This is a homework, I would appreciate any kind of answer.
Im trying to figure out why i keep getting a NullPointerException when i call the equals method. I have instantiated the object if im not mistaken, but it still doesn't work.
Exception in thread "main" 8
java.lang.NullPointerException
at labbfyra.TextBuilder.equals(TextBuilder.java:69)
at labbfyra.SkapaOrd.main(SkapaOrd.java:17)
Is this the stacktrace?
public class TextBuilder {
private static class Node{
public char inChar;
public Node next;
public Node(char c, Node nästa){
inChar = c;
next = nästa;
}
}
private Node first = null;
private Node last = null;
public TextBuilder(){
first = null;
last = null;
}
public void append(String s){
int x = s.length();
for(int i=0;i<x;i++){
Node n = new Node(s.charAt(i),null);
if(first ==null){
first = n;
last = n;
}else{
last.next = n;
last = n;
}
}
}
public int ShowSize(){
int counter = 0;
Node n = first;
while(n!=null){
counter++;
n=n.next;
}
return counter;
}
public boolean equals(String s){
boolean eq = false;
int counter = 0;
char[] cArray = s.toCharArray();
char[] cArrayComp = new char[10];
Node n = first;
cArrayComp[counter] = n.inChar;
while(n!=null){
counter++;
n=n.next;
cArrayComp[counter] = n.inChar; //THIS IS LINE 69
}
if(cArrayComp==cArray){
eq = true;
}
else{
eq=false;
}
return eq;
}
}
In your while loop, you check that n is not null, but then you assign n.next to n just before accessing n. The problem is that you do not ensure that the assigned value (n.next) is not null.
At a quick glance, looks like the counter variable in your while loop is going past the 10 you set your cArrayComp size to. Perhaps the string parameter being passed is longer than 10 chars?
public boolean equals(String s){
boolean eq = false;
int counter = 0;
char[] cArray = s.toCharArray();
char[] cArrayComp = new char[10];
Node n = first;
while(n!=null){
System.out.println(counter);
cArrayComp[counter] = n.inChar;
System.out.println(cArrayComp[counter]);
System.out.println(n.inChar);
n=n.next;
counter++;
}
if(cArrayComp==cArray){
eq = true;
}
else{
eq=false;
}
return eq;
}
This is the corrected version, i found a bug in your loop. Just check my version. Works at 100%
Related
my assignment involves using recursive method. Write a program that reverses a LinkedList. This is the code i have done below, can anyone see what i am doing wrong, thank you very much!
PS: this is done in jGRASP
// Java program for reversing the linked list
class MyLinkedList {
static Node head;
static class Node {
int data;
Node next;
Node(int d) {
data = d;
next = null;
}
}
/* Function to reverse the linked list */
Node reverse(Node node) {
Node prev = null;
Node current = node;
Node next = null;
while (current != null) {
next = current.next;
current.next = prev;
prev = current;
current = next;
}
node = prev;
return node;
}
// prints content of double linked list
void printList(Node node) {
while (node != null) {
System.out.print(node.data + " ");
node = node.next;
}
}
public static void main(String[] args) {
MyLinkedList list = new MyLinkedList();
list.head = new Node(85);
list.head.next = new Node(15);
list.head.next.next = new Node(4);
list.head.next.next.next = new Node(20);
System.out.println("Given Linked list");
list.printList(head);
head = list.reverse(head);
System.out.println("");
System.out.println("Reversed linked list ");
list.printList(head);
}
}
I cannot get the loop to work in the buildDimArray method to store the number combinations "11+11", "11+12", "11+21", "11+22", "12+11", "12+12", "12+21", "12+22", "21+11", "21+12", "21+21", "21+22", "22+11", "22+12", "22+21", and "22+22" into the 2d arraylist with each expression going into one column of the index dimBase-1 row. The loop may work for other people, but for some reason mine isn't functioning correctly. The JVM sees the if dimBase==1 condition, but refuses to check the other conditions. The "WTF" not being printed as a result from the buildDimArray method. If dimBase=1, it prints successfully, but doesn't for the other integers. The dimBase==3 condition needs a loop eventually. The "WTF" is for illustrative purposes. I could get away with a 1d arraylist, but in the future I will likely need the 2d arraylist once the program is completed.
package jordanNumberApp;
import java.util.Scanner;
import java.util.ArrayList;
/*
* Dev Wills
* Purpose: This code contains some methods that aren't developed. This program is supposed to
* store all possible number combinations from numbers 1-dimBase for the math expression
* "##+##" into a 2d arraylist at index row dimBase-1 and the columns storing the
* individual combinations. After storing the values in the arraylist, the print method
* pours the contents in order from the arraylist as string values.
*/
public class JordanNumberSystem {
// a-d are digits, assembled as a math expression, stored in outcomeOutput, outcomeAnswer
public static int dimBase, outcomeAnswer, a, b, c, d;
public static String inputOutcome, outcomeOutput;
public static final int NUM_OF_DIMENSIONS = 9; //Eventually # combinations go up to 9
public static ArrayList<ArrayList<String>> dimBaseArray;
public static Scanner keyboard;
/*
* Constructor for JordanNumber System
* accepts no parameters
*/
public JordanNumberSystem() // Defunct constructor
{
// Declare and Initialize public variables
this.dimBase = dimBase;
this.outcomeOutput = outcomeOutput;
this.outcomeAnswer = outcomeAnswer;
}
// Set all values of variable values
public static void setAllValues()
{
// Initialize
dimBase = 1;
outcomeAnswer = 22; // variables not used for now
outcomeOutput = "1"; // variables not used for now
//a = 1;
//b = 1;
//c = 1;
//d = 1;
dimBaseArray = new ArrayList<ArrayList<String>>();
keyboard = new Scanner(System.in);
}
public static void buildDimArray(int dim)
{
dimBase = dim;
try
{
//create first row
dimBaseArray.add(dimBase-1, new ArrayList<String>());
if( dimBase == 1)
{
a = b = c = d = dimBase ;
dimBaseArray.get(0).add(a+""+b+"+"+c+""+d);
System.out.println("WTF"); // SHOWS
}
else if (dimBase == 2)
{ // dim = 2
a = b = c = d = 1 ;
System.out.println("WTF"); // doesn't show
// dimBaseArray.get(dimBase-1).add(a+""+b+"+"+c+""+d);
for( int i = 1 ; i <= dim ; i++)
a=i;
for( int j = 1 ; j <= dim ; j++)
b=j;
for( int k = 1 ; k <= dim ; k++)
c=k;
for( int l = 1 ; l <= dim ; l++)
{
d=l;
dimBaseArray.get(dim-1).add(a+""+b+"+"+c+""+d);
}
}
else if (dimBase == 3)
{
a = b = c = d = dimBase;
dimBaseArray.get(2).add(a+""+b+"+"+c+""+d);
System.out.println("WTF");
}
}catch (IndexOutOfBoundsException e)
{
System.out.println(e.getMessage());
}
}
public static void printArray(int num) // Prints the contents of the array
{ // Fixing the printing method
try
{
int i = num-1;
for( String string : dimBaseArray.get(i))
{
System.out.println(string);
System.out.println("");
}
} catch (IndexOutOfBoundsException e)
{
System.out.println(e.getMessage());
}
}
public static void main(String[] args) throws java.lang.IndexOutOfBoundsException
{
setAllValues(); // sets the initial a,b,c,d values and dimBase, initializes 2d arraylist
// Get the Dimension Base number
System.out.println("Enter Dimension Base Number. Input an integer: ");
int dimBaseInput = keyboard.nextInt(); // Receives integer
dimBase = dimBaseInput;
if( dimBase != 1 && dimBase != 2 && dimBase != 3)
{// Error checking
System.out.println("invalid Dimension Base Number should be 1 or 2 ");
System.exit(1);
}
// Build the arraylist, print, clear, exit
buildDimArray(dimBase);
printArray(dimBase);
dimBaseArray.clear();
System.exit(1);
}
}// End of class
I am am trying to call a variable in another method to my array.
var Com = the difficulty for the game. But the method below I'm trying to call the var Com, for: var c = Com.GetChoice();
Not sure why I can not figure out how to call it.
public object SetDiff()
{
Console.WriteLine("Enter difficulty #: (1 = Easy, 2 = Normal, 3 = Impossible)");
var diff = Console.ReadLine();
int mode;
int.TryParse(diff, out mode);
if (mode == 1)
{
Console.Clear();
var Com = new Easy();
return Com;
}
if (mode == 2)
{
Console.Clear();
var Com = new Medium();
return Com;
}
if (mode == 3)
{
Console.Clear();
var Com = new Hard();
return Com;
}
else
{
Console.WriteLine("That is not a valid input.");
return SetDiff();
}
} // Apparently you can't set variables in a switch.
public int[] FaceOff(int num)
{
int PlayerWin = 0;
int ComWin = 0;
int Tie = num + 1;
// TODO : Get rid of TIES!
for (int i = 0; i < num; i++)
{
var p = p1.GetChoice();
var c = Com.GetChoice();
You have many different options:
Pass as parameter
public int[] FaceOff(int num, int Com){...}
make a "global" variable
private int Com;
I would also recommend you to learn OOP (Object Orientated Programming) basics.
Here is a program that is supposed to find out whether a sequence of numbers match the mathematical formula a[n+1] = a[n]*b+c for any combination of b and c in the integer range -9:9.
import java.util.Scanner;
public class Nastaord{
private static int[] lasTal(){
int[] tallista; //Det vi ska ha talföljden i
int i = 0; //räknare för tallista
while(true){
System.out.print("Ange tal, eller tryck enter om du är klar: ");
int nytt_tal = scanner.nextLine();
if(nytt_tal == ""){
return tallista;}
tallista[i] = nytt_tal;
i++;
}
}
private static boolean bcFinns(int[] tallista){
boolean OK = true;
for(int b = -9; b <= 9; b++){
for(int c = -9; c <= 9; c++){
for(int i = tallista.length; i > 0;i--){
OK = tallista[i] == tallista[i-1]*b+c;
if(OK == false){
break;}
}
if(OK == true){
public int b = b;
public int c = c;
return true;}
}
}
return false;
}
public static void main(String[] args){
boolean OK = bcFinns(lasTal());
if (OK == true){
System.out.print(tallista[tallista.length-1]*b+c);
}
if (OK == false){
System.out.print("No");
}
}
}
The program, on a principal level, works. The only thing is that I do not know how to save the correct numbers b and c for the sequence once they are found. I tried creating two public variables so that I can access them in the main method, but I get the following error:
Nastaord.java:30: error: illegal start of expression
public int b = b;
^
Nastaord.java:31: error: illegal start of expression
public int c = c;
Could you help me save these variables b and c in some way?
public class Nastaord{
public static int bFinal,cFinal;
Then later on:
bFinal = b;
cFinal = c;
import java.util.*;
public class MyTwoWayLinkedList<E> extends java.util.AbstractSequentialList<E> {
private Node<E> head, tail;
private int size = 0;
private List<E> list;
/** Create a default list */
public MyTwoWayLinkedList() {
list = new LinkedList<E>();
}
public MyTwoWayLinkedList(E[] objects) {
list = new LinkedList<E>();
for (int i = 0; i < objects.length; i++)
add(objects[i]);
}
/** Return the head element in the list */
public E getFirst() {
if (size == 0) {
return null;
}
else {
return head.element;
}
}
/** Return the last element in the list */
public E getLast() {
if (size == 0) {
return null;
}
else {
return tail.element;
}
}
/** Add an element to the beginning of the list */
public void addFirst(E e) {
Node<E> newNode = new Node<E>(e); // Create a new node
newNode.next = head; // link the new node with the head
head.previous = newNode; //link the old node with new head
head = newNode; // head points to the new node
size++; // Increase list size
if (tail == null) // the new node is the only node in list
tail = head;
}
/** Add an element to the end of the list */
public void addLast(E e) {
Node<E> newNode = new Node<E>(e); // Create a new for element e
if (tail == null) {
head = tail = newNode; // The new node is the only node in list
}
else {
tail.next = newNode;// Link the new with the last node
newNode.previous = tail;
tail = tail.next; // tail now points to the last node
}
size++; // Increase size
}
#Override /** Add a new element at the specified index
* in this list. The index of the head element is 0 */
public void add(int index, E e) {
if (index == 0) {
addFirst(e);
}
else if (index >= size) {
addLast(e);
}
else {
Node<E> current = tail;
for (int i = size - 1; i > index; i--) {
current = current.previous;
}
Node<E> temp = current.next;
current.next = new Node<E>(e);
(current.next).previous = current;
(current.next).next = temp;
size++;
}
}
/** Remove the head node and
* return the object that is contained in the removed node. */
public E removeFirst() {
if (size == 0) {
return null;
}
else {
Node<E> temp = head;
head = head.next;
head.previous = null;
size--;
if (head == null) {
tail = null;
}
return temp.element;
}
}
/** Remove the last node and
* return the object that is contained in the removed node. */
public E removeLast() {
if (size == 0) {
return null;
}
else if (size == 1) {
Node<E> temp = head;
head = tail = null;
size = 0;
return temp.element;
}
else {
Node<E> temp = tail;
tail = tail.previous;
tail.next = null;
size--;
return temp.element;
}
}
#Override /** Remove the element at the specified position in this
* list. Return the element that was removed from the list. */
public E remove(int index) {
if (index < 0 || index >= size) {
return null;
}
else if (index == 0) {
return removeFirst();
}
else if (index == size - 1) {
return removeLast();
}
else {
Node<E> previous = tail;
for (int i = size - 1; i > index; i--) {
previous = previous.previous;
}
Node<E> current = previous.next;
(current.next).previous = previous;
previous.next = current.next;
size--;
return current.element;
}
}
#Override /** Override toString() to return elements in the list */
public String toString() {
StringBuilder result = new StringBuilder("[");
Node<E> current = tail;
for (int i = size - 1; i > 0; i--) {
result.append(current.element);
current = current.previous;
if (current != null) {
result.append(" ,"); // Separate two elements with a comma
}
else {
result.append("["); // Insert the closing ] in the string
}
}
return result.toString();
}
#Override /** Clear the list */
public void clear() {
size = 0;
head = tail = null;
}
#Override /** Override iterator() defined in Iterable */
public ListIterator<E> listIterator() {
Node<E> current = head; // Current index
return list.listIterator();
}
#Override /** Override iterator() defined in Iterable */
public ListIterator<E> listIterator(int index) {
Node<E> current = head; // Current index
for (int i = 0; i < index; i++) { // sets current int to the parameter
current = current.next;
}
return list.listIterator();
}
#Override
public int size()
{
return size;
}
public class Node<E> {
E element;
Node<E> next;
Node<E> previous;
public Node(E element) {
this.element = element;
}
}
}
This is my original class, I will include my test case below but first let me explain my problem. I am trying to create a Doubly linked list and iterate backwards through it. However I am getting a Null Pointer Exception by just adding elements to the list. I have looked over the section of code for my addFirst method for about 2 hours now and don't see any logic errors(doesn't mean there arent any), please help!
Here is my test case as promised.
public class TestMyLinkedList {
/** Main method */
public static void main(String[] args) {
// Create a list for strings
MyTwoWayLinkedList<String> list = new MyTwoWayLinkedList<String>();
// Add elements to the list
list.add("America"); // Add it to the list
System.out.println("(1) " + list);
list.add(0, "Canada"); // Add it to the beginning of the list
System.out.println("(2) " + list);
list.add("Russia"); // Add it to the end of the list
System.out.println("(3) " + list);
list.addLast("France"); // Add it to the end of the list
System.out.println("(4) " + list);
list.add(2, "Germany"); // Add it to the list at index 2
System.out.println("(5) " + list);
list.add(5, "Norway"); // Add it to the list at index 5
System.out.println("(6) " + list);
list.add(0, "Poland"); // Same as list.addFirst("Poland")
System.out.println("(7) " + list);
// Remove elements from the list
list.remove(0); // Same as list.remove("Australia") in this case
System.out.println("(8) " + list);
list.remove(2); // Remove the element at index 2
System.out.println("(9) " + list);
list.remove(list.size() - 1); // Remove the last element
System.out.print("(10) " + list + "\n(11) ");
for (String s: list)
System.out.print(s.toUpperCase() + " ");
list.clear();
System.out.println("\nAfter clearing the list, the list size is "
+ list.size());
}
}
I'm not completely sure why you are using a LinkedList within your own implementation of a Double Linked List. In regards to your question about your addFirst method however, I have the following comments and an example of how I would approach this solution.
Head is null when you call the addFirst method.
Head has not been initialized as a new Node.
Therefore newNode.next = head; is actually newNode.next = null; There is your null pointer exception, I would imagine!
public void addFirst (E e)
{
Node<E> newNode = new Node<E>(e); //create new node
if (head != null){ //if head exists
newNode.next = head; //the new node's next link becomes the old head
}
head = newNode; //the new head is the new node
if (tail == null){ //if the tail is non existent ie head the only object in list
tail = head; //the head and the tail are the same
head.next = tail; //the 'next' value of head will be tail
}
head.prev = tail; //the previous node to head will always be tail
size++;
}
}