Handle Errors and Timeouts

public class CurrencyConversionException extends Exception {}

public Decimal convert(Decimal amountInZar, String targetCurrency) {
    HttpRequest request = new HttpRequest();
    request.setEndpoint('callout:Currency_API/convert?from=ZAR&to=' + targetCurrency + '&amount=' + amountInZar);
    request.setMethod('GET');

    HttpResponse response = new Http().send(request);

    if (response.getStatusCode() != 200) {
        throw new CurrencyConversionException(
            'Currency API returned status ' + response.getStatusCode() + ': ' + response.getBody()
        );
    }

    ConversionResponse parsed = (ConversionResponse) JSON.deserialize(response.getBody(), ConversionResponse.class);
    return parsed.convertedAmount;
}

Module 22's custom exception pattern, once more — CurrencyConversionException carries the actual status code and response body, so whoever catches it (or reads the debug log, Module 33) knows exactly what the external API said went wrong, e.g. an unrecognized currency code (Lesson 1's edge case).

public Decimal convert(Decimal amountInZar, String targetCurrency) {
    HttpRequest request = new HttpRequest();
    request.setEndpoint('callout:Currency_API/convert?from=ZAR&to=' + targetCurrency + '&amount=' + amountInZar);
    request.setMethod('GET');
    request.setTimeout(5000); // 5 seconds, well under the 10-second callout limit

    HttpResponse response;
    try {
        response = new Http().send(request);
    } catch (CalloutException e) {
        throw new CurrencyConversionException('Could not reach the currency service: ' + e.getMessage());
    }

    if (response.getStatusCode() != 200) {
        throw new CurrencyConversionException('Currency API returned status ' + response.getStatusCode());
    }

    ConversionResponse parsed = (ConversionResponse) JSON.deserialize(response.getBody(), ConversionResponse.class);
    return parsed.convertedAmount;
}

A CalloutException (Module 20's try/catch, applied here) means the request never even got a response at all — a genuine network failure, a DNS problem, or exceeding setTimeout's limit — a fundamentally different failure from "the API responded, but with an error status." Both get wrapped in the same CurrencyConversionException for a consistent interface to callers, but they're detected and handled at different points in the method.