Here's a complete method in the ForecastResource.java file:

@GET
@Produces(MediaType.APPLICATION_JSON)
public void getLocationsWithTemperature(@Suspended AsyncResponse asyncResponse) {
    asyncResponse.setTimeout(5, TimeUnit.MINUTES);

// initial completed stage to reduce all stages into one
    CompletionStage<List<Forecast>> initialStage
        = CompletableFuture.completedFuture(new ArrayList<>());
    CompletionStage<List<Forecast>> finalStage = locations.stream()
// for each location, call a service and return a CompletionStage
        .map(location -> {
            return temperatureTarget
                .resolveTemplate("city", location.getName())
                .request()
                .rx()
                .get(Temperature.class)
                .thenApply(temperature -> new Forecast(location, temperature));
        })
// reduce stages using thenCombine, which joins 2 stages into 1
        .reduce(initialStage,
            (combinedStage, forecastStage) -> {
                return combinedStage.thenCombine(forecastStage, 
                    (forecasts, forecast) -> {
                        forecasts.add(forecast);
                        return forecasts;
                    });
            }, (stage1, stage2) -> null); // a combiner is not needed

// complete the response with forecasts
    finalStage.thenAccept(forecasts -> {
            asyncResponse.resume(Response.ok(forecasts).build());
        })
// handle an exception and complete the response with it
        .exceptionally(e -> {
// unwrap the real exception if wrapped in CompletionException)
            Throwable cause = (e instanceof CompletionException) ? e.getCause() : e;
            asyncResponse.resume(cause);
            return null;
        });
}