java.net.URLStreamHandler::openConnection(java.net.URL,java.net.Proxy)
implementation says :
throw new UnsupportedOperationException("Method not implemented.");

but spec says :
/**
     * Same as openConnection(URL), except that the connection will be
     * made through the specified proxy; Protocol handlers that do not
     * support proxying will ignore the proxy parameter and make a
     * normal connection.
     *
     * Calling this method preempts the system's default
     * {@link java.net.ProxySelector ProxySelector} settings.
     *
     * @param u the URL that this connects to.
     * @param p the proxy through which the connection will be made.
     * If direct connection is desired, Proxy.NO_PROXY
     * should be specified.
     * @return a {@code URLConnection} object for the {@code URL}.
     * @exception IOException if an I/O error occurs while opening the
     * connection.
     * @exception IllegalArgumentException if either u or p is null,
     * or p has the wrong type.
     * @exception UnsupportedOperationException if the subclass that
     * implements the protocol doesn't support this method.
     * @since 1.5
     */

So following is expected to thrown IllegalArgumentException but UnsupportedOperationException is the one that is always thrown

CustomURLStreamHandler customURLStreamHandler = new CustomURLStreamHandler();
customURLStreamHandler.openConnection(null, null);
// above should throw IllegalArgumentException

class CustomURLStreamHandler extends URLStreamHandler {
    public CustomURLStreamHandler() {
    }

    public URLConnection openConnection(URL var1) {
        return null;
    }

    public URLConnection openConnection(URL url, Proxy proxy) throws IOException {
        return super.openConnection(url, proxy);
    }
}