Review and Refactor

public class CurrencyConversionException extends Exception {}

public class ConversionResponse {
    public Decimal convertedAmount;
    public Decimal rate;
    public String timestamp;
}

public class CurrencyConversionService {
    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);

        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;
    }
}

Every lesson's piece is present: the contract-matching request (Lessons 1-2), the Named Credential (Lesson 3), typed JSON parsing (Lesson 4), and both categories of error handling (Lesson 5) — one small, complete, well-tested integration.

• Matches the request shape defined in Lesson 1? Yes — GET, the exact query parameters, in order.
• Matches the response shape defined in Lesson 1? Yes — ConversionResponse mirrors it field for field.
• Handles the unrecognized-currency edge case named in Lesson 1? Yes — surfaces as a clear CurrencyConversionException with the API\'s own error detail included.

• Requirements/contract defined precisely before coding? Yes (Lesson 1).
• Secrets kept out of Apex code? Yes, via Named Credential (Lesson 3).
• Both failure categories handled distinctly? Yes — API errors and network failures, each clear (Lesson 5).
• Custom exception with a clear message? Yes, CurrencyConversionException (Lesson 5).
• Comprehensive tests, including both failure paths? Yes (Lesson 6).

This closes out every "single external system" integration project this course will build — the next modules move into asynchronous Apex, where a callout's synchronous 10-second limit (Lesson 1's opening constraint) finally gets a real alternative.