// Create a HTTPClient instance (currently Jetty Client is supported
// as it is non-blocking
HttpClient httpClient = ...

// Create a Rest Client instance. This instance will have the right
// ObjectMapper inside, so conversion of date-times should be OK.
RestClient restClient =
    JiraCloudRestClientFactory.createRestClient(httpClient);

// Create a reusable API instance. E.g.:
VersionApi versionApi = new VersionApi(restClient);

// If needed, create a RestRequstEnhancer. Such enhancer can be
// used to attach an act-as-user or a call-as-addon token to the
// request, or replace the basePath to the URL of the Jira Cloud
// instance of the client.
RestRequestEnhancer enhancer = ...

// Call any function on the created API
Single<Version> versionSingle = versionApi.getVersion(id,
    Optional.empty(), Optional.of(enhancer));

// Use the functions of RXJava Single to receive the version synchronously
// or asynchronously. E.g.:
single.subscribe(version -> {...}, error -> {...});