code: public java.security.cert.Certificate[] getCertificates()
           throws IOException
      {
          JarEntry e = getJarEntry();
          return e != null ? e.getCertificates() : null;
      }
docs: Return the Certificate object for this connection if the URL
  for it points to a JAR file entry, null otherwise. This method
  can only be called once
  the connection has been completely verified by reading
  from the input stream until the end of the stream has been
  reached. Otherwise, this method will return {@code null}
  
  @return the Certificate object for this connection if the URL
  for it points to a JAR file entry, null otherwise.
  
  @exception IOException if getting the JAR entry causes an
  IOException to be thrown.
  
  @see #getJarEntry