CurrencyApp.java

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.util.*;

/**
 * CUSTOM EXCEPTION: Specifically for API or Network failures.
 */
class ApiException extends Exception {
    public ApiException(String message) { super(message); }
}

/**
 * SERVICE LAYER: CurrencyService
 * Responsibility: Fetching data from the Frankfurter API (European Central Bank data).
 */
class CurrencyService {
    // Standard User-Agent header to identify our traffic (Professional Standard)
    private static final String USER_AGENT = "JavaCurrencyConverter/1.0 (Student Project)";
    private static final String BASE_URL = "https://api.frankfurter.app/latest?from=USD&to=";

    /**
     * Fetches rate dynamically for a specific target currency.
     * @param targetCurrency The 3-letter code (e.g., EUR, JPY)
     * @return The exchange rate
     */
    public double getRate(String targetCurrency) throws Exception {
        String url = BASE_URL + targetCurrency;

        HttpClient client = HttpClient.newHttpClient();
        HttpRequest request = HttpRequest.newBuilder()
                .uri(URI.create(url))
                .header("User-Agent", USER_AGENT) // Identify our app
                .GET()
                .build();

        HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());

        if (response.statusCode() != 200) {
            throw new ApiException("API Error: Received status code " + response.statusCode());
        }

        return parseRate(response.body(), targetCurrency);
    }

    /**
     * Helper Method: Parses the JSON string manually.
     */
    private double parseRate(String responseBody, String currency) {
        String searchKey = "\"" + currency + "\":";
        int startIndex = responseBody.indexOf(searchKey) + searchKey.length();
        int endIndex = responseBody.indexOf("}", startIndex);
        return Double.parseDouble(responseBody.substring(startIndex, endIndex));
    }
}

/**
 * MAIN EXECUTION: The Command Line Interface
 */
public class CurrencyApp {
    public static void main(String[] args) {
        // --- DATA SETUP ---
        // Major Currencies
        Set<String> majors = new HashSet<>(Arrays.asList("EUR", "GBP", "JPY", "CAD", "AUD"));
        // Minor/Emerging Currencies (Supported by Frankfurter API)
        Set<String> minors = new HashSet<>(Arrays.asList("INR", "BRL", "ZAR", "TRY", "THB"));

        Scanner scanner = new Scanner(System.in);
        CurrencyService service = new CurrencyService();

        System.out.println("=== PROFESSIONAL MULTI-CURRENCY CONVERTER ===");
        System.out.println("Available Majors: " + majors);
        System.out.println("Available Minors: " + minors);
        
        try {
            System.out.print("\nEnter Target Currency Code: ");
            String target = scanner.next().toUpperCase();

            // Validation logic
            if (!majors.contains(target) && !minors.contains(target)) {
                System.out.println("Error: Currency not supported.");
                return;
            }

            System.out.print("Enter Amount in USD: ");
            double amount = scanner.nextDouble();

            System.out.println("Fetching live rate for " + target + "...");
            double rate = service.getRate(target);

            // Output Results
            System.out.println("\n" + "=".repeat(20));
            System.out.printf("Result: %.2f USD = %.2f %s\n", amount, (amount * rate), target);
            System.out.printf("Exchange Rate: 1 USD = %.4f %s\n", rate, target);
            System.out.println("=".repeat(20));

        } catch (InputMismatchException e) {
            System.err.println("Input Error: Please enter numbers only.");
        } catch (Exception e) {
            System.err.println("System Error: " + e.getMessage());
        } finally {
            scanner.close();
            System.out.println("Application Closed.");
        }
    }
}