Check java on users PC and give message

Question

The following code has been written with Java 1.4 and works great, but if a user has Java 1.3 they get an error message and if a user has Java 1.2 nothing happens at all. Is there a way to check if the user is using a version older than 1.4 and if so pop up a message telling them they need to download a newer version of Java?
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.util.List;
import java.io.*;
import java.util.*;
import javax.xml.parsers.*;
import javax.xml.transform.*;
import javax.xml.transform.dom.*;
import javax.xml.transform.stream.*;
import org.w3c.dom.*;
public class Sametime extends JFrame implements ActionListener {
    private int indentation = -1;
    JPanel panel = new JPanel();
    JTextArea jta = new JTextArea(
    //Instructions for user
    "For a successful buddy list migration do the following:\n"
    + "1. Save your current Sametime Buddy List to your PC.\n   "
    + "The default location should be: C:/Program Files/Lotus/Sametime Client.\n"
    + "  A. Open the Sametime Client.\n"
    + "  B. Click on People\n"
    + "  C. Click on Save List.\n"
    + "  D. Save as your first.last.dat\n"
    + "     Ex. john.doe.dat\n"
    + "NOTE: If you have AOL contacts in your Sametime buddy list they will not be migrated.\n");
    JButton browse = new JButton("Continue");
    JButton exit = new JButton("Exit");
    public Sametime() {
        super("Sametime Buddy List Migration");
        setSize(610, 245);
        Container c = this.getContentPane();
        c.add(panel);
        browse.addActionListener(this);
        exit.addActionListener(this);
        panel.add(jta);
        panel.add(browse);
        panel.add(exit);
        jta.setEditable(false);
        setLookAndFeel();
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    } //end Sametime
    public class DATFilter extends javax.swing.filechooser.FileFilter {
        public boolean accept(File f) {
            //if it is a directory -- we want to show it so return true.
            if (f.isDirectory())
                return true;
            String extension = getExtension(f);//get the extension of the file
            //check to see if the extension is equal to "dat"
            if ((extension.equals("dat")))
                return true;
            //default -- fall through. False is return on all
            //occasions except:
            //a) the file is a directory
            //b) the file's extension is what we are looking for.
            return false;
        }//end accept
        public String getDescription() {
            return "dat files";
        }//end getDescription
         * Method to get the extension of the file, in lowercase
        private String getExtension(File f) {
            String s = f.getName();
            int i = s.lastIndexOf('.');
            if (i > 0 &&  i < s.length() - 1)
                return s.substring(i+1).toLowerCase();
            return "";
        }//end getExtension
    }//end class DATFilter
    public void actionPerformed(ActionEvent e) {
        //Default Location for JFileChooser search
        String error = "The file selected is not a .dat file!\n"
        + "Please select your recently saved .dat file and try again.";
        JFileChooser fc = new JFileChooser("/Program Files/Lotus/Sametime Client");
        fc.setFileFilter(new DATFilter());
        fc.setFileSelectionMode( JFileChooser.FILES_ONLY);
        String user = System.getProperty("user.name");// finds who the current user is
        if (e.getSource() == browse) {
            int returnVal = fc.showSaveDialog(Sametime.this);
            if (returnVal == JFileChooser.APPROVE_OPTION) {
                //if (fc.getSelectedFile().getName().equals(".dat")){
                if (fc.getSelectedFile().getName().endsWith(".dat")){ // checks to see if selected file is .dat
                }else{
                    JOptionPane.showMessageDialog(null, error, "Wrong File", JOptionPane.ERROR_MESSAGE);
                    return;
                }//end else
                try {
                    String[] contactArray = parseDatFile(fc.getSelectedFile());
                    Document document = createXMLDocument(contactArray);
                    saveToXMLFile(
                    document,
                    new File(
                    "C:/Documents and Settings/" + user +"/My Documents/OLCS/",// looks for directory for list
                    "contacts-list_migration.ctt"));
                } catch (Exception exc) {
                    File f = new File("C:/Documents and Settings/" + user +"/My Documents/OLCS/");// setting directory for list if not there
                    boolean yes = true;
                    yes = f.mkdir();// creating directory
                    try {
                        String[] contactArray = parseDatFile(fc.getSelectedFile());
                        Document document = createXMLDocument(contactArray);
                        saveToXMLFile(
                        document,
                        new File(
                        "C:/Documents and Settings/" + user +"/My Documents/OLCS/",// used only if the directory didn't exist
                        "contacts-list_migration.ctt"));
                        //exc.printStackTrace();// not sure if this is needed?
                    } catch (Exception exc1) {
                        exc1.printStackTrace();
                    }//end inner catch
                }// end catch
            }// end if
            if(returnVal==JFileChooser.CANCEL_OPTION){
                String Warning = "You did not migrate your Sametime buddy list at this time.";
                JOptionPane.showMessageDialog(null, Warning, "Migration Canceled", JOptionPane.WARNING_MESSAGE);
                return;
            }else{
                String thankyou = "Thank You for Migrating your Sametime buddy list to OLCS"
                + "\nYour new OLCS buddy list has been saved to:"
                + "\nC:/Documents and Settings/" + user +"/My Documents/OLCS"
                + "\n as: Contact-List_migration.ctt"
                + "\n\n To be able to use Contact-List_migration.ctt for Windows Messenger:"
                + "\n1. Log into Windows Messenger."
                + "\n2. Click on File"
                + "\n3. Click on 'Import Contacts from a Saved File...'"
                + "\n4. Open OLCS in My Documents"
                + "\n5. Click on 'Contact-list_migration.ctt'"
                + "\n6. Click Open to import the list."
                + "\n   A window will pop up confirming that you want to add all of the contacts"
                + "\n   Click 'yes'"
                + "\n   Your buddy list is ready to be used.";
                JOptionPane.showMessageDialog(null, thankyou, "Migration Completed", JOptionPane.INFORMATION_MESSAGE);//Change this when defualt directory is known.
            }//end if else statement
        } //end if
        System.exit( 0 );
        if (e.getSource() == exit) {
            System.exit( 0 );
        } //end if
    } //end actionPerformed
    String[] parseDatFile(File datFile)
    throws Exception    {
        List list = new ArrayList();
        BufferedReader br = new BufferedReader(new FileReader(datFile));
        String line;
        while ((line = br.readLine()) != null) {
            line = line.trim();
            if (line.indexOf("U") != 0)
                continue;
            int p = line.indexOf("::");
            if (p == -1)
                continue;
            line = line.substring(p + 2).trim();
            if (line.indexOf("AOL") == 0)
                continue;
            p = line.indexOf(",");
            if (p != -1)
                line = line.substring(0, p);
            line = line.trim() + "@mci.com";
            if (list.indexOf(line) == -1)
                list.add(line);
        }//end while
        br.close();
        String[] contactArray = new String[list.size()];
        list.toArray(contactArray);
        return contactArray;
    }// end String
    // setting up the XML file
    Document createXMLDocument(String[] contactArray) throws Exception {
        DocumentBuilderFactory dBF = DocumentBuilderFactory.newInstance();
        DocumentBuilder builder = dBF.newDocumentBuilder();
        DOMImplementation domImpl = builder.getDOMImplementation();
        Document document = domImpl.createDocument(null, "messenger", null);
        Element root = document.getDocumentElement();
        Element svcElm = document.createElement("service");
        Element clElm = document.createElement("contactlist");
        svcElm.setAttribute("name", "Microsoft RTC Instant Messaging");
        svcElm.appendChild(clElm);
        root.appendChild(svcElm);
        for (int i = 0; i < contactArray.length; i++) {
            Element conElm = document.createElement("contact");
            Text conTxt = document.createTextNode(contactArray);
conElm.appendChild(conTxt);
clElm.appendChild(conElm);
}//end for
return document;
}// end Document
void saveToXMLFile(Document document, File xmlFile) throws Exception {
OutputStream os =
new BufferedOutputStream(new FileOutputStream(xmlFile));
TransformerFactory tFactory = TransformerFactory.newInstance();
Transformer transformer = tFactory.newTransformer();
transformer.setOutputProperty(OutputKeys.INDENT, "yes");//puts information on seperate lines
transformer.setOutputProperty(OutputKeys.METHOD, "xml");
transformer.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "2");//gives the XML file indentation
transformer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "no");
DOMSource source = new DOMSource(document);
StreamResult result = new StreamResult(os);
transformer.transform(source, result);
os.close();
}//end saveToXMLFile
public static void main(String[] args) {
Sametime st = new Sametime();
ImageIcon picIcon = new ImageIcon(st.getClass().getResource("/ST_Migration/images/mci.gif"));//Change when default is known!
st.setIconImage(picIcon.getImage());
} //end main
private void setLookAndFeel() {
try {
UIManager.setLookAndFeel(
UIManager.getSystemLookAndFeelClassName());
SwingUtilities.updateComponentTreeUI(this);
} catch (Exception e) {
System.err.println("Could not use Look and Feel: " + e);
} //end catch
} //end void setLookAndFeel
} //end public class Sametime

Answer 1

Well, if you don't want to use Package.isCompatibleWith and the version numbers will always be of the form x.y where y is at most one digit, you can use Double.parseDouble() and then compare the numbers. Otherwise you need to write a method of your own.