A resource class to perform currency conversion

ConverterResource.java

@Path("/converter")
public class ConverterResource {

    /**
     * The name of the path parameter for the API key.
     */
    public static final String APP_ID = "app_id";
    /**
     * A part of an error message.
     */
    public static final String UNKNOWN_CURRENCY = "Unknown currency ";
    /**
     * Error message when no amount to convert was presented.
     */
    public static final String AMOUNT_IS_NECESSARY = "Amount is necessary.";

    /**
     * A Jersey client to connect to external API to get exchange rate.
     */
    private final Client client;
    /**
     * The URL to access exchange rate API.
     */
    private final String apiURL;
    /**
     * The key to access exchange rate API.
     */
    private final String apiKey;

    /**
     * A constructor.
     *
     * @param client Jersey client.
     * @param apiURL the URL to access exchange rate API.
     * @param apiKey he key to access exchange rate API.
     */
    public ConverterResource(Client client, String apiURL, String apiKey) {
        this.client = client;
        this.apiURL = apiURL;
        this.apiKey = apiKey;
    }

    /**
     * A subresource method used to convert currencies.
     *
     * The URI to access it looks like
     * <em>localhost:8080/converter/1.5?from=EUR&amp;to=CAD</em> where a double
     * number is provided as a path parameter and initial and final currencies
     * are provided as query parameters.
     *
     * @param amount a whole number, an amount to convert.
     * @param from the currency to be converted.
     * @param to the currency to which to convert
     * @return the result of conversion rounded to Cents.
     */
    @GET
    @Path("/{amount}")
    @Produces(MediaType.TEXT_PLAIN)
    @Consumes(MediaType.TEXT_PLAIN)
    //localhost:8080/converter/1.5?from=EUR&to=CAD
    public double convert(
            @PathParam("amount") Optional<Double> amount,
            @DefaultValue("USD") @QueryParam("from") String from,
            @DefaultValue("EUR") @QueryParam("to") String to
    ) {

        //A variable to store convertion rate of initial currency to US Dollar.
        Double fromRate;
        //A variable to store convertion rate of the desired currency
        //to US Dollar.
        Double toRate;
        //A string to store an error message if an incorrect currency
        //was provided.
        String errorMessage;

        //Check that amount to convert is present.
        if (!amount.isPresent()) {
            throw new BadRequestException(AMOUNT_IS_NECESSARY);
        }

        //Obtain data from external API.
        CurrencyData currencyData = client
                // returns WebTarget
                .target(apiURL)
                // returns WebTarget
                .queryParam(APP_ID, apiKey)
                //Can be done only in Enterprise and unlimited versions.
                //.queryParam("base", to)
                //returns Invocation.Builder
                .request(MediaType.APPLICATION_JSON)
                //returns CurrencyData
                .get(CurrencyData.class);

        //Obtain rates.
        fromRate = currencyData.getRates().get(from);
        toRate = currencyData.getRates().get(to);

        //Check that valid currency symbols were provided.
        if (fromRate == null || toRate == null) {
            errorMessage = UNKNOWN_CURRENCY
                    + (fromRate == null ? from : to);
            throw new BadRequestException(errorMessage);
        }

        //Convert and return the result.
        return convert(fromRate, toRate, amount.get());
    }

    ...