code: public X509Extension get(String oid) {
  	if (oid == null) {
  	    throw new IllegalArgumentException(i18n.getMessage("oidNull"));
  	}
  	return (X509Extension)this.extensions.get(oid);
      }
docs: Retrieves X509Extension by given oid.
  
  @param oid the oid of the extension to retrieve.
  @return the extension with the specified oid. Can be null if
  there is no extension with such oid.