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<Club> clubs = new ArrayList<Club>();

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)

Nilgai

What you are seeing Club@c5d8jdj is the result of the Object class' toString() method. How would you like the list to be saved in the file? What format? – Coleorhiza

toString() is not suitable for writing to file. You want Serialization. – Ostiary

If you want to do it this way, you have to override the toString() method of the Club() class so that it contains all the information and then a Constructor, which is able to recreate the object. Or have a look at Serializable interface and ObjectStreams: docs.oracle.com/javase/tutorial/essential/io/objectstreams.html – Semiquaver

I have no preference what file format it is saved in as long as it can be loaded later on the application.. – Nilgai

Answer 1

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();
}

Coleorhiza

for a small data set, it seems to be ok but I very much doubt that its efficient given a huge dataset. – Allin

Answer 2

You should use Java's built in serialization mechanism. To use it, you need to do the following:

1. 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.

2. 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();
   

3. To read the list from a file, do the following:

   FileInputStream fis = new FileInputStream("t.tmp");
   ObjectInputStream ois = new ObjectInputStream(fis);
   List<Club> clubs = (List<Club>) ois.readObject();
   ois.close();
   

Businessman

If you are getting a "Read-only file system" exception, you can change "t.tmp" to file, where File file = new File(getFilesDir(), "t.tmp"); or File file = new File(getExternalFilesDir(null), "t.tmp"); but the latter may need the WRITE_EXTERNAL_STORAGE permission. – Analysis

i want to add more items into arraylist containig file how can i do that ? and thnx your answers works well – Intermediary

Answer 3

In Java 8 you can use Files.write() method with two arguments: Path and List<String>, something like this:

List<String> 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);
}

Fumikofumitory

Answer 4

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<Clubs>)ois.readObject();
fin.close();
}

Davao

Answer 5

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?

Feminacy

Answer 6

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<User>)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 

Stillborn