save/load - serialization/deserialization

[Gatsu]

Hi, I have a chess game that I am trying to save to a .ser file and then load.

My main class is serialized like this:

public class Board extends JFrame implements Serializable

My save class has a method that looks like this:

public void saveGameAction()
    {
        try
        {
            FileOutputStream fs = new FileOutputStream("test.ser");
            ObjectOutputStream os = new ObjectOutputStream(fs);
            os.writeObject(b);
            os.close();
        }
        catch(Exception ex)
        {
            ex.printStackTrace();
        }
    }

Here variable b contains the Board class which we want to restore.

My load class has this method:

public void loadGameAction()
    {
        try
        {
            FileInputStream fileStream = new FileInputStream("test.ser");
            ObjectInputStream is = new ObjectInputStream(fileStream);
            Board restore = (Board) is.readObject();
        }
        catch(Exception ex)
        {
            ex.printStackTrace();
        }
    }

Trying to load test.ser which I think I save correctly in the save class, now do I need a method in Board which I call like this to make this work?

restore.restoreMethod();

If I do like this, does the variables after loading contain the saved values that I in this method just use to place my chess pieces on its old positions again? I ask this because the current code does nothing, all chess pieces are at their starting positions if I load a file that had its pieces moved and a variable just containing an int is also not restored to its supposedly saved value.

[Ronin]

Hi Gatsu,

The code looks ok but it doesn't look as though the board being painted is the same one being created in the loadGameAction() method.

I'm a little confused by the mention of the three classes, main, save and load. At the least I would expect there to be a single class which constructs an instance of board. This same class would contain the save and load methods and reference the single board instance.

Regards.

[jim829]

There can be a lot of issues regarding serialization. For one, are your chess pieces stored in static variables or variables marked transient. If they are, then they will not be serialized.

Jim

[Gatsu]

Hi Gatsu,

The code looks ok but it doesn't look as though the board being painted is the same one being created in the loadGameAction() method.

I'm a little confused by the mention of the three classes, main, save and load. At the least I would expect there to be a single class which constructs an instance of board. This same class would contain the save and load methods and reference the single board instance.

Regards.

In the Board class I also have the main method that does:

Board b = new Board();

And then delcare this in the start of the class:

Board b;

So this b is not the same as the one I create in the main method in the same class?

[Tolls]

Board restore = (Board) is.readObject();

You read the saved Board and store the reference in 'restore'.
Then do nothing with that reference, which goes out of scope and so the object is probably quickly reclaimed during garbage collection.

[Gatsu]

Board restore = (Board) is.readObject();

You read the saved Board and store the reference in 'restore'.
Then do nothing with that reference, which goes out of scope and so the object is probably quickly reclaimed during garbage collection.

I tried doing anything with 'restore'.
I add this in Board class:

public void aa()
    {
        System.out.println("aa");
    }

then try restore.aa(); in the load method but it gives me this error at that line:
java.lang.NullPointerException

FileInputStream fis = new FileInputStream("test.ser");
ObjectInputStream ois = new ObjectInputStream(fis);
Board restore = (Board) ois.readObject();
restore.aa();
ois.close();

[Tolls]

What's the full stack trace?
Is it the restore.aa() call giving the exception?

[Gatsu]

I'm sorry, a full stack trace would look like this and yes that is the line causing it:

java.lang.NullPointerException
at chess.LoadGame.loadGameAction(LoadGame.java:34)
at Chess.Board$3.actionPerformed(Board.java:137)
at javax.swing.AbstractButton.fireActionPerformed(Abs tractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed (AbstractButton.java:2341)
at javax.swing.DefaultButtonModel.fireActionPerformed (DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultB uttonModel.java:259)
at javax.swing.AbstractButton.doClick(AbstractButton. java:376)
at javax.swing.plaf.basic.BasicMenuItemUI.doClick(Bas icMenuItemUI.java:833)
at javax.swing.plaf.basic.BasicMenuItemUI$Handler.mou seReleased(BasicMenuItemUI.java:877)
at java.awt.Component.processMouseEvent(Component.jav a:6505)
at javax.swing.JComponent.processMouseEvent(JComponen t.java:3321)
at java.awt.Component.processEvent(Component.java:627 0)
at java.awt.Container.processEvent(Container.java:222 9)
at java.awt.Component.dispatchEventImpl(Component.jav a:4861)
at java.awt.Container.dispatchEventImpl(Container.jav a:2287)
at java.awt.Component.dispatchEvent(Component.java:46 87)
at java.awt.LightweightDispatcher.retargetMouseEvent( Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(C ontainer.java:4492)
at java.awt.LightweightDispatcher.dispatchEvent(Conta iner.java:4422)
at java.awt.Container.dispatchEventImpl(Container.jav a:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2719 )
at java.awt.Component.dispatchEvent(Component.java:46 87)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.j ava:723)
at java.awt.EventQueue.access$200(EventQueue.java:103 )
at java.awt.EventQueue$3.run(EventQueue.java:682)
at java.awt.EventQueue$3.run(EventQueue.java:680)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPri vilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPri vilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:696)
at java.awt.EventQueue$4.run(EventQueue.java:694)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPri vilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java: 693)
at java.awt.EventDispatchThread.pumpOneEventForFilter s(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(E ventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarch y(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThre ad.java:91)

I have a load button in my menu that runs LoadGame.loadGameAction() which is the load method containing restore.aa().

[Tolls]

So you've saved a null object.
That's the only thing that can have happened, based on the code in #6.

[Gatsu]

Ok thanks, I moved the save method to the Board class to make it save correctly, and now I can do restore.aa();

Now I have to create a method that will do something to restore all variables? I cant find any tutorial that does anything else then this step to restore an object:

Board restore = (Board) ois.readObject();

I get no error message nor anything is hapening when I load.

[Tolls]

So now you've restored your Board.
But you haven't displayed it anywhere (I think the visible flag is transient), so I would pack() and setVisible(true).

Remember, you have saved the entire Window, here, so this will open a new one (if it works).

[Gatsu]

thank yo uvery much, aparently actionListeners also wont get serialized but I solved it by making a new method for adding all those and calling that method when loading, it made my code more clean also, thanks it's working great!

[Tolls]

This is one of the reasons most saves are done using just a data model, rather than the model and the GUI.