import java.util.jar.*;
import java.util.*;
import java.io.*;

public class PackageUtils {

 private static boolean debug = true;

 private PackageUtils() {}

 public static List getClasseNamesInPackage
     (String jarName, String packageName){
   ArrayList classes = new ArrayList ();

   packageName = packageName.replaceAll("\\." , "/");
   if (debug) System.out.println
        ("Jar " + jarName + " looking for " + packageName);
   try{
     JarInputStream jarFile = new JarInputStream
        (new FileInputStream (jarName));
     JarEntry jarEntry;

     while(true) {
       jarEntry=jarFile.getNextJarEntry ();
       if(jarEntry == null){
         break;
       }
       if((jarEntry.getName ().startsWith (packageName)) &&
            (jarEntry.getName ().endsWith (".class")) ) {
         if (debug) System.out.println
           ("Found " + jarEntry.getName().replaceAll("/", "\\."));
         classes.add (jarEntry.getName().replaceAll("/", "\\."));
       }
     }
   }
   catch( Exception e){
     e.printStackTrace ();
   }
   return classes;
}

/**
*
*/
  public static void main (String[] args){
   List list =  PackageUtils.getClasseNamesInPackage
        ("C:/j2sdk1.4.1_02/lib/mail.jar", "com.sun.mail.handlers");
   System.out.println(list);
   /*
   output :

    Jar C:/j2sdk1.4.1_02/lib/mail.jar looking for com/sun/mail/handlers
    Found com.sun.mail.handlers.text_html.class
    Found com.sun.mail.handlers.text_plain.class
    Found com.sun.mail.handlers.text_xml.class
    Found com.sun.mail.handlers.image_gif.class
    Found com.sun.mail.handlers.image_jpeg.class
    Found com.sun.mail.handlers.multipart_mixed.class
    Found com.sun.mail.handlers.message_rfc822.class
    [com.sun.mail.handlers.text_html.class,
    com.sun.mail.handlers.text_xml.class,  com
    .sun.mail.handlers.image_jpeg.class,
    , com.sun.mail.handlers.message_rfc822.class]

   */
  }
}