Correctly implementing save with JFileChooser

[Chris Waguespack]

I'm currently using JFileChooser to save a file from my application, and would like to make sure the file name the user is giving does not already exist and does not contain any OS-disallowed filename characters. So far I have

                final JFileChooser fileChooser = new JFileChooser();
fileChooser.setFileFilter(new FileNameExtensionFilter(Messages.getString("StartScreen.browseSimulationFileNameExtensionFilterText") + " (*.ssim)", "ssim"));
fileChooser.setSelectedFile(new File("Simulation.ssim"));
final int returnValue = fileChooser.showSaveDialog(this);
if (returnValue == JFileChooser.APPROVE_OPTION) {
File selectedFile = fileChooser.getSelectedFile();
String fileName = fileChooser.getSelectedFile().toString();
                        //Make sure it has the right extension
if (!fileName.endsWith(".ssim")) {
fileName += ".ssim";
}
                        //Does it already exist
if (selectedFile.exists()) {
int overwriteReturnValue = JOptionPane.showConfirmDialog(null, 
                                   selectedFile.toString() 
                                        + " already exists." 
                                        + Constants.LINE_SEPARATOR 
                                        + "Do you want to replace it?", 
                                   "File Already Exists", 
                                   JOptionPane.YES_NO_OPTION, 
                                   JOptionPane.WARNING_MESSAGE);
if (overwriteReturnValue == JOptionPane.YES_OPTION) {
//Create object here
                                        // ...

try {
ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream(fileName));
objectOutputStream.writeObject(theObjectICreated);
objectOutputStream.close();
} catch (FileNotFoundException ex) {
ex.printStackTrace();
} catch (IOException ex) {
ex.printStackTrace();
}
}
}
}

The problem with this code, is that it does not check for bad filename characters (which I can't figure out how to do) and that if the file exists already, it won't take you back to the JFileChooser to correct it (like it will in most other applications ex. Notepad).
Any ideas?

Thanks a bunch,
Chris
[ May 16, 2008: Message edited by: Chris Waguespack ]