Node<E> head;
private int nodeCount;
public static void main(String args[]) {
}
public E peek() {
return this.head.getData();
}
public E pop() {
E item;
item = head.getData();
head = head.getNext();
nodeCount--;
return item;
}
public boolean empty() {
if (head==null) {
return true;
} else {
return false;
}
}
public void push(E data) {
Node<E> head = new Node<E>(data);
nodeCount++;
}
public int search(Object o) {
// todo
}
}
public class Node<E>
{
E data;
Node<E> next;
// getters and setters
public Node(E data)
{
this.data = data;
this.next = null;
}
public E getData() {
return data;
}
public void setData(E data) {
this.data = data;
}
public Node<E> getNext() {
return next;
}
public void setNext(Node<E> next) {
this.next = next;
}
}
public class MyQueue<E> implements QueueInterface<E>
{
private Node<E> head;
private int nodeCount;
Node<E> rear;
public MyQueue()
{
this.head = this.rear = null;
}
public void add(E item){
Node<E> temp = new Node<E>(item);
if (this.rear == null) {
this.head = this.rear = temp;
return;
}
this.rear.next = temp;
this.rear = temp;
}
public E peek(){
return this.head.getData();
}
public E remove(){
E element = head.getData();
Node<E> temp = this.head;
this.head = this.head.getNext();
nodeCount--;
return element;
}
}
After working on it based off of the first comment I have this:
public int search(Object o){
int count=0;
Node<E> current = new Node<E> (head.getData());
while(current.getData() != o){
current.getNext();
count++;
}
return count;
}
It doesn't have any errors but I cannot tell if it is actually working correctly. Does this seem correct?

It needs the following improvements,
search method should have parameter of type 'E'. So, the signature should look like public int search(E element)
start the count with 1 instead of 0.As you have mentioned topmost item is considered to be at distance 1
initialize current with head, because creating a new node with data value of head(new node(head.getData())) will create an independent node with data same as head node; and the while will run only for the head node as current.getNext() will be null always. Node<E> current = head will create another reference variable pointing to the head.
Instead of != in condition, use if( !current.getData().equals(element.getData())) )
If using your own class as data type, don't forget to override equals method.
Change current.getNext(); to current = current.getNext();

You have problems with other method. Pay attention on top == null. To calculate search() all you need is just iterate over the elements and find position of required value:
public class MyStack<E> {
private Node<E> top;
private int size;
public void push(E val) {
Node<E> node = new Node<>(val);
node.next = top;
top = node;
size++;
}
public E element() {
return top == null ? null : top.val;
}
public E pop() {
if (top == null)
return null;
E val = top.val;
top = top.next;
size--;
return val;
}
public boolean empty() {
return size == 0;
}
public int search(E val) {
int res = 1;
Node<E> node = top;
while (node != null && node.val != val) {
node = node.next;
res++;
}
return node == null ? -1 : res;
}
private static final class Node<E> {
private final E val;
private Node<E> next;
public Node(E val) {
this.val = val;
}
}
}

I assume your MyStack class should be compatible with the Stack class provided by Java as you mention it in your question. This means that your signature public int search(Object o) matches the signature of java.util.Stack#search (apart from synchronised).
To implement the search method using your Node class, we need to traverse the stack and return the index of the first (uppermost) match. First, assign head to a local variable (current). Then you can create a loop where you current.getNext() at the end to get the next element. Stop if the next element is null as we have reached the end of the stack. In the loop, you either count up the index or return this index when the current element's data matches the argument o.
The evaluation needs to be able to deal with null values for your argument o. Therefore, you need to check for null first and adjust your logic accordingly. When o is null, do a null-check against current.getData(). If o is not null, check if current.getData() is equal to o with equals().
Here is a working example: (compatible with java.util.Stack#search)
public int search(Object o) {
int index = 1;
Node<E> current = head;
while (current != null) {
if (o == null) {
if (current.getData() == null) {
return index;
}
} else {
if (o.equals(current.getData())) {
return index;
}
}
current = current.getNext();
index++;
}
return -1; // nothing found
}
To test this, you can write a simple unit test with JUnit like this:
#Test
public void testMyStackSearch() {
// initialize
final MyStack<String> stack = new MyStack<>();
stack.push("e5");
stack.push("e4");
stack.push(null);
stack.push("e2");
stack.push("e1");
// test (explicitly creating a new String instance)
assertEquals(5, stack.search(new String("e5")));
assertEquals(3, stack.search(null));
assertEquals(2, stack.search(new String("e2")));
assertEquals(1, stack.search(new String("e1")));
assertEquals(-1, stack.search("X"));
}
Since you have already a reference implementation, you can replace MyStack with Stack (java.util.Stack) and see if your asserts are correct. If this runs successfully, change it back to MyStack and see if your implementation is correct.
Note: I do not recommend to actually use the Stack implementation in Java. Here, it just serves as a reference implementation for the java.util.Stack#search method. The Deque interface and its implementations offer a more complete and consistent set of LIFO stack operations, which should be used in preference to Stack.

Cannot cast from inner class to class

I have a BinaryTree class which contains an inner class Node.
What I would like to do is to be able to insert some nodes in my BinaryTree tree by calling tree.insert(node). However, to keep it clean and consistent, I dont want to create an insert() method inside Node inner class. So I tried the code below, but I have an error: Cannot cast from BinaryTree.Node to BinaryTree.
What should I do?
BinaryTree class
public class BinaryTree {
Node root = null;
private class Node {
int value;
Node left;
Node right;
}
public BinaryTree(int v) {
root.value = v;
root.left = null;
root.right = null;
}
public void insert(Node n) {
/* Error */
if(n.value > root.value) ((BinaryTree) root.right).insert(n);
}
}
Main class
public class Main {
public static void main(String[] args) {
// TODO Auto-generated method stub
Scanner sc = new Scanner(System.in);
String[] str = sc.nextLine().split(" ");
BinaryTree tree;
for(int i = 0; i < str.length-1; i++) {
int val = Integer.parseInt(str[i]);
//tree.insert(node);
}
}
}
Thanks,

You don't need typecasting inside insert method. It should be like this:
public void insert(Node n) {
if(n.value > root.value)
insert(root.right);
}

To insert a node in a tree you need to define where to insert it, so your insert methods should be something like:
//insert a new node right to a node. not null safe
public void insert(Node newNode, Node rightTo) {
newNode.right = rightTo.right;
newNode.left = rightTo;
rightTo.right = newNode;
}
which does not require casting.
To find the rightTo node you could use:
//get the last node which has a value lower than `value`
//may return null
public Node getNodeWithValueLowerThan(int value) {
if(root == null) return null;
return getNodeWithValueLowerThan(root, value);
}
//recursive method. null safe
private Node getNodeWithValueLowerThan(Node node, int value) {
if(node == null) return null;
if(node.value > value) return node.left; //return previous node. may be null
return getNodeWithValueLowerThan(node.right, value);
}
To insert a node as a last node, you could use:
//insert a new node as last
public void insertLastNode(Node newNode) {
Node lastNode = getTail();
if(lastNode == null) {//empty tree
root = newNode;
return;
}
newNode.left = lastNode;
lastNode.right = newNode;
}
where getTail is something like:
//find last node
private Node getTail() {
if(root == null) return null;
return getTail(root);
}
//recursive method to find last node. not null safe
private Node getTail(Node node) {
if(node.right == null) return node;
return getTail(node.right);
}
Note: code was not tested so debug carefully.

Need guidance on creating Node class (java)?

I need to implement a Node class, where the basic methods are: getItem(), getNext(), setItem() and setNext(). I want the nodes to be able to store at least the default integer range in Java as the “item”; the “next” should be a reference or pointer to the next Node in a linked list, or the special Node NIL if this is the last node in the list.I also want to implement a two-argument constructor which initializes instances with the given item (first argument) and next node (second argument) , I've kind of hit a brick wall and need some guidance about implementing this , any ideas ?
I have this so far:
class Node {
public Node(Object o, Node n) {
}
public static final Node NIL = new Node(Node.NIL, Node.NIL);
public Object getItem() {
return null;
}
public Node getNext() {
return null;
}
public void setItem(Object o) {
}
public void setNext(Node n) {
}
}

While implementing the custom LinkedList/Tree, we need Node. Here is demo of creating Node and LinkedList. I have not put in all the logic. Just basic skeleton is here and you can then add more on yourself.

I can give you a quick hint on how to do that:
Class Node{
//these are private class attributes, you need getter and setter to alter them.
private int item;
private Node nextNode;
//this is a constructor with a parameter
public Node(int item)
{
this.item = item;
this.nextNode = null;
}
// a setter for your item
public void setItem(int newItem)
{
this.item = newItem;
}
// this is a getter for your item
public int getItem()
{
return this.item;
}
}
You can create a Node object by calling:
Node newNode = Node(2);
This is not a complete solution for your problem, the two parameter constructor and the last node link are missing, but this should lead you in the correct direction.

Below is a simple example of the Node implementation, (i renamed Item to Value for readability purpose). It has to be implemented somehow like this, because methods signatures seems to be imposed to you. But keep in mind that this is definely not the best way to implement a LinkedList.
public class Node {
public static final Node NIL = null;
private Integer value;
private Integer next;
public Node(Integer value, Node next) {
this.value = value;
this.next = next;
}
public Integer getValue() {
return this.value;
}
public Node getNext() {
return this.next;
}
public void setValue(Integer value) {
this.value = value;
}
public void setNext(Node next) {
this.next = next;
}
public boolean isLastNode() {
return this.next == Node.NIL || Node;
}
}
public class App {
public static void main(String[] args) {
Node lastNode = new Node(92, Node.NIL);
Node secondNode = new Node(64, lastNode);
Node firstNode = new Node(42, secondNode);
Node iterator = firstNode;
do () {
System.out.println("node value : " + iterator.getValue());
iterator = iterator.getNext();
} while (iterator == null || !iterator.isLastNode());
}
}

The node class that will be implemented changes according to the linked list you want to implement. If the linked list you are going to implement is circular, then you could just do the following:
public class Node {
int data;
Node next = null;
public Node(int data){
this.data = data;
}
}
Then how are you going to implement the next node?
You are going to do it in the add method of the circularLinkedList class. You can do it as follows:
import java.util.*;
public class CircularLinkedList {
public CircularLinkedList() {}
public Node head = null;
public Node tail = null;
public void add(int data) {
Node newNode = new Node(data);
if(head == null) {
head = newNode;
}
else {
tail.next = newNode;
}
tail = newNode;
tail.next = head;
}
public void displayList() {
System.out.println("Nodes of the circular linked list: ");
Node current = head;
if(head == null) {
System.out.println("Empty list...");
}
else {
do {
System.out.print(" " + current.data);
current = current.next;
}while(current != head);
System.out.println();
}
}
}

Add nodes in linked list

Trying to implement single-linked-list in below program, i am really not able to undertsand how to add a node in an Linked list (for start, m trying it on empty linked list).
To put it plain simple,i tried to setData and setNext but getSizeofList() return 0 everytime....its really looking like a rocket science to me now!!
Question : Can some-one tell me how to implement it....or rather, add a node to existing linked list....
What i have tried so far and why they dint worked out: i referenced multiple programs but they were too complex for me to understand(rocket science), so wrote below program from what i understood from algorithms....but even in algo's, they just show methods on how to implement and this is where i failed, as, i dont understand,what data-type and value is to be passed for adding a node...
please not that m not a java guy, so please go easy, this question comes in as an attempt to learn
package Data_S;
public class Linked_List {
private int data;
private Linked_List next_ptr;
private Linked_List headNode = null;
/**
* #param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
Linked_List ll = new Linked_List();
//ll.setnext(25);
ll.insert_node(24);
ll.traverse();
ll.getSizeofList();
}
//size of list
public void getSizeofList()
{
int l = 0;
Linked_List curr = headNode;
while(curr != null)
{
l++;
curr = curr.getnext();
}
System.out.print("Size of list is = "+l);
}
//insert node
public void insert_node(/*Linked_List node, */int data)
{
if(headNode == null)
{
System.out.println("in insert"); // checking
this.setnext(headNode);
this.setData(data);
System.out.print("value = "+this.getData());
}
}
//set data for this node
public void setData(int data)
{
this.data = data;
}
//return the data
public int getData()
{
return this.data;
}
//set next pointer
public void setnext(Linked_List next_ptr)
{
this.next_ptr = next_ptr;
}
//get next pointer
public Linked_List getnext()
{
return this.next_ptr;
}
}

You have to make a distinction between the single chains (Node) of a linked list, and the entire container (LinkedList).
public class LinkedList {
Node head;
int size; // Maybe
public void insertAtEnd(int data) {
Node previous = null;
for (Node current = head; current != null; current = current.next) {
previous = current;
}
Node baby = new Node(data);
if (previous == null) {
head = baby;
} else {
previous.next = baby;
}
++size;
}
public void insertInSortedList(int data) {
Node previous = null;
Node current = null;
for (current = head; current != null && data < current.data;
current = current.next) {
previous = current;
}
Node baby = new Node(data);
baby.next = current;
if (previous == null) {
head = baby;
} else {
previous.next = baby;
}
++size;
}
}
class Node {
int data;
Node next;
Node(int data) {
this.data = data;
}
}
One may sometimes see encapsulation as:
public class LinkedList {
private static class Node {
}
...
}

You never set headnode. In insertnode you just use setnext which does not set headnode. You are mixing the top class and the node implementation together.
Here is an example of how to implement a linked list in java for further reference:
How do I create a Linked List Data Structure in Java?