Section 17.15. File Selection Dialog

17.15. File Selection Dialog

A JFileChooser is a standard file-selection box. As with other Swing components , JFileChooser is implemented in pure Java, so it can look and act the same on different platforms or take on the native appearance of the operating system, depending on what look and feel is in effect.

Selecting files all day can be pretty boring without a greater purpose, so we'll exercise the JFileChooser in a mini-editor application. Editor provides a text area in which we can load and work with files. (The JFileChooser created by Editor is shown in Figure 17-14.) We'll stop just shy of the capability to save and let you fill in the blanks (with a few caveats).

Figure 17-14. Using a JFileChooser

Here's the code:

import java.awt.*;
    import java.awt.event.*;
    import java.io.*;
    import javax.swing.*;

    public class Editor extends JFrame implements ActionListener
    {
      private JEditorPane textPane = new JEditorPane(  );

      public Editor(  ) {
        super("Editor v1.0");
        Container content = getContentPane(  );  // unnecessary  in 5.0+
        content.add(new JScrollPane(textPane), BorderLayout.CENTER);
        JMenu menu = new JMenu("File");
        menu.add(makeMenuItem("Open"));
        menu.add(makeMenuItem("Save"));
        menu.add(makeMenuItem("Quit"));
        JMenuBar menuBar = new JMenuBar(  );
        menuBar.add(menu);
        setJMenuBar(menuBar);
        setSize(300, 300);
    setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
      }

      public void actionPerformed(ActionEvent e) {
        String command = e.getActionCommand(  );
        if (command.equals("Quit")) System.exit(0);
        else if (command.equals("Open")) loadFile(  );
        else if (command.equals("Save")) saveFile(  );
      }

      private void loadFile (  ) {
        JFileChooser chooser = new JFileChooser(  );
        int result = chooser.showOpenDialog(this);
        if (result == JFileChooser.CANCEL_OPTION) return;
        try {
          File file = chooser.getSelectedFile(  );
          java.net.URL url = file.toURL(  );
          textPane.setPage(url);
        }
        catch (Exception e) {
          textPane.setText("Could not load file: " + e);
        }
      }

      private void saveFile(  ) {
        JFileChooser chooser = new JFileChooser(  );
        chooser.showSaveDialog(this);
        // Save file data...
      }

      private JMenuItem makeMenuItem( String name ) {
        JMenuItem m = new JMenuItem( name );
        m.addActionListener( this );
        return m;
      }

      public static void main(String[] s) {
    new Editor(  ).setVisible(true);
      }
    }

Editor is a JFrame that lays itself out with a JEditorPane (which is covered in Chapter 18) and a pull-down menu. From the pull-down File menu, we can Open, Save, or Quit. The actionPerformed( ) method catches the events associated with these menu selections and takes the appropriate action.

The interesting parts of Editor are the private methods loadFile( ) and saveFile( ) . The loadFile( ) method creates a new JFileChooser and calls its showOpenDialog( ) method.

A JFileChooser does its work when the showOpenDialog( ) method is called. This method blocks the caller until the dialog completes its job, at which time the file chooser disappears. After that, we can retrieve the designated file with the getFile( ) method. In loadFile( ) , we convert the selected File to a URL and pass it to the JEditorPane , which displays the selected file. As you'll learn in the next chapter, JEditorPane can display HTML and RTF files.

You can fill out the unfinished saveFile( ) method if you wish, but it would be prudent to add the standard safety precautions . For example, you could use one of the confirmation dialogs we just looked at to prompt the user before overwriting an existing file.