java - Reading unknown number of objects from file -

the assignment

create program accepts , stores data winter olympics. program should accept , store 6 competitor names each event name of event participated in. should sort scores or times of events showing medal winners medal won.

i have created "event" class going smoothly, run problems when trying write "event" objects file , read them.

here method create new event:

public static void addevent() {     file events = new file("events.dat");     scanner input = new scanner(system.in);     string eventname;     arraylist<event> olympics = new arraylist<event>();     boolean fileexists = false;      if (events.exists()) {         fileexists = true;     } else {         try {             events.createnewfile();             system.out.println("file created");         } catch (ioexception e) {             system.out.println("file not created");             system.err.println("ioexception: " + e.getmessage());         }     }      system.out.print("enter name of event: ");     eventname = input.nextline();     olympics.add(new event(eventname));      //write objects     try {         fileoutputstream out = new fileoutputstream(events, fileexists);         objectoutputstream writeevent = new objectoutputstream(out);         writeevent.writeobject(olympics);         writeevent.close();     } catch (filenotfoundexception e) {         system.out.println("file not found.");         system.err.println("filenotfoundexception: " + e.getmessage());     } catch (ioexception e) {         system.out.println("problem input/output.");         system.err.println("ioexception: " + e.getmessage());     }     menu(); }// end addevent() 

and method display file:

public static void displayfile(file datfile) {     file events = datfile;     string output ="";      //read objects     try {         fileinputstream in = new fileinputstream(events);         objectinputstream readevents = new objectinputstream(in);         arraylist<event> recoveredevents = new arraylist<event>();          while (readevents.readobject() != null) {             recoveredevents.add((event)readevents.readobject());         }         readevents.close();         (event e: recoveredevents) {             //system.out.println(e + "\n");             output += e + "\n";         }     } catch(filenotfoundexception e) {         system.out.println("file not exist or not found");         system.err.println("filenotfoundexception: " + e.getmessage());     } catch (ioexception e) {         system.out.println("problem reading file");         system.err.println("ioexception: " + e.getmessage());     } catch (classnotfoundexception e) {         system.out.println("class not used cast object.");         system.err.println("classnotfoundexception: " + e.getmessage());     }     system.out.println(output);     menu(); }// end displayfile() 

this beginning class , handling files has been difficult wrap head around far. edit: forgot mention throws ioexception message "null" when try call displayfile().

1. you're getting eofexception. when readobject() gets end of stream doesn't return null. check javadoc. throws eofexception. should catch separately , not treat error needs reporting.

2. you're throwing away every odd element. read loop should like:

   while (true) {     recoveredevents.add((event)readobject.readobject()); } 

3. you can't append file written objectoutputstream. ioexception: invalid type code ac when join when reading. there ways around non-trivial. keep file open.

4. new fileoutputstream() creates file. exists/createnewfile crud before waste of time , space.