How can I hold data stored in an object?

Question

Hi everyone!
I have a problem with a Java-Program that I am trying to write for university.
The program is the following:
import java.io.*;
public class Ergasia4
{     public static void main (String args[])throws IOException
     {     int i,epilogi;
          boolean first1=true, first2=true, first3=true, first4=true;
          Ptisi A3510,A3511,A3512,A3513;
          A3510 = new Ptisi();
          A3511 = new Ptisi();
          A3512 = new Ptisi();
          A3513 = new Ptisi();
          i=0;
          for(;;)
          {     do     
               {     if (i==1) System.out.println ("Lathos epilogi");
                    System.out.println ("EPILOGI PTISIS");
                    System.out.println ("1. Ptisi A3510, ATH-SKG");
                    System.out.println ("2. Ptisi A3511, SKG-ATH");
                    System.out.println ("3. Ptisi A3512, ATH-SKG");
                    System.out.println ("4. Ptisi A3513, SKG-ATH");
                    System.out.println ("0. Exodos");
                    System.out.print ("Epilogi: ");
                    epilogi=my.readInt();
                    System.out.println ();
                    i=1;
               while ((epilogi!=0)&&(epilogi!=1)&&(epilogi!=2)&&(epilogi!=3)&&(epilogi!=4));
               if (epilogi==0) break;
               else
               {     switch (epilogi)
                    {     case 1: System.out.println("Ptisi A3510, ATH-SKG"); A3510.menu(first1); break;
                         case 2: System.out.println("Ptisi A3511, SKG-ATH"); A3511.menu(first2); break;
                         case 3: System.out.println("Ptisi A3512, ATH-SKG"); A3512.menu(first3); break;
                         case 4: System.out.println("Ptisi A3513, SKG-ATH"); A3513.menu(first4); break;
                         default : break;
class Ptisi
{     int epilogi,i;
     private boolean availiability[];
     private long telephones[];
     public void menu (boolean a) throws IOException     
     {     if (a==true) arxikopoiisi(availiability);
          a=false;
          for (;;)
          {     do
               {     if (i==1) System.out.println ("Lathos epilogi");
                    System.out.println ("1. Kratisi thesewn");
                    System.out.println ("2. Akyrwsi kratisis");
                    System.out.println ("3. Emfanisi sygentrwtikwn stoixeiwn");
                    System.out.println ("0. Exodos");
                    System.out.print ("Epilogi: ");
                    epilogi=my.readInt();
                    System.out.println ();
                    i=1;
               while ((epilogi!=0)&&(epilogi!=1)&&(epilogi!=2)&&(epilogi!=3));
               if (epilogi==1)
               {     kratisi(availiability,telephones);
                    i=0;
               else if (epilogi==2)
               {     akyrwsi(availiability,telephones);
                    i=0;
               else if (epilogi==3)
               {     emfanisi(availiability);
                    i=0;
               else if (epilogi==0) break;
     public static void kratisi (boolean availiability[],long telephones[]) throws IOException
     {     int theseis;
          long tilefwno;
          int j,i=0,x=-1;
          boolean k=false;
          i=0;
          do
          {     if (i==1) System.out.println ("Arithmos ektos oriwn");
               System.out.println ("Poses theseis theleis na kratiseis ?");
               System.out.print ("Theseis: ");     
               theseis=my.readInt ();
               System.out.println ();
               i=1;
          while ((theseis<0)||(theseis>36));
          for (i=0;i<availiability.length;i++)
          {     if (availiability==true)
               {     for (j=i;j<=(i+theseis);j++)
                    {     if (availiability[j]==false)
                         {     break;
                         else if (j==(i+theseis))
                         {     x=i;
                              break;
               if (x>-1) break;
          if (x>-1);
          {     i=0;
               do
               {     if (i==1) System.out.println ("Lathos arithmos tilefwnou");
                    System.out.println ("Eisagete ton arithmo tilefwnou");
                    System.out.print ("Arithmos: ");     
                    tilefwno=my.readInt ();
                    System.out.println ();
                    i=1;
               while (tilefwno<0);
          if (x>-1)
          {     for (i=x;i<(x+theseis);i++)
               {     availiability[i]=false; telephones[i]=tilefwno;
               System.out.println("I kratisi "+theseis+" thesewn egine epityxws");
               System.out.println();
          else System.out.println("Den yparxoun "+theseis+" diathesimes synexomenes theseis");
     public static void akyrwsi (boolean availiability[],long telephones[]) throws IOException
     {     int i,thesi,plithos,x=-1;
          i=0;
          do
          {     if (i==1) System.out.println ("Arithmos ektos oriwn");
               System.out.println ("Dwse tin prwti thesi tis akyrwsis");
               System.out.print ("Prwti thesi: ");     
               thesi=my.readInt ();
               i=1;
          while ((thesi<0)||(thesi>36));
          i=0;
          do
          {     if (i==1) System.out.println ("Arithmos ektos oriwn");
               System.out.println ("Poses theseis theleis na akyrwseis ?");
               System.out.print ("Prwti thesi: "+thesi+" Theseis: ");     
               plithos=my.readInt ();
               System.out.println ();
               i=1;
          while ((plithos<0)||(plithos>36-thesi));
          for (i=thesi;i<=(thesi+plithos-1);i++)
          {     if (availiability[i]==true)
               {     break;
               else if (i==(thesi+plithos-1))
               {     x=i;
                    break;
          if (x>-1)
          {     for (i=thesi;i<(thesi+plithos);i++)
               {     availiability[i]=true; telephones[i]=0;
               System.out.println ("I akyrwsi egine epityxws");
               System.out.println ();
          else
          {     System.out.println ("I epilegmenes theseis einai kenes");
     public static void emfanisi (boolean availiability []) throws IOException
     {     char seatmap[][]=new char [10][10];
          int k,i,j,plirotita=0;
          for (i=0;i<availiability.length;i++)
          {     if (availiability[i]==false) plirotita++;
          plirotita=plirotita*100/36;
          System.out.println("Plirotita :"+plirotita+"%");
          i=0;               
          for (j=0;j<4;j++)
          {     for (k=0;k<9;k++)
               {     if (availiability[i]==false) seatmap[j][k]='X';
                    else seatmap[j][k]='_';
                    i++;
          for (j=0;j<4;j++)
          {     for (k=0;k<9;k++)
               {     System.out.print (seatmap[j][k]+" ");
                    if ((k+1)%4==0) System.out.println();
     public void arxikopoiisi (boolean availiability[])
     {     int i;
          for (i=0;i<availiability.length;i++)
          {     availiability[i]=true;
So, this program is supposed to simulate an airline's reservations system.
There are four flights, numbered 510-513, each with an aircraft of 36 seats (9 rows of 4 seats each).
On class "Ptisi" there are 5 classes, which do the following:
a. class "menu" shows a menu, where the user can select if he wants to make a reservation, a cancellation or if he wants to view the seat map.
b. class "kratisi" performs a reservation.
c. class "akyrwsi" performs a cancellation.
d. class "emfanisi" shows the seat map.
e. class "arxikopoiisi" initializes the array "availability[36], making it all "true".
Class "Ergasia4" is supposed to create four objects, A3510-A3513, for each of the 4 flights. Then there is a menu for the user to select between the four flights. Depending on the choice of the user, the program must show the menu for one of the four available flights, as programmed on class "Ptisi" and then perform the action(s) requested by the user. On both menus, "0" exits.
So, what is my problem?
Well, when I run the program, if I select a flight, then all actions are performed without a problem, I can make a reservation, cancel some seats or view the seat map. The problem comes when I exit a flight, then work on another flight and then return on the original one. All seat reservations are gone!
Can anybody help me solve this problem?
Thank you in advance,
philimonas
P.S. If you think it would be helpful, I can omit the code of the functions that work!

Answer 1

Thanks for your help, doremifasollatido!!
So, the code now looks as follows (I am posting a "smaller" version of it)
import java.io.*;
public class Ergasia4
{     public static void main (String args[])throws IOException
     {     int epilogi;
          boolean secondTry;
          Ptisi A3510,A3511,A3512,A3513;
          A3510 = new Ptisi();
          A3511 = new Ptisi();
          A3512 = new Ptisi();
          A3513 = new Ptisi();
          secondTry=false;
          for(;;)
          {     do     
               {     if (secondTry)
                    {     System.out.println ("Lathos epilogi");
                    System.out.println ("EPILOGI PTISIS");
                    System.out.println ("1. Ptisi A3510, ATH-SKG");
                    System.out.println ("2. Ptisi A3511, SKG-ATH");
                    System.out.println ("3. Ptisi A3512, ATH-SKG");
                    System.out.println ("4. Ptisi A3513, SKG-ATH");
                    System.out.println ("0. Exodos");
                    System.out.print ("Epilogi: ");
                    epilogi=my.readInt();
                    System.out.println ();
                    secondTry=true;
               while ((epilogi<0) || (epilogi>4));
               if (epilogi==0) break;
               else
               {     switch (epilogi)
                    {     case 1:
                         {     System.out.println("Ptisi A3510, ATH-SKG");
                              A3510.menu();
                              secondTry=false;
                              break;
                         case 2:
                         {     System.out.println("Ptisi A3511, SKG-ATH");
                              A3511.menu();
                              secondTry=false;
                              break;
                         case 3:
                         {     System.out.println("Ptisi A3512, ATH-SKG");
                              A3512.menu();
                              secondTry=false;
                              break;
                         case 4:
                         {     System.out.println("Ptisi A3513, SKG-ATH");
                              A3513.menu();
                              secondTry=false;
                              break;
                         default : break;
class Ptisi
     private boolean availability[];
     private long telephones[];
     public Ptisi()
     {     availability = new boolean[36]; // Do you do this somewhere?
             arxikopoiisi();
     //Displays the menu for action-selection     
     public void menu () throws IOException     
     {      //Variables
             //Shows the menu
     //Method to reserve seats
     public void kratisi () throws IOException
     {     //Variables
          //Asks the number of seats to be booked and checks for input errors          
          //Finds, if available, the first seat of the booking
          //Asks for the telephone number to be used at the booking, if a booking is to be made
                if (x>-1);
          {     i=0;
               do
               {     if (i==1) System.out.println ("Lathos arithmos tilefwnou");
                    System.out.println ("Eisagete ton arithmo tilefwnou");
                    System.out.print ("Arithmos: ");     
                    tilefwno=my.readInt ();
                    System.out.println ();
                    i=1;
               while (tilefwno<0);
          //Performs the booking, or displays an error message
                if (x>-1)
          {     for (i=x;i<(x+theseis);i++)
               {     availability=false; telephones[i]=tilefwno;
               System.out.println("I kratisi "+theseis+" thesewn egine epityxws");
               System.out.println();
          else System.out.println("Den yparxoun "+theseis+" diathesimes synexomenes theseis");
     //Method to make a cancellation
     public void akyrwsi () throws IOException
     {     //Variables
          //Asks for the first seat of the cancellation and checks for input errors
          //Asks for the total seats to be canceled and checks for input errors
          //Checks if the seats to be canceled are really booked
          //Performs the cancellation, or displays an error message
     //Method to display the % of occupancy and the seatmap
     public void emfanisi () throws IOException
     {     //Variables
          //Calculates the % of occupancy          
          //Displays of the % of occupancy
          //Creation of the seatmap
          //Display of the seatmap
//Method to initialize the array "arxikopoiisi", giving it "true" as a value
     public void arxikopoiisi ()
     {     for (int i=0;i<availability.length;i++)
          {     availability[i]=true;
My problem now is an exception in thread "main" java.lang.NullPointerException at this line:
{     availability=false; telephones[i]=tilefwno;
Any more ideas?
Regards,
philimonas