Create JarFile and use Pack200.Packer : JarFile « File « Java Tutorial

1. Java Tutorial
2. File
3. JarFile

import java.io.FileOutputStream;
import java.io.OutputStream;
import java.util.jar.JarFile;
import java.util.jar.Pack200;

public class MainClass {

  public static void main(String[] args) throws Exception {

    OutputStream out = null;

    JarFile f = new JarFile(args[0]);
    Pack200.Packer packer = Pack200.newPacker();
    out = new FileOutputStream(args[0] + ".pack");
    packer.pack(f, out);
    out.close();
  }
}