我写了一个方法：

public static double zamien(int ilosc, String waluta, String kursy) {
        double result = 0.0;

        Pattern pattern = Pattern.compile("(?<kraj>([A-Z]){3}\"):(?<kurs>[0-9]+\\.[0-9]+)");

        if(!kursy.contains(waluta)) {
            throw new IllegalArgumentException("Nie ma takiej waluty!");
        }

        int firstIndex = kursy.indexOf(waluta);
        String krajIkurs = kursy.substring(firstIndex, firstIndex + 11);
        //System.out.println(krajIkurs);

        Matcher matcher = pattern.matcher(krajIkurs);

        if (matcher.matches()) {
            double rate = Double.parseDouble(matcher.group("kurs"));
            result = ilosc * rate;
        } else {
            throw new IllegalArgumentException("zly pattern!");
        }

        return result;

    }

对我来说很好，但我能做得更好吗？

import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;

public class CurrencyConverter {
    private static String rates = "{\"rates\":{\"CAD\":1.5563,\"HKD\":9.1212,\"MYR\":4.9194},\"base\":\"EUR\",\"date\":\"2020-08-21\"}";
private static JsonNode root;

static {
    try {
        root = new ObjectMapper().readTree(rates);
    } catch (Exception e) {
        throw new RuntimeException("Error while parsing rates JSON string", e);
    }
}

public static double change(int value, String country) throws Exception {
    JsonNode ratesNode = root.path("rates");
    if (!ratesNode.has(country)) {
        throw new Exception("Country not found in rates");
    }

    double rate = ratesNode.get(country).asDouble();
    return value * rate;
}

}