title: adjust selected File to FileFilter in a JFileChooser
body: <p>I'm writing a diagram editor in java. This app has the option to export to various standard image formats such as .jpg, .png etc. When the user clicks File->Export, you get a <code>JFileChooser</code> which has a number of <code>FileFilter</code>s in it, for <code>.jpg</code>, <code>.png</code> etc.</p>
  
  <p>Now here is my question:</p>
  
  <p>Is there a way to have the extension of the default adjust to the selected file filter? E.g. if the document is named "lolcat" then the default option should be "lolcat.png" when the png filter is selected, and when the user selects the jpg file filter, the default should change to "lolcat.jpg" automatically.</p>
  
  <p>Is this possible? How can I do it?</p>
  
  <p>edit:
  Based on the answer below, I wrote some code. But it doesn't quite work yet. I've added a <code>propertyChangeListener</code> to the <code>FILE_FILTER_CHANGED_PROPERTY</code>, but it seems that within this method <code>getSelectedFile()</code> returns null. Here is the code. </p>
  
  <pre><code>package nl.helixsoft;
  
  import java.awt.event.ActionEvent;
  import java.awt.event.ActionListener;
  import java.beans.PropertyChangeEvent;
  import java.beans.PropertyChangeListener;
  import java.io.File;
  import java.util.ArrayList;
  import java.util.List;
  
  import javax.swing.JButton;
  import javax.swing.JFileChooser;
  import javax.swing.JFrame;
  import javax.swing.filechooser.FileFilter;
  
  public class JFileChooserTest {
      public class SimpleFileFilter extends FileFilter {
          private String desc;
          private List&lt;String&gt; extensions;
          private boolean showDirectories;
  
          /**
           * @param name example: "Data files"
           * @param glob example: "*.txt|*.csv"
           */
          public SimpleFileFilter (String name, String globs) {
              extensions = new ArrayList&lt;String&gt;();
              for (String glob : globs.split("\\|")) {
                  if (!glob.startsWith("*.")) 
                      throw new IllegalArgumentException("expected list of globs like \"*.txt|*.csv\"");
                  // cut off "*"
                  // store only lower case (make comparison case insensitive)
                  extensions.add (glob.substring(1).toLowerCase());
              }
              desc = name + " (" + globs + ")";
          }
  
          public SimpleFileFilter(String name, String globs, boolean showDirectories) {
              this(name, globs);
              this.showDirectories = showDirectories;
          }
  
          @Override
          public boolean accept(File file) {
              if(showDirectories &amp;&amp; file.isDirectory()) {
                  return true;
              }
              String fileName = file.toString().toLowerCase();
  
              for (String extension : extensions) {   
                  if (fileName.endsWith (extension)) {
                      return true;
                  }
              }
              return false;
          }
  
          @Override
          public String getDescription() {
              return desc;
          }
  
          /**
           * @return includes '.'
           */
          public String getFirstExtension() {
              return extensions.get(0);
          }
      }
  
      void export() {
          String documentTitle = "lolcat";
  
          final JFileChooser jfc = new JFileChooser();
          jfc.setDialogTitle("Export");
          jfc.setDialogType(JFileChooser.SAVE_DIALOG);
          jfc.setSelectedFile(new File (documentTitle));
          jfc.addChoosableFileFilter(new SimpleFileFilter("JPEG", "*.jpg"));
          jfc.addChoosableFileFilter(new SimpleFileFilter("PNG", "*.png"));
          jfc.addPropertyChangeListener(JFileChooser.FILE_FILTER_CHANGED_PROPERTY, new PropertyChangeListener() {
              public void propertyChange(PropertyChangeEvent arg0) {
                  System.out.println ("Property changed");
                  String extold = null;
                  String extnew = null;
                  if (arg0.getOldValue() == null || !(arg0.getOldValue() instanceof SimpleFileFilter)) return;
                  if (arg0.getNewValue() == null || !(arg0.getNewValue() instanceof SimpleFileFilter)) return;
                  SimpleFileFilter oldValue = ((SimpleFileFilter)arg0.getOldValue());
                  SimpleFileFilter newValue = ((SimpleFileFilter)arg0.getNewValue());
                  extold = oldValue.getFirstExtension();
                  extnew = newValue.getFirstExtension();
                  String filename = "" + jfc.getSelectedFile();
                  System.out.println ("file: " + filename + " old: " + extold + ", new: " + extnew);
                  if (filename.endsWith(extold)) {
                      filename.replace(extold, extnew);
                  } else {
                      filename += extnew;
                  }
                  jfc.setSelectedFile(new File (filename));
              }
          });
          jfc.showDialog(frame, "export");
      }
  
      JFrame frame;
  
      void run() {
          frame = new JFrame();
          JButton btn = new JButton ("export");
          frame.add (btn);
          btn.addActionListener (new ActionListener() {
              public void actionPerformed(ActionEvent ae) {
                  export();
              }
          });
          frame.setSize (300, 300);
          frame.pack();
          frame.setVisible(true);
      }
  
      public static void main(String[] args) {
          javax.swing.SwingUtilities.invokeLater(new Runnable() {     
              public void run() {
                  JFileChooserTest x =  new JFileChooserTest();
                  x.run();
              }
          });     
      }
  }
  </code></pre>