Welcome to the RingCentral Community

Check out and combine these 2 dev guide examples:

1. Authorization flow
2. Webhooks

See the sample code below where I added the "/subscribe" case to handle webhook subscription.

case "/test":
  String api = request.getParameter("api");
  String result = "";
  switch (api) {
    case "extension":
      result = JSON.toJSONString(rc.restapi().account().extension().list(), SerializerFeature.PrettyFormat);
      break;
    case "extension-call-log":
      result = JSON.toJSONString(rc.restapi().account().extension().calllog().list(), SerializerFeature.PrettyFormat);
      break;
    case "account-call-log":
      result = JSON.toJSONString(rc.restapi().account().calllog().list(), SerializerFeature.PrettyFormat);
      break;
  }
  response.getWriter().println("<pre>" + result + "</pre>");
  break;
case "/subscribe":
  String result = ""; 
  var eventFilters = new String[]{
         "/restapi/v1.0/account/~/extension/~/message-store/instant?type=SMS"
         }
  CreateSubscriptionRequest createSubscriptionRequest = new CreateSubscriptionRequest()
      .eventFilters(eventFilters)
      .deliveryMode( new NotificationDeliveryModeRequest()
        .transportType("WebHook")
        .address(DELIVERY_ADDRESS)
      );
  var result = rc.restapi().subscription().post(createSubscriptionRequest);