How to add files to an existing zip file?

I need to add files to an already existing zip file, here's what I have so far, this searches for files of a certain ext and zips them up.

I would like to maybe pass in a file name, and add that to an already existing zip file.

I have no idea where to start.

help.

thanks.

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import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FilenameFilter;
import java.io.IOException;
import java.util.zip.Deflater;
import java.util.zip.ZipEntry;
import java.util.zip.ZipOutputStream;

public class ZipIt
{
    private static final String dir = "C:\\Temp\\";
    private static final String ext = ".TMP";

    public ZipIt()
    {        
    }

    public static void main(String[] args)
    {
        ZipIt z = new ZipIt();
        String[] filesToZip = z.getFilesToZip();

        byte[] buffer = new byte[18024];
        String zipFileName = dir + "example.zip";
        try
        {
            ZipOutputStream out = new ZipOutputStream(new FileOutputStream(zipFileName));
            out.setLevel(Deflater.DEFAULT_COMPRESSION);
            for (int i = 0; i < filesToZip.length; i++)
            {
                FileInputStream in = new FileInputStream(dir + filesToZip[i]);
                out.putNextEntry(new ZipEntry(dir + filesToZip[i]));
                int len;
                while ((len = in.read(buffer)) > 0)
                {
                    out.write(buffer, 0, len);
                }
                out.closeEntry();
                in.close();
            }
            out.close();
        }
        catch (IllegalArgumentException iae)
        {
            iae.printStackTrace();
        }
        catch (FileNotFoundException fnfe)
        {
            fnfe.printStackTrace();
        }
        catch (IOException ioe)
        {
            ioe.printStackTrace();
        }
    }

    private String[] getFilesToZip()
    {
        return new File(dir).list(
                new FilenameFilter()
                {
                    public boolean accept(File dir, String name)
                    {
                        return name.toUpperCase().endsWith(ext);
                    }
                });
    }
}