Question

public class Storage implements Serializable{

    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    public static List<Message> MessageList = Collections.synchronizedList(new ArrayList<Message>()); //Fail safe if multiple threads modify them.
    public static List<Group> GroupList = Collections.synchronizedList(new ArrayList<Group>());

    protected Storage() {
        super();
    }

    static private Storage _instance = null;

    //initialized: Storage.instance();
    static public Storage instance() {
        if(_instance == null) {
            _instance = new Storage();
        }
     return _instance;
    }



}

I have the upper class which creates a single object. I want to save this object with its Lists to a file. Then when my app starts and I instantiate Storage I want it to read the file and if it is empty create a new Storage, but if its not then read the previous instance of Storage and create this new based on the old. Basically meaning that I want the contents of GroupList and MessageList to be persistent.

EDIT: because I didn’t make it clear enough.

Where do I place the code needed to check and read a previous instance of this class in? I guess in the constructor, but then will my Lists also get the values of the other object? I dont know where/how to code that.

EDIT2: pasting solution.

public class Storage implements Serializable{

    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    public static List<Message> MessageList = Collections.synchronizedList(new ArrayList<Message>()); //Fail safe if multiple threads modify them.
    public static List<Group> GroupList = Collections.synchronizedList(new ArrayList<Group>());

    protected Storage() {
        super();
    }

    static private Storage _instance = null;

    //initialized: Storage.instance();
    public static synchronized Storage instance(){
        initialize();
        if(_instance == null) {
            _instance = new Storage();
        }
        return _instance;
    }

    public static synchronized void persist(){
        FileOutputStream fos = null;
        ObjectOutputStream out = null;
        try{
            fos = new FileOutputStream("Storage.txt");
            out = new ObjectOutputStream(fos);
            out.writeObject(instance());
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }finally{
            try {
                out.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }



    protected static synchronized void initialize(){
        FileInputStream fis = null;
        ObjectInputStream in = null;
        try{
            fis = new FileInputStream("Storage.txt");
            in = new ObjectInputStream(fis);
            _instance = (Storage)in.readObject();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (OptionalDataException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }finally{
            try {
                in.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    public static synchronized void addElement(Message message){
        if(!MessageList.contains(message)){
            MessageList.add(message);
            persist();
            Log.i("STORAGE-addElement", "Added: " + message);
        }
    }

    public static synchronized void addElement(Group group){
        if(!GroupList.contains(group)){
            GroupList.add(group);
            persist();
            Log.i("STORAGE-addElement", "Added: " + group);
        }
    }

    public static synchronized void removeElement(Message message){
        if(!MessageList.contains(message)){
            MessageList.remove(message);
            persist();
            Log.i("STORAGE-removeElement", "Removed: " + message);
        }
    }

    public static synchronized void removeElement(Group group){
        if(!GroupList.contains(group)){
            GroupList.remove(group);
            persist();
            Log.i("STORAGE-removeElement", "Removed: " + group);
        }
    }

    public static synchronized void wipeAll(){
        MessageList.clear();
        GroupList.clear();
        persist();
        Log.i("STORAGE-wipeAll", "Wiped all data");
    }

}

Thanks for your help! 🙂

Answer 1

You can add the following methods to your Storage object:

public void persist() throws IOException{

  FileOutputStream fos = null;
  ObjectOutputStream out = null;
  try{
   fos = new FileOutputStream(FILE_NAME); //assumes filename is a constant you've defined
   out = new ObjectOutputStream(fos);
   out.writeObject(time);
  }finally{
    out.close();
  }
}



protected static void initialize() throws IOException{
   FileInputStream fis = null;
   ObjectInputStream in = null;
   try{
     fis = new FileInputStream(FILE_NAME);
     in = new ObjectInputStream(fis);
     instance  = (PersistentTime)in.readObject();
   }finally{
     in.close();
   }
}

You can call initialize() from your static instance method instead of calling the constructor directly.