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The goal with my program is to have it save when closed, and reloaded when opened.

I have a driver (which contains all of the graphics) and I want it to create and save a file. I've seen numerous tutorials on the internet, but what they fail to explain is how to correctly implement this system in an actual program.

Do I create the OutputStrema in my main method? Do I need to check if a file has been created and if not create one, and if so read from it? How do I accomplish all of this? Should I have a WindowListener for quitting so that it can save all of the data?

Thanks

flyinghigh
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2 Answers2

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Yes, a WindowListener sounds like a good idea. One way to save the state of a program is to use Properties. Please have a look at the tutorial linked to above. I'm not sure what to recommend for your "graphics". Please tell us more details about just what you mean here.

Edit

Do I create the OutputStrema in my main method?

you're question re creating the OutputStream in the main method is asking about program structure that we know nothing about. My gut tells me that i wouldn't create anything like this in the main method, even if the JFrame is created there (or in a static method called from main). The OutputStream would be created in the class that implements the WindowListener, and this can be its own stand alone class. You want to make the main method as small as possible.

Do I need to check if a file has been created and if not create one, and if so read from it?

Possibly. This question revolves around what you're going to do with the stored information once created. Perhaps you'll search for it and read it on program start up? I don't know as it all depends on your needs, something you'll have to figure out.

How do I accomplish all of this? Should I have a WindowListener for quitting so that it can save all of the data?

If this is a Swing application, then yes, a WindowListener would help you control your application's closing.

Hovercraft Full Of Eels
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  • Nothing to recommend for graphics? Will look at tutorial. – flyinghigh May 27 '12 at 00:49
  • @user1419623: your question is a bit vague on this point making it difficult for me to make any recommendations yet. Again please clarify your question. What exactly do you mean by the "graphics"? Do you have images in your GUI that you wish to save? – Hovercraft Full Of Eels May 27 '12 at 00:58
  • Oh! Sorry. No. I have an object in the graphics driver that I need to save, not the graphics themselves. I have a StateManager object. – flyinghigh May 27 '12 at 01:08
  • @user14: If you need to save a complex object, then you may need serialization of some sort, either standard serialization or xml serialization. – Hovercraft Full Of Eels May 27 '12 at 01:17
  • Thanks the main method is pretty bare. I'm going to try a few things, I'll let you know how they go. Thanks for your help, guys! – flyinghigh May 27 '12 at 02:10
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Do I create the OutputStrema in my main method?

It would be better to create the stream at the point where you are saving the state.

When my program runs it is going to take in the saved data file with its ObjectInputStream. Do I put the code to accomplish this in my Main method?

Sounds like a good choice. Basically, you need to do this before you attempt to do something that needs that data. Anything that achieves this will work (though doing it using static initialization is a bad idea ...)

Do I need to check if a file has been created and if not create one, and if so read from it?

This question is confusing writing the state file and reading it. They occur at different points in the lifecycle, and use different code to do the task.

There is no point checking to see if a save file exists before creating one ... unless you propose to rename the existing save file.

Conversely, if a save file doesn't exist when you start the application, then the constructor that attempts to open it will throw a FileNotFoundException. Just catch the exception and skip the code that reads the saved state.

... if there has not been a file created yet, will this cause an error?

Yes. A FileNotFoundException; see above. So your code has to deal with that, or test to see if the file exists before attempting to open in; see File.exists() and related methods.

Should I have a WindowListener for quitting so that it can save all of the data?

That sounds like part of the solution. (But what about the case where the program crashes or is killed? Do you want to save state in those cases ... or not?)

A couple of things to beware of:

  • Many GUI-related objects do not implement Serializable, and therefore cannot be saved using an ObjectOutputStream. Threads, streams, sockets and a few other things can't be serialized either.

  • When you save instances of classes using ObjectOutputStream, change the classes or their names, and then attempt to reload them using ObjectInputStream, you are liable to run into trouble with saved instances not matching the new versions of the classes. If you have to be able to read that serialized state you are in trouble.

Stephen C
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  • Okay, I apologize for assuming you'd know every detail of my project. I do have a VERY specific idea of what I want to accomplish. So here is my new question: When my program runs it is going to take in the saved data file with its ObjectInputStream. Do I put the code to accomplish this in my Main method? And if I do, if there has not been a file created yet, will this cause an error? – flyinghigh May 27 '12 at 02:02