Last Project -- Need Help, Please

Question

Okay, this is my last project, and I am struggling with it. The specification we were given was to write a program that reads in a file of dates and outputs the dates in descending order, using a priority queue. The PriorityQueue should implement the PriorityQueueInterface interface, and should implements a priority queue using a singly linked list. The list should be maintained in sorted order. This implementation should not use a heap.
I have the code built, but when I run it is not reading my dates fully, and I get an error message. The LinkList is already built for us, but here is the rest of my code. First I input from my dates.txt:
11/12/2000
04/04/2004
01/02/1998
import java.io.*;
public class DateDriver {
   public static void main(String[] args) throws IOException {
      PriorityQueue pq = new PriorityQueue();
      // ... read all Date objects from the file and enqueue them in the pq object
      BufferedReader in = new BufferedReader (new FileReader("C:/Users/April/Desktop/UMUC/CMIS241/Project4/Dates.txt"));
      Date date = new Date();
      date.input(in);
      while (!date.isNull()){
           pq.add(date);
           date = new Date();
           date.input(in);
           System.out.println(date);
      // ... dequeue the pq object and display the Date objects
      System.out.println("The dates in descending order are: ");
      while (!pq.isEmpty()){
           date = (Date) pq.dequeue();
           date.output(System.out);
import java.io.BufferedReader;
import java.io.IOException;
import java.io.PrintStream;
public class Date implements InOutputable, Comparable {
   // ... define monthNames as a ?static final? array of String objects
   //  initialize the array with the first 3 characters of each month
     static final String[] monthNames = {null, "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep",
          "Oct", "Nov", "Dec"};
   // ... define the instance variables of class Date, i.e. day, month, year
     private String monthAbr, date;
     private int month, day, year;
   // ... define the default constructor (initializes the instance variables to zero)
     public Date(){
          this.date = "";
          month = 00;
          day = 00;
          year = 0000;
   // ... define the method toString
   //     This method should return the corresponding Date string in the following format: Nov, 11, 2006
     public String toString() {
           return (monthAbr + ", " + day + ", " + year);
   // ... define the methods specified by the interface InOutputable
public boolean equals(InOutputable other) {
     Date otherDate = (Date) other;
     if (month == otherDate.month && day == otherDate.day && year == otherDate.year)
          return true;
     else
          return false;
public void input(BufferedReader in) throws IOException {
     date = in.readLine();
     if (date == null)
          return;
     month = Integer.parseInt(date.substring(0, 2));
     day = Integer.parseInt(date.substring(3, 5));
     year = Integer.parseInt(date.substring(6, 10));
public boolean isNull() {
     if (month == 0 && day == 0 && year == 0)
          return true;
     else
          return false;
public void output(PrintStream writer) {
     writer.println(toString());
   // ... define the method specified by the interface Comparable
public int compareTo(Object o) {
     Date otherDate = (Date) o;
     if(month == otherDate.month && day == otherDate.day && year == otherDate.year)
          return 0;
     else if (month >= otherDate.month && day >= otherDate.day && year >= otherDate.year)
          return 1;
     else
          return -1;
public class PriorityQueue implements PriorityQueueInterface {   
   private RefSortedList lst = new RefSortedList();
   // ... define the methods specified by the interface PriorityQueueInterface
public Comparable dequeue() throws EmptyQueue {
     return null;
public void enqueue(Comparable key) throws FullQueue {
public boolean isEmpty() {
     return false;
   // ... Use the lst object (by invoking list specific methods on it) when
   //     defining the PriorityQueueInterface methods.
   //     For example, method enqueue should be implemented by invoking list method add.
public void add(Date date){
     lst.add(date);
public void remove(Date date){
     lst.remove(date);
}My output:
Exception in thread "main" java.lang.NullPointerException
     at DateDriver.main(DateDriver.java:27)
null, 4, 2004
null, 2, 1998
null, 0, 0
The dates in descending order are:
Can anybody please point me in the right direction? I know I am missing something, but I am unable to pinpoint it. I don't understand why the date.output(System.out) is throwing a null pointer exception. I am still working, but I thought maybe a fresh pair of eyes would help me.

Answer 1

Dates implement Comparable... no need to sort them yourself
SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy");
LinkedList<Date> dtes = new LinkedList<Date>();
BufferedReader in = new BufferedReader (new FileReader("C:/dates.txt"));
while (in.read()>0){
dtes.add(sdf.parse(in.readLine()));
}Edited by: wpp289 on Dec 11, 2008 1:27 PM