Sorting linked lists - Help
I am writing a program for an assignment that reads a file containing population growth for all the Counties in the U.S. I'm required to use a link list to store the data. They want me to sort the data based on certain criteria. My first problem is that as each new County instance is created, it is to be inserted into this linked list in alphabetical order first by county name.
My understanding of linked lists is that they cannot be randomly accessed like a vector or an array. So is it possible to do this without using a vector or an array? And how would I go about it?
thanks.
Can you create a second, sorted linked list?The prof. didn't specify whether or not I can use a second list.
Are you prohibited from using the collections framework (probably, if it is a school assignment!)Not really sure on this one. Again it is not specified in the assignment
(Why would they have you store this data in a linked list??)I their reasoning is to have us learn how to implement linked list because it can grow or shrink when necessary.(Other than that I don't understand the practicality of it either)
Are you using a doubly linked list (forwards and backwards)?The assignment does not specify a certain linked list to use.
Did your prof mention any sort algorithms you might use? He states later in the assignment that I have to generate different growth reports by using MergeSort.
I appreciate your help and comments. Unfortunately my prof. is very vague about the assignment and its implementation in his outline. He just kind of throws us into it hoping that we will figure it out.
Similar Messages
-
I need help sorting a linked list.
So far, in my sort method I have...
public LinkedList sort();
LinkedList newList = new LinkedList();
Node ref;
Node temp = new Node(head.getempID(); head.getempLast(), head.getempFirst(); head.getempDept(), head.getempSalary());
for(ref = head; ref != null; ref.getNext());
if(ref.getempID() <= temp.getEmpID());
temp = (ref.getempID(), ref.getempLast(), ref.getempFirst(), ref.getempDept(), ref.getempSalary());
newList.insert(temp); //???????
return newList();
// As you can tell, each one of my node's consists of an employee's id number, last name, first name, department, and salary...I need to sort the linked list by the employee's ID number. I can not figure out how to swap nodes though. PLEASE HELP A KID OUT!!!I was worng, my last data structure lecture today(I hope).
| | | | | | | | | |
head->| Node1 |---->| Node2 |---->| Node3 |---->| Node4 |---etc-->| NodeN |->null
| | | | | | | | | |
Q: How do you swap node2 with node3?
A: Think about it! Look at a picture if thats what you need. Draw the lines of reference to figure it out.
1. Where does node1.next need to point to, how about node2.next, or node3.next?
3. How do you change these without losing a node, are you going to need a temp?
4. What about special case(switching node1 and node2)?
This is simple problem solving skills. I assume you have built the list yourself,
so you have the basic knowledge base needed. -
Alphabetizing a linked list, HELP!
To all:
Firstly, I'm not looking for someone to do my assignment for me. I just need a push in the right direction...
I need to create a linked list that contains nodes of a single string (a first name), but that list needs to be alphabetized and that's where I am stuck.
I can use an iterator method that will display a plain-old "make a new node the head" linked list with no problem. But I'm under the impression that an iterator method is also the way to go in alphabetizing this list (that is using an iterative method to determine the correct place to place a new node in the linked list). And I am having zero luck doing that.
The iterator method looks something like this:
// ---- MyList: DisplayList method ----
void DisplayList()
ListNode p;
for (ResetIterator(); (p = Iterate()) != null; )
p.DisplayNode();
// ---- MyList: ResetIterator method ----
void ResetIterator()
current = head;
// ---- MyList: Iterate method ----
ListNode Iterate()
if (current != null)
current = current.next;
return current;
Can I use the same iterator method to both display the final linked list AND alphabetize my linked list or do I need to create a whole new iterator method, one for each?
Please, someone give me a hint where to go with this. I've spent something like 15 hours so far trying to figure this thing out and I'm just stuck.
Thanks so much,
John
[email protected]I'll try and point you in the right direction without being too explicit as you request.
Is your "linked list" an instance of the Java class java.util.LinkedList or a class of your own?
If it is the Java class, then check out some of the other types of Collections that provide built-in sorting. You should be able to easily convert from your List to another type of Collection and back again to implement the sorting. (hint: you can do this in two lines of code).
If this is your own class and you want to code the sort yourself, implement something simple such as a bubble sort (should be easy to research on the web).
Converting to an array, sorting that and converting back is another option.
Good Luck. -
Hi,
I'm developing a circular linked list class.. However everytime I add to the front of the list it overwrites the first element in the list.. I pretty confused at the moment. If anyone could assist me in what I'm doing wrong it would be greatly appreciated..
My Class:
public class CNode<T>
public T nodeValue; //data held by the node
public CNode<T> next; //next node in the list
//default constructor; next references node itself
public CNode()
nodeValue = null;
next = this;
//constructor; initializes nodeValue to item
//and sets the next to reference the node itself
public CNode(T item)
nodeValue = item;
next = this;
}My addFirst:
public static <T> void addFirst(CNode<T> header, T item)
CNode<T> curr;
CNode<T> newNode = new CNode<T>(item);
//header.next = newNode;
//newNode.next = header;
curr = header;
newNode.next = header;
curr.next = newNode;
}You need a Node class and a class that manages the nodes. The class the manages the nodes is typically called something like MyLinkedList. The MyLinkedList class will typically have a member called 'head' or 'first' and another member called 'last', 'end', or 'tail'. Those members will contain references to the first node and the last node respectively.
The methods like add(), remove(), find(), etc. will be members of the MyLinkedList class--not the Node class. The add() method of the MyLinkedList class will create a node, set the value of the node, and then set the next member to refer to the proper node or null if the node is added to the end of the list. Therefore, there is really no need for a default constructor in the node class. The MyLinkedList class will always set the value of the node, and then set it's next member to refer to something.
You might want to try to write a linear linked list first, and get that working, and then modify it to make it a circular linked list. In addition, you should be drawing pictures of nodes with arrows connecting the different nodes to understand what's going on. Whenver the next member of a node refers to another node, draw an arrow starting at the next member and ending at the node it refers to. The pictures will be especially helpful when you write functions like add().
Message was edited by:
7stud -
Hey,
I am having trouble understanding the Linked List implementation code given under the Node Operations part of this pdf.
http://www.cs.berkeley.edu/~jrs/61b/lec/07.pdf
public ListNode(int item, ListNode next) {
this.item = item;
this.next = next;
public ListNode(int item) {
this(item, null);
ListNode l1 = new ListNode(7, new ListNode(0, new ListNode(6)));
I understand the earlier implementation, but this one has me confused. Would someone please go through/explain the flow of this implementation with me?
ThanksWell the code is pretty self explanitory.
a ListNode class that has two fields. an int field item, and a ListNode field next.
so when you create a ListNode object you can do so by one of two means passing one parameter or passing two parameters.
if you pass one parameter then you have a ListNode which has an int say 1 and a ListNode object who's value is null.
if you pass two parameters then you have a ListNode which has an int say 2 and a ListNode object who's value is the ListNode you pass to it.
ex ListNode myNode = ListNode(1,new ListNode(2));
so myNode now has an integer 1 and a reference to another ListNode which has an integer value of 2 and a reference to a null ListNode.
hope this helps. -
Hi,
I have an assignment on linked lists, as a beginner I am really struggling with it for hours...
I have to write the following methods without importing any libraries.
"Write an iterative method delete() for LinkedStackOfStrings that takes an integer parameter k and deletes the kth element (assuming it exists).
Write a method find() that takes an instance of LinkedStackOfStrings and a string key as arguments and returns true if some node in the list has key as its item field, false otherwise."
It must be implemented in the code on this site:
[http://www.cs.princeton.edu/introcs/43stack/LinkedStackOfStrings.java.html]
Help would be appreciated! I have been trying for hours :(vdL wrote:
Thanks for the reply! Yes, I forgot add what my actual problem is.
If I am correct, I will need to be able to traverse through the list up to a certain point, redirect and redirect the pointers.
My problem is I am able to traverse through the list using 'for(Node x = first; x != null; x= x.next)' but I do not know how make it only run up to a particular position in the list.You can keep an integer counter, increment it each time through the loop, and break out of the loop when the counter is big enough. Example of breaking:
if (n == k) break;
And also, how not to loose the previous Node when traversing trough the list.You're allowed to have multiple references. So you could keep one reference to the current node, and one reference to the one before it.
Or, rather than looping through the list until you find the kth element, loop until you find the (k - 1)th element, then (if it exists) remove the one after it. -
//would anyone know how to sort this, tried to put in a bubble sort but i got some many errors i deleted it
import uuInOut;
class studentlist
public static void main(String [] args)
final int LENGTH = 10; //the length of the list
//declare variables
String name, temp3, searchName;
int age, id, current;
boolean present = false;
//create new linked list
LinkedList list = new LinkedList();
//open data
uuTextFile data = new uuTextFile("data.txt");
data.OpenInputFile();
//add data from data file to the linked list
while (!data.EOF)
name = data.ReadString();//read name
age = data.ReadInt();//read age
id = data.ReadInt();//read id
//new instance of studentrecord to add to list
StudentRecord temp = new StudentRecord(name, age, id);
list.insert(0, temp);//add to list
//Print The whole list
for (int x = 0; x < list.length(); x ++)
StudentRecord temp2 = (StudentRecord)list.elementAt(x);
temp2.printdetails();
System.out.println("First in list: ");
StudentRecord First = (StudentRecord)list.getHead().getElement();
First.printdetails();//print the first entered in the list
System.out.println("Last in List: ");
int x = list.length() - 1;
StudentRecord Last = (StudentRecord)list.elementAt(x);
Last.printdetails();//print the last entered in the list
System.out.println("Please Enter Current Position: ");
current = uuInOut.ReadInt();
StudentRecord next = (StudentRecord) list.elementAt(current + 1);
next.printdetails();//prints the next position in the list
System.out.println("Please Enter Name: ");
temp3 = uuInOut.ReadString();//reads in a name to search for in the list
//loop to search through the list for the name entered
for (int j = 0; j < list.length(); j++){
StudentRecord temp4 = (StudentRecord)list.elementAt(j);
searchName = temp4.getname();
if (temp3.equals(searchName))
present = true;
System.out.println(present);
if (present == true)
System.out.println("Student Name Found");
else System.out.println("Not in list" );
}You could scrap this implementation and use a Collection class, then use Collections.sort. Or you could have your implementation implement the List interface and use Collections.sort. But more than likely you are supposed to do this for homework, so in that case I would try to implement the bubble sort and fix it until you don't get any errors. There are other sort routines you could use, but they are all more complicated than the bubble sort, so you will likely get many errors doing that too.
Good luck
Lee -
Hi, I am implementing a graph for my Java project and part of the coding requires link list for the edges.
I wrote the code, but I just don't understand why it does not function the way like it is in C++.
Bascilly, I have trouble adding data into a linklist
My codes are as follows:
public int insertLink(String fCity, String tCity, int distance)
int location = find(fCity);
if(location == -1)
return -1;
else
insertNode(new Link(tCity,distance), location);
return 0;
private void insertNode(Link node, int index)
Link head = vertex[index].head;
while(head != null)
head = head.next;
head = node;
This is to add stuff to the list, but for some reason, when i try to display the data, i keep getting null. somehow when
head = node
head = new Link(data, data);
the head points to somwhere else, and not refer to the linklist vertex[index].head anymore...
Here is my code to display my data:
public void printVertex()
for(int i = 0; i < vertex.length; i++)
if(vertex[i] != null)
System.out.print(vertex[i]+" --");
Link node = vertex.head;
while(node != null)
System.out.print(node.toCity+","+node.distance+"--");
node = node.next;
System.out.println();
Am i doing something wrong so my linklist isn't implement correct?
THank you.Hi, the "fix" for head.next != null didn't work
private void insertNode(Link node, int index)
Link head = vertex[index].head;
while(head != null)
head = head.next;
head = node;
System.out.println(head.toCity+","+head.distance);
System.out.println(vertex[index].head.toCity+","+vertex[index].head.distance);
}basiclly. When i do System.out.println(head.toCity+","+head.distance);, it can display data
but System.out.println(vertex[index].head.toCity+","+vertex[index].head.distance);
it throow null's error.
Btw, yes , i know Java have build in Link list, but unfortunately, my professor likes to reinvent the wheel and want us to do everything from scratch...
So any other help will be useful, thank you. -
I am fairly new to Java, and Linked Lists. Can you tell me how I can fix my code? Why does my code print infinite "3" s. That is all I am trying to do is create a linked list and put that linked list into index 2 of the array. In the loop, I am setting up a whole bunch on null Node's, is it even possible to link all these nodes together in that same loop? If something does not make sense, I will try to elaborate. I will eventually impliment an insert method in the Node class, are there any pointers on how to implement it? What are the special cases I need to be looking for when implementing the insert method?
* To change this template, choose Tools | Templates
* and open the template in the editor.
package testlinkedlist;
* @author Ben
public class Main {
* @param args the command line arguments
public static Node l;
public static Node p = new Node();
public static Node Front;
public static void main(String[] args) {
Node[] anArray = new Node[4];
for(int i = 0; i < 4; i++){
anArray[i] = new Node();
l = new Node(i);
l.setLink(l);
anArray[2] = l;
while(anArray[2] != null){
System.out.println(anArray[2].dataInNode());
anArray[2] = anArray[2].next;
}l = new Node(i);
l.setLink(l);The node l is created in line one, in line two a link is created from l to l. In other words here is the presence of your infinite loop which is magnified in the following piece of code:
anArray[2] = l;
while(anArray[2] != null){
System.out.println(anArray[2].dataInNode());
anArray[2] = anArray[2].next;
}Mel -
Linked lists help... how exactly is this a NullPointerException?
So I have a linked list class with variables head and tail both initialized to "null", with variables for size, declare, and traveler (not really relevant to this though). We have to create methods to manipulate said list.
For a test I declared an instance of this linked list:
static SinglyLinkedList list = new SinglyLinkedList();One of the methods we have to create is an append method, here's the code for that:
public void append(Object dataToAdd)
if(head == null & tail == null)
head = new Node(dataToAdd, null);
tail = head;
else
tail.next = new Node(dataToAdd, null);
tail = tail.next;
}Testing it with...
list.append("A");Gives null pointer exceptions at the if statement in the append method as well as at the list.append line. But why? What is exactly pointing to null?Ah... I didn't think of head not being null but tail being null.
I initiated the traveler node in the linked list class to head.
then I added this in between the if and else statements I had:
else if(head != null & tail == null)
while(traveler.next != null)
traveler = traveler.next;
tail = traveler;
tail.next = new Node(dataToAdd, null);
tail = tail.next;
}But it's still giving me the same errors.
Edited by: klawson88 on Feb 12, 2009 12:32 AM -
Merging two linked lists of integers that are sorted
hey guys,
i need to write a method to merge two linked lists of integers that are sorted into ascending order. the results should be a third linked list that is the sorted combination of the original lists.
can someone help me start this? Basically, the method will be taking in two sorted linked lists of integers and merge them into one big sorted linked list.
Im just looking to get some algorithm ideas and maybe one or two lines of pseudocode to get me started.
thanksi can't destroy the original lists so im gonna need
to create a new sorted list. then since both of the
lists im using are sorted, i'll just have to copy one
of them into the new list i created. Then i will get
an item from the unused list and search that node in
the new copied list and insert in appropriate
position using the two Nodes...example(prev and
curr).That can work. I'd probably do it by initializing a new list and compare the first items in each of the original lists, inserting the smaller in my new list and advancing to the next element in the original list from which I took the item, repeating the process until I copied each item from both original lists. Don't forget to take into account that you will reach the end of one list before the other. -
Comparable linked list sorting
i have been trying to find a way to sort a link listed. After some reading i found out that it is probably best to use collections.sort(list). I think i understand how to implement it , i need to declare a compareto and equals method. However, most of the reading i have done shows how to create a collection from a linked list and add items to the collection and sort them, however it doesn show how to copy this sorted collection back inot a linked list. I need to remove items and add items to the sorted linked list. Is it possible to create a linked list, sort it using the collection sort and return the sorted linked list? I have looked in the api's but got confused!!
You don't need to copy anything.
List list = new LinkedList(); // or ArrayList
// list.add stuff
Collections.sort(list); Assuming whatever you added to the list implements Comarable, the list is now sorted. -
Need help regarding Linked List
I'm a beginner who just spent ages working on the following code.. but need help on re-implementing the following using a linked list, i.e. no array is allowed for customer records but you still can use arrays for names, address, etc.. Hopefully I've inserted enough comments..
Help very much appreciated!! Thanks! =]
import java.util.Scanner;
import java.io.*;
public class Bank
/* Private variables declared so that the data is only accessible to its own
class, but not to any other class, thus preventing other classes from
referring to the data directly */
private static Customer[] customerList = new Customer[30];
//Array of 30 objects created for storing information of each customer
private static int noOfCustomers;
//Integer used to store number of customers in customerList
public static void main(String[] args)
Scanner sc = new Scanner(System.in);
menu();
public static void menu()
char choice;
String filename;
int custId,counter=0;
double interestRate;
Scanner sc = new Scanner(System.in);
do
//Displaying of Program Menu for user to choose
System.out.println("ABC Bank Customer Management System Menu");
System.out.println("========================================");
System.out.println("(1) Input Data from File");
System.out.println("(2) Display Data");
System.out.println("(3) Output Data to File");
System.out.println("(4) Delete Record");
System.out.println("(5) Update Record");
System.out.println("(Q) Quit");
System.out.println();
System.out.print("Enter your choice: ");
String input = sc.next();
System.out.println();
choice = input.charAt(0);
//switch statement used to assign each 'selection' to its 'operation'
switch(choice)
case '1': int noOfRecords;
System.out.print("Enter file name: ");
sc.nextLine();
filename = sc.nextLine();
System.out.println();
noOfRecords = readFile(filename);
System.out.println(+noOfRecords+" records read.");
break;
case '2': displayRecords();
break;
case '3': writeFile();
break;
case '4': System.out.print("Enter account ID to be deleted: ");
sc.nextLine();
custId = sc.nextInt();
deleteRecord(custId);
break;
case '5': if(counter==0)
System.out.print("Enter current interest rate for saving account: ");
sc.nextLine();
interestRate = sc.nextDouble();
update(interestRate);
counter++;
else
System.out.println("Error: Accounts have been updated for the month.");
break;
}System.out.println();
}while(choice!='Q' && choice!='q');
/* The method readFile() loads the customer list of a Bank from a specified
text file fileName into customerList to be stored as array of Customer
objects in customerList in ascending alphabetical order according to the
customer names */
public static int readFile(String fileName)
int custId,i=0;
String custName,custAddress,custBirthdate,custPhone,custAccType;
double custBalance,curRate;
boolean d;
/* Try block to enclose statements that might throw an exception, followed by
the catch block to handle the exception */
try
Scanner sc = new Scanner(new File(fileName));
while(sc.hasNext())
/* sc.next() gets rid of "Account", "Id" and "=" */
sc.next();sc.next();sc.next();
custId = sc.nextInt();
d=checkDuplicate(custId);
/* checkDuplicate() is a method created to locate duplicating ids in array */
if(d==true)
/* A return value of true indicates duplicating record and the sc.nextLine()
will get rid of all the following lines to read the next customer's record */
sc.nextLine();sc.nextLine();sc.nextLine();
sc.nextLine();sc.nextLine();sc.nextLine();
continue;
/* A return value of false indicates no duplicating record and the following
lines containing the information of that customer's record is being read
in */
if(d==false)
/* sc.next() gets rid of "Name" and "=" and name is changed to upper case*/
sc.next();sc.next();
custName = sc.nextLine().toUpperCase();
/* sc.nextLine get rids of the following lines to read the next customer's
record if length of name is more than 20 characters*/
if(custName.length()>21)
System.out.println("Name of custId "+custId+" is more than 20 characters");
System.out.println();
sc.nextLine();sc.nextLine();sc.nextLine();sc.nextLine();
continue;
/* sc.next() gets rid of "Address" and "=" */
sc.next();sc.next();
custAddress = sc.nextLine();
/* sc.nextLine get rids of the following lines to read the next customer's
record if length of address is more than 80 characters*/
if(custAddress.length()>81)
System.out.println("Address of custId "+custId+" is more than 80 characters");
System.out.println();
sc.nextLine();sc.nextLine();sc.nextLine();sc.nextLine();
continue;
/* sc.next() gets rid of "DOB" and "=" */
sc.next();sc.next();
custBirthdate = sc.nextLine();
/* sc.nextLine get rids of the following lines to read the next customer's
record if length of date of birth is more than 10 characters*/
if(custBirthdate.length()>11)
System.out.println("D.O.B of custId "+custId+" is more than 10 characters");
System.out.println();
sc.nextLine();sc.nextLine();sc.nextLine();sc.nextLine();
continue;
/* sc.next() gets rid of "Phone", "Number" and "=" */
sc.next();sc.next();sc.next();
custPhone = sc.nextLine();
/* sc.nextLine get rids of the following lines to read the next customer's
record if length of phone number is more than 8 characters*/
if(custPhone.length()>9)
System.out.println("Phone no. of custId "+custId+" is more than 8 characters");
System.out.println();
sc.nextLine();sc.nextLine();sc.nextLine();sc.nextLine();
continue;
/* sc.next() gets rid of "Account", "Balance" and "=" */
sc.next();sc.next();sc.next();
custBalance = sc.nextDouble();
/* sc.next() gets rid of "Account", "Type" and "=" */
sc.next();sc.next();sc.next();
custAccType = sc.next();
if(custAccType.equals("Saving"))
customerList[noOfCustomers] = new Account1(custId,custName,custAddress,custBirthdate,custPhone,custBalance,custAccType);
sc.nextLine();
noOfCustomers++;
i++;
else if(custAccType.equals("Checking"))
customerList[noOfCustomers] = new Account2(custId,custName,custAddress,custBirthdate,custPhone,custBalance,custAccType);
sc.nextLine();
noOfCustomers++;
i++;
else if(custAccType.equals("Fixed"))
sc.next();sc.next();sc.next();sc.next();
curRate = sc.nextDouble();
Account3 temp = new Account3(custId,custName,custAddress,custBirthdate,custPhone,custBalance,custAccType,curRate);
customerList[noOfCustomers]=temp;
sc.nextLine();
noOfCustomers++;
i++;
else
System.out.println("Account type not defined.");
if(noOfCustomers==30)
System.out.println("The customer list has reached its maximum limit of 30 records!");
System.out.println();
return noOfCustomers;
//Exceptions to be caught
catch (FileNotFoundException e)
System.out.println("Error opening file");
System.exit(0);
catch (IOException e)
System.out.println("IO error!");
System.exit(0);
/* Bubblesort method used to sort the array in ascending alphabetical order
according to customer's name */
bubbleSort(customerList);
return i;
/* The method displayRecords() displays the data of the customer records on
screen */
public static void displayRecords()
int k;
/* Displaying text using the printf() method */
for(k=0;k<noOfCustomers;k++)
System.out.printf("Name = %s\n", customerList[k].getName());
System.out.printf("Account Balance = %.2f\n", customerList[k].getBalance());
System.out.printf("Account Id = %d\n", customerList[k].getId());
System.out.printf("Address = %s\n", customerList[k].getAddress());
System.out.printf("DOB = %s\n", customerList[k].getBirthdate());
System.out.printf("Phone Number = %s\n", customerList[k].getPhone());
String type = customerList[k].getAccType();
System.out.println("Account Type = " +type);
if(type.equals("Fixed"))
System.out.println("Fixed daily interest = "+((Account3)customerList[k]).getFixed());
System.out.println();
/* The method writeFile() saves the content from customerList into a
specified text file. Data is printed on the screen at the same time */
public static void writeFile()
/* Try block to enclose statements that might throw an exception, followed by
the catch block to handle the exception */
try
int i;
int n=0;
//PrintWriter class used to write contents of studentList to specified file
FileWriter fwStream = new FileWriter("newCustomers.txt");
BufferedWriter bwStream = new BufferedWriter(fwStream);
PrintWriter pwStream = new PrintWriter(bwStream);
for(i=0;i<noOfCustomers;i++)
pwStream.println("Account Id = "+customerList.getId());
pwStream.println("Name = "+customerList[i].getName());
pwStream.println("Address = "+customerList[i].getAddress());
pwStream.println("DOB = "+customerList[i].getBirthdate());
pwStream.println("Phone Number = "+customerList[i].getPhone());
pwStream.printf("Account Balance = %.2f\n", customerList[i].getBalance());
pwStream.println("Account Type = "+customerList[i].getAccType());
if(customerList[i].getAccType().equals("Fixed"))
pwStream.println("Fixed Daily Interest = "+((Account3)customerList[i]).getFixed());
pwStream.println();
n++;
//Closure of stream
pwStream.close();
System.out.println(+n+" records written.");
catch(IOException e)
System.out.println("IO error!");
System.exit(0);
//Deletes specified record from list
public static void deleteRecord(int id)
int i;
i=locate(id);
if(i==200)
//checking if account to be deleted does not exist
System.out.println("Error: no account with the id of "+id+" found!");
//if account exists
else
while(i<noOfCustomers)
customerList[i] = customerList[i+1];
i++;
System.out.println("Account Id: "+id+" has been deleted");
--noOfCustomers;
//Updates the accounts
public static void update(double interest)
int i,j,k;
double custBalance,addition=0;
for(i=0;i<noOfCustomers;i++)
if(customerList[i] instanceof Account1)
for(j=0;j<30;j++)
addition=customerList[i].getBalance()*interest;
custBalance=customerList[i].getBalance()+addition;
customerList[i].setBalance(custBalance);
else if(customerList[i] instanceof Account2)
continue;
else if(customerList[i] instanceof Account3)
for(j=0;j<30;j++)
addition=customerList[i].getBalance()*((Account3)customerList[i]).getFixed();
custBalance=customerList[i].getBalance()+addition;
customerList[i].setBalance(custBalance);
else
System.out.println("Account type not defined");
System.out.println("The updated balances are: \n");
for(k=0;k<noOfCustomers;k++)
System.out.printf("Name = %s\n", customerList[k].getName());
System.out.printf("Account Balance = %.2f\n", customerList[k].getBalance());
System.out.println();
/* ================== Additional methods ==================== */
/* Bubblesort method to sort the customerList in ascending alphabetical
order according to customer's name */
public static void bubbleSort(Customer[] x)
int pass, index;
Customer tempValue;
for(pass=0; pass<noOfCustomers-1; pass++)
for(index=0; index<noOfCustomers-1; index++)
if(customerList[index].getName().compareToIgnoreCase(customerList[index+1].getName()) > 0)
tempValue = x[index];
x[index] = x[index+1];
x[index+1]= tempValue;
/* Method used to check for duplicated ids in array */
public static boolean checkDuplicate(int id)
int i;
for(i=0;i<noOfCustomers;i++)
if(id == customerList[i].getId())
System.out.println("Account Id = "+id+" already exists");
System.out.println();
return true;
}return false;
/* Method to seach for account id in array */
public static int locate(int id)
int j;
for(j=0;j<noOfCustomers;j++)
if(customerList[j].getId()==id)
return j;
j=200;
return j;
import java.util.Scanner;
public class Customer
/* The following private variables are declared so that the data is only
accessible to its own class,but not to any other class, thus preventing
other classes from referring to the data directly */
protected int id;
protected String name,address,birthdate,phone,accType;
protected double balance;
// Null constructor of Customer
public Customer()
id = 0;
name = null;
address = null;
birthdate = null;
phone = null;
balance = 0;
accType = null;
/* The following statements with the keyword this activates the Customer
(int id, String name String address, String birthdate, String phone, double
balance) constructor that has six parameters of account id, name, address,
date of birth, phone number, account balance and assign the values of the
parameters to the instance variables of the object */
public Customer(int id, String name, String address, String birthdate, String phone, double balance, String accType)
//this is the object reference that stores the receiver object
this.id = id;
this.name = name;
this.address = address;
this.birthdate = birthdate;
this.phone = phone;
this.balance = balance;
this.accType = accType;
/* The following get methods getId(), getName(), getAddress(), getBirthdate(),
getPhone(), getBalance() return the values of the corresponding instance
properties */
public int getId()
return id;
public String getName()
return name;
public String getAddress()
return address;
public String getBirthdate()
return birthdate;
public String getPhone()
return phone;
public double getBalance()
return balance;
public String getAccType()
return accType;
/* The following set methods setId(), setName(), setAddress(), setBirthdate(),
setPhone and setBalance() set the values of the corresponding instance
properties */
public void setId (int custId)
id = custId;
public void setName(String custName)
name = custName;
public void setAddress (String custAddress)
address = custAddress;
public void setBirthdate (String custBirthdate)
birthdate = custBirthdate;
public void setPhone (String custPhone)
phone = custPhone;
public void setBalance (double custBalance)
balance = custBalance;
public void setAccType (String custAccType)
accType = custAccType;
class Account1 extends Customer
public Account1(int id, String name, String address, String birthdate, String phone, double balance, String accType)
super(id,name,address,birthdate,phone,balance,accType);
this.id = id;
this.name = name;
this.address = address;
this.birthdate = birthdate;
this.phone = phone;
this.balance = balance;
this.accType = accType;
class Account2 extends Customer
public Account2(int id, String name, String address, String birthdate, String phone, double balance, String accType)
super(id,name,address,birthdate,phone,balance,accType);
this.id = id;
this.name = name;
this.address = address;
this.birthdate = birthdate;
this.phone = phone;
this.balance = balance;
this.accType = accType;
class Account3 extends Customer
protected double fixed=0;
public Account3(int id, String name, String address, String birthdate, String phone, double balance, String accType, double fixed)
super(id,name,address,birthdate,phone,balance,accType);
this.id = id;
this.name = name;
this.address = address;
this.birthdate = birthdate;
this.phone = phone;
this.balance = balance;
this.accType = accType;
this.fixed = fixed;
public double getFixed()
return fixed;
Example of a customers.txt
Account Id = 123
Name = Matt Damon
Address = 465 Ripley Boulevard, Oscar Mansion, Singapore 7666322
DOB = 10-10-1970
Phone Number = 790-3233
Account Balance = 405600.00
Account Type = Fixed
Fixed Daily Interest = 0.05
Account Id = 126
Name = Ben Affleck
Address = 200 Hunting Street, Singapore 784563
DOB = 25-10-1968
Phone Number = 432-4579
Account Balance = 530045.00
Account Type = Saving
Account Id = 65
Name = Salma Hayek
Address = 45 Mexican Boulevard, Hotel California, Singapore 467822
DOB = 06-04-73
Phone Number = 790-0000
Account Balance = 2345.00
Account Type = Checking
Account Id = 78
Name = Phua Chu Kang
Address = 50 PCK Avenue, Singapore 639798
DOB = 11-08-64
Phone Number = 345-6780
Account Balance = 0.00
Account Type = Checking
Account Id = 234
Name = Zoe Tay
Address = 100 Blue Eyed St, Singapore 456872
DOB = 15-02-68
Phone Number = 456-1234
Account Balance = 600.00
Account Type = Saving1) When you post code, please use[code] and [/code] tags as described in Formatting tips on the message entry page. It makes it much easier to read.
2) Don't just post a huge pile of code and ask, "How do I make this work?" Ask a specific question, and post just enough code to demonstrate the problem you're having.
3) Don't just write a huge pile of code and then test it. Write a tiny piece, test it. Then write the piece that will work with or use the first piece. Test that by itself--without the first piece. Then put the two together and test that. Only move on to the next step after the current step produces the correct results. Continue this process until you have a complete, working program. -
Alright, here's the problem. I have my linked list, either imported from a text file or inputted by the user. Now, I need to sort it by the date and the time strings... Yet, every time I do I get an NullPointException. Can anyone help? Here's my code for the list:
public class ListReferenceBased implements ListInterface{
//references to the linked list items
private Node head;
private int numItems; //number of items that exist in list
//Default Constructor
//sets both items to null
public ListReferenceBased(){
numItems = 0;
head = null;
//if the numItems in the list is empty
//returns true.
public boolean isEmpty(){
return numItems == 0;
// returns numItems, aka the size of the list
public int size(){
return numItems;
//finds a node by setting a node to the 'head'
// and loops through to the index 'resetting'
// curr to the 'next' node.
private Node find(int index){
Node curr = head;
for( int skip = 1; skip < index; skip++){
curr = curr.getNext();
return curr;
public Object get(int index)
throws ListIndexOutOfBoundsException{
//catches if list does not reach the index given
//loops through the index
if(index >= 1 && index <= numItems){
//sets curr to the index through find()
Node curr = find(index);
//sets dataitem to the current node
Object dataItem = curr.getItem();
return dataItem;
}else{
throw new ListIndexOutOfBoundsException("The List index is out of bounds on get");
public void add(int index, Object item)
throws ListIndexOutOfBoundsException{
//checks to make sure there are no nodes yet
//if there are inserts them in the list
//by moving
if(index >=1&& index<= numItems+1){
if(index==1){
// if no list exists
//creates a new node and sets it to head
Node newNode = new Node(item, head);
head = newNode;
}else{
//inserts the node at the index
//by finding it first
Node prev = find(index-1);
//creates a new node by placing the Object item
//and the 'new prev' node into a node
//'deleting' the old 'bonds'
Node newNode = new Node(item, prev.getNext());
rev.setNext(newNode);
//increments number of items in list
numItems++;
}else{
throw new ListIndexOutOfBoundsException("List index out of bounds");
public void remove(int index)
throws ListIndexOutOfBoundsException{
//catches the list if it doesn't exist
//checks to make sure list exists
if(index>=1&& index<= numItems){
//if only the head exists
//calls next node else moves
//the node away and moves the prev
//on up.
if(index==1){
head = head.getNext();
}else{
Node prev = find(index-1);
Node curr = prev.getNext();
prev.setNext(curr.getNext());
//decreases the number of items in list
numItems--;
}else{
throw new ListIndexOutOfBoundsException("List index out of bounds");
//sets the numItem to null telling the list it
//does not exist
public void removeAll(){
head = null;
numItems=0;
Yeah, unfortunately, I had to design my own linked list for my assignment (that's taken almost 40+ hours). So, now every alogrithm I've tried to write seems to give me a null pointer, for example this one
public void sort(){
//loop through all existing nodes
for(Node curr = head; curr != null; curr = curr.getNext()){
//parse the string of the current node
//to a string value
Node current = curr.getNext();
Node next = current.getNext();
Node temp;
String thisline = current.getItem().toString();
System.out.println(thisline);
StringTokenizer parser = new StringTokenizer ( thisline, "\t" );
String thisdate = parser.nextToken();
String thistime = parser.nextToken();
String thisline2 = next.getItem().toString();
System.out.println(thisline2);
StringTokenizer parser2 = new StringTokenizer ( thisline2, "\t" );
String thisdate2 = parser.nextToken();
String thistime2 = parser.nextToken();
//compares the dates of next in list
if(thisdate.compareTo(thisdate2)==1){
next.setNext(current);
current.setNext(next.getNext());
printList();
//if dates equal, compare the times
//if equal move to new position
//delete old position
}Pretty much I believe moving the positions is causing the problems after the first run around. Any suggestions? -
Linked List for custom insertion sorting not listing
Hello fellow humans! :3 this is an assignment; the little menu's options are the tasks it's supposed to perform. Sadly, case 1 only records the last valid (non -1) entry instead of building a list. case 2 does the same, except for actually recording the ending value (-1) instead of the last valid entry. Cases 3 and 4 are completely useless without the first two; I've included them because being quite new I'd rather not exclude anything important.
class with main method follows:
package batch2;
import java.io.*;
class e21
public static void main (String []args) throws IOException, NumberFormatException
BufferedReader input = new BufferedReader (new InputStreamReader (System.in));
FromKeyboard key = new FromKeyboard();
int a, option = 0, lrg;
MyList alist, blist, clist;
char fin = 'v';
boolean avac, bvac, cvac, bord;
NodeLD nodeaux = null;
alist = new MyList();
blist = new MyList();
clist = new MyList();
do{
System.out.println("Please choose an option:");
System.out.println("1. Create list 'A' sorting by insertion");
System.out.println("2. Create list 'B', unsorted");
System.out.println("3. Sort list 'B'");
System.out.println("4. Mix lists 'A' & 'B' into a new sorted list 'C'");
System.out.println("9. Exit");
option = key.enterInt();
switch (option){
case 1: //this only records the last valid (non -1) entry D:
System.out.println("Enter the integers you wish you sort in list 'A'");
System.out.println("To finish, enter '-1'");
a = key.enterInt();
if (a != -1){
alist.insertToList(a);
System.out.println("Test 1");
while (a != -1){
a = key.enterInt();
if (a != -1){
System.out.println("Test 1.1");
nodeaux = new NodeLD();
nodeaux.theData(a);
alist.sortInserList(nodeaux);
System.out.println("Test 2, a="+a+"");
System.out.println("List 'A', sorted by insertion:");
alist.seeList();
break;
case 2: //this also keeps the last value, yet not a valid one but the '-1' for the ending condition
System.out.println("Enter the integers you wish to add to list 'B'");
System.out.println("Enter '-1' to finish");
a = 0;
while (a != -1){
if (a != -1){
a = key.enterInt();
clist.insertToList(a);
System.out.println("Unsorted list:");
clist.seeList();
break;
case 3:
while (clist.front != null){
if (clist.front != null){
blist.sortInserList(clist.front);
clist.front = clist.front.givePrevious();
System.out.println("List 'B', sorted");
blist.seeList();
break;
case 4:
while (blist.front != null){
if (blist.front != null){
alist.sortInserList(blist.front);
blist.front = blist.front.givePrevious();
System.out.println("List 'C', combining 'A' & 'B', sorted");
clist.seeList();
break;
case 9:
System.out.println("Program ended");
break;
default:
System.out.println("Invalid option");
break;
while (option != 9);
}Custom list class follows:
package batch2;
class MyList
boolean tag = false;
protected NodeLD front;
protected int size;
public MyList(){
front = null;
size = 0;
public void insertToList(int x){
NodeLD t;
t = new NodeLD();
t.theData(x);
t.theNext(front);
if(front!=null){
front.thePrevious(t);
front = t;
size++;
public void seeList(){
NodeLD g;
g = front;
System.out.println();
while (g!=null){
System.out.println(""+g.dat+"");
g = g.givePrevious();
System.out.println("=============================");
public int sortInserList(NodeLD g){
NodeLD pointer = new NodeLD();
pointer = front;
NodeLD prepointer = new NodeLD();
System.out.println("Test A");
while (g.dat > pointer.dat && pointer != null){
if (pointer.giveNext()!= null){
pointer = pointer.giveNext();
tag = false;
System.out.println("Test A.b");
else if (pointer.giveNext() == null) {
tag = true;
System.out.println("Test A.c");
break;
prepointer = pointer.givePrevious();
if (pointer == null || tag){
insertToList(g.dat);
System.out.println("Test B. Pointer == null or tag == true");
if (prepointer != null && !tag){
g.thePrevious(prepointer);
prepointer.theNext(g);
System.out.println("Test C. prepointer != null && !tag");
if (pointer != null && !tag){
g.theNext(pointer);
pointer.thePrevious(g);
System.out.println("Test D. pointer !=null && !tag");
System.out.println("Test E. Right before return");
return g.dat;
}And here comes the accompanying Node class:
package batch2;
class NodeLD
private int data;
private NodeLD next;
private NodeLD previous;
public int dat;
NodeLD(){
data = 0;
next = null;
previous = null;
dat = data;
public void theData(int x){
data = x;
dat = data;
public void theNext(NodeLD y){
next = y;
public void thePrevious(NodeLD z){
previous = z;
public int giveData(){
return data;
public NodeLD giveNext(){
return next;
public NodeLD givePrevious(){
return previous;
}And last but not least, the part of FromKeyboard I'm using for this exercise, just in case:
package batch2;
import java.io.*;
public class FromKeyboard
public BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
public int enterInt(){
System.out.flush();
int integ = -1;
try{
integ = Integer.valueOf(input.readLine());
catch(NumberFormatException e){
System.err.println("Error: "+e.getMessage()+"");
catch(IOException t){
System.err.println("Error: "+t.getMessage()+"");
return integ;
}All help with regards as to why my lists aren't working and general comments/nudges on the sorting are welcome and appreciated.Hi Kevin, thank you for replying! I'll trim my code in a moment; as for a specific inquiry:
>Sadly, case 1 only records the last valid (non -1) entry instead of building a list. case 2 does the same, except for actually recording the ending value (-1) instead of the last valid entry. >
More direct approach to asking for help: These classes compile correctly and run, although the result is not what I'm aiming for: I get a single-noded list instead of the full list. Why isn't it keeping all the nodes but the last one only?
Edited by: SquaredCircle on 09-Sep-2010 16:46
That was fun :D this is my SSCCE for the main class:
class e21SSCCE
public static void main (String []args) throws NumberFormatException
int a[] = {14, 25, 11, 43, 33, -1}, key[] = {1,2,9}, option = 0, i = 0, e = 0;
MyList alist, clist;
char fin = 'v';
NodeLD nodeaux = null;
alist = new MyList();
clist = new MyList();
do{
System.out.println("Please choose an option:");
System.out.println("1. Create list 'A' sorting by insertion");
System.out.println("2. Create list 'C', unsorted");
System.out.println("9. Exit");
option = key[e];
e++;
switch (option){
case 1: //this only records the last valid (non -1) entry D:
System.out.println("Enter the integers you wish you sort in list 'A'");
System.out.println("To finish, enter '-1'");
i = 0;
if (a[i] != -1){
alist.insertToList(a);
i++;
while (a[i] != -1){
if (a[i] != -1){
nodeaux = new NodeLD();
nodeaux.theData(a[i]);
alist.sortInserList(nodeaux);
i++;
System.out.println("List 'A', sorted by insertion:");
alist.seeList();
break;
case 2: //this also keeps the last value, yet not a valid one but the '-1' for the ending condition
System.out.println("Enter the integers you wish to add to list 'B'");
System.out.println("Enter '-1' to finish");
i = 0;
while (a[i] != -1){
if (a[i] != -1){
clist.insertToList(a[i]);
i++;
System.out.println("Unsorted list:");
clist.seeList();
break;
case 9:
System.out.println("Program ended");
break;
default:
System.out.println("Invalid option");
break;
while (option != 9);
The cleaner version of the list class:class MyList
boolean tag = false;
protected NodeLD front;
protected int size;
public MyList(){
front = null;
size = 0;
public void insertToList(int x){
NodeLD t;
t = new NodeLD();
t.theData(x);
t.theNext(front);
if(front!=null){
front.thePrevious(t);
front = t;
size++;
public void seeList(){
NodeLD g;
g = front;
System.out.println();
while (g!=null){
System.out.println(""+g.dat+"");
g = g.givePrevious();
System.out.println("=============================");
public int sortInserList(NodeLD g){
NodeLD pointer = new NodeLD();
pointer = front;
NodeLD prepointer = new NodeLD();
while (g.dat > pointer.dat && pointer != null){
if (pointer.giveNext()!= null){
pointer = pointer.giveNext();
tag = false;
else if (pointer.giveNext() == null) {
tag = true;
break;
prepointer = pointer.givePrevious();
if (pointer == null || tag){
insertToList(g.dat);
if (prepointer != null && !tag){
g.thePrevious(prepointer);
prepointer.theNext(g);
if (pointer != null && !tag){
g.theNext(pointer);
pointer.thePrevious(g);
return g.dat;
And the node class:class NodeLD
private int data;
private NodeLD next;
private NodeLD previous;
public int dat;
NodeLD(){
data = 0;
next = null;
previous = null;
dat = data;
public void theData(int x){
data = x;
dat = data;
public void theNext(NodeLD y){
next = y;
public void thePrevious(NodeLD z){
previous = z;
public int giveData(){
return data;
public NodeLD giveNext(){
return next;
public NodeLD givePrevious(){
return previous;
I followed the SSCCE guideline, and tried my best with it. The indentation checker was a broken link, though :c.
Edited by: SquaredCircle on 09-Sep-2010 17:12
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