A DESCRIPTION OF THE PROBLEM :
For the moment, it is impossible to make an HTTP PATCH request with HttpURLConnection object. PATCH is a valid HTTP Verb and is described on RFC 5789.

HTTP PATCH method RFC: https://tools.ietf.org/html/rfc5789

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Use this main :

public static void main(String[] args) throws IOException {
    HttpURLConnection httpURLConnection = (HttpURLConnection) new URL("https://httpbin.org/patch").openConnection();
    httpURLConnection.setRequestMethod("PATCH");
    System.out.println(httpURLConnection.getResponseCode());
}

ACTUAL -
Exception in thread "main" java.net.ProtocolException: Invalid HTTP method: PATCH

---------- BEGIN SOURCE ----------
HttpURLConnection httpURLConnection = (HttpURLConnection) new URL("https://httpbin.org/patch").openConnection();
httpURLConnection.setRequestMethod("PATCH");
assertEquals(200, httpURLConnection.getResponseCode());
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
It's possible to bypass this bug by using reflection: rewrite the HttpUrlConnections.methods field to add the "PATCH" string into the array.

FREQUENCY : always