import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.util.jar.JarOutputStream;
import java.util.jar.Pack200;
import java.util.zip.GZIPInputStream;
public class Main {
public static void main(String[] args) throws Exception {
String inName = "a.pack.gz";
String outName = "a.unpacked";
Pack200.Unpacker unpacker = Pack200.newUnpacker();
JarOutputStream out = new JarOutputStream(new FileOutputStream(outName));
InputStream in = new FileInputStream(inName);
in = new GZIPInputStream(in);
unpacker.unpack(in, out);
out.close();
}
}