Java - How Can I Write My ArrayList to a file and Read load that file to the original ArrayList

Question

I am writing a program in Java which displays a range of afterschool clubs  E G  Football  Hockey - entered by user   The clubs are added into the following ArrayList     private ArrayList lt Club gt  clubs   new ArrayList lt Club gt       By the followng Method   public void addClub String clubName        Club club   findClub clubName       if  club    null          clubs add new Club clubName         Club  is a class with a constructor - name   public class Club        private String name       public Club String name            this name   name               There are more methods in my program but don t affect my query       My program is working - it lets me add a new Club Object into my arraylist  i can view the arraylist  and i can delete any that i want etc    However  I now want to save that arrayList  clubs  to a file  and then i want to be able to load the file up later and the same arraylist is there again   I have two methods for this  see below   and have been trying to get it working but havent had anyluck  any help or advice would be appreciated   Save Method  fileName is chosen by user   public void save String fileName  throws FileNotFoundException       String tmp   clubs toString        PrintWriter pw   new PrintWriter new FileOutputStream fileName        pw write tmp       pw close        Load method  Current code wont run - File is a string but needs to be Club   public void load String fileName  throws FileNotFoundException       FileInputStream fileIn   new FileInputStream fileName       Scanner scan   new Scanner fileIn       String loadedClubs   scan next        clubs add loadedClubs       I am also using a GUI to run the application  and at the moment  i can click my Save button which then allows me to type a name and location and save it  The file appears and can be opened up in Notepad but displays as something like Club c5d8jdj  for each Club in my list

User · Answer

In Java 8 you can use Files write   method with two arguments  Path and List lt String gt   something like this   List lt String gt  clubNames   clubs stream        map Club  getName       collect Collectors toList     try       Files write Paths get fileName   clubNames     catch  IOException e        log error  Unable to write out names   e

User · Answer

As an exercise  I would suggest doing the following   public void save String fileName  throws FileNotFoundException       PrintWriter pw   new PrintWriter new FileOutputStream fileName        for  Club club   clubs          pw println club getName         pw close        This will write the name of each club on a new line in your file    Soccer Chess Football Volleyball        I ll leave the loading to you  Hint  You wrote one line at a time  you can then read one line at a time   Every class in Java extends the Object class  As such you can override its methods  In this case  you should be interested by the toString   method  In your Club class  you can override it to print some message about the class in any format you d like   public String toString         return  Club     name      You could then change the above code to   public void save String fileName  throws FileNotFoundException       PrintWriter pw   new PrintWriter new FileOutputStream fileName        for  Club club   clubs           pw println club      call toString   on club  like club toString       pw close

User · Answer

You should use Java s built in serialization mechanism  To use it  you need to do the following    Declare the Club class as implementing Serializable   public class Club implements Serializable               This tells the JVM that the class can be serialized to a stream  You don t have to implement any method  since this is a marker interface  To write your list to a file do the following   FileOutputStream fos   new FileOutputStream  t tmp    ObjectOutputStream oos   new ObjectOutputStream fos   oos writeObject clubs   oos close     To read the list from a file  do the following   FileInputStream fis   new FileInputStream  t tmp    ObjectInputStream ois   new ObjectInputStream fis   List lt Club gt  clubs    List lt Club gt   ois readObject    ois close

User · Answer

ObjectOutputStream writeObject clubs  ObjectInputStream readObject      Also  you  add  logic is logically equivalent to using a Set instead of a List  Lists can have duplicates and Sets cannot  You should consider using a set  After all  can you really have 2 chess clubs in the same school

User · Answer

This might work for you  public void save String fileName  throws FileNotFoundException   FileOutputStream fout  new FileOutputStream  fileName   ObjectOutputStream oos   new ObjectOutputStream fout   oos writeObject clubs   fout close        To read back you can have   public void read String fileName  throws FileNotFoundException   FileInputStream fin  new FileInputStream  fileName   ObjectInputStream ois   new ObjectInputStream fin   clubs   ArrayList lt Clubs gt  ois readObject    fin close

User · Answer

To save and load an arraylist of  public static ArrayList  data   new ArrayList      I used  to write      static void saveDatabase     try            FileOutputStream fos   new FileOutputStream  mydb fil            ObjectOutputStream oos   new ObjectOutputStream fos           oos writeObject data           oos close            databaseIsSaved   true                  catch  IOException e            e printStackTrace            End of saveDatabase   And used  to read       static void loadDatabase      try                      FileInputStream fis   new FileInputStream  mydb fil            ObjectInputStream ois   new ObjectInputStream fis                    data    ArrayList lt User gt  ois readObject            ois close                             catch  IOException e            System out println     catch ERROR               e printStackTrace                  catch  ClassNotFoundException e            System out println     catch ERROR               e printStackTrace                  End of loadDatabase

[java] Java - How Can I Write My ArrayList to a file, and Read (load) that file to the original ArrayList?

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