如何将JSON转换为Java集合

Question

I'm trying to consume the following API result into a Java object. The code can be seen below.

public Map<String, List<CurrencyData>> gsonCurrency(String answer) {

    Gson g = new Gson();
    CurrencyData currencyData = null;

    Map<String, List<CurrencyData>> object;
    try {
        object = g.fromJson(answer,
                new TypeToken<Map<String, List<CurrencyData>>>().getType());

    } catch (Exception e) {
        throw new OutputFromApiException("HistoricalFlight API output is empty ", e.toString());

    }

    return object;

public class CurrencyData {

    @SerializedName("rates")
    @Expose
    private Rates rates;

    @SerializedName("base")
    @Expose
    private String base;

    @SerializedName("date")
    @Expose
    private String date;

    // Getters & Setters
}

I get the following error.

Expected BEGIN_ARRAY but was BEGIN_OBJECT at line 1 column 11 path $.

Answer 1

Your CurrencyData is not representative of the payload from the API endpoint you are calling. The quickest fix is to change Rates rate to a Map of String currency keys to BigDecimal 's. Don't use Double as you will get precision errors.

    public static class CurrencyData {
        @SerializedName("rates")
        @Expose
        private Map<String, BigDecimal> rates;
        ...
    }

You are also deserializing the wrong data structure. You only need to deserialize an instance of CurrencyData.

    public CurrencyData gsonCurrency(String answer) {
        try {
            return new Gson().fromJson(answer, CurrencyData.class);
        } catch (Exception e) {
            throw new OutputFromApiException("HistoricalFlight API output is empty ", e.toString());
        }
    }

Answer 2

You need to modify your CurrencyData class to look like the following for it to be able to consume your API result.

public final class CurrencyData {

    @SerializedName("rates")
    private Map<String, BigDecimal> rates = new HashMap<>();

    @SerializedName("base")
    private String base;

    @SerializedName("date")
    private String date;

    public Map<String, BigDecimal> getRates() {
        return rates;
    }

    public void setRates(Map<String, BigDecimal> rates) {
        this.rates = rates;
    }

    public String getBase() {
        return base;
    }

    public void setBase(String base) {
        this.base = base;
    }

    public String getDate() {
        return date;
    }

    public void setDate(String date) {
        this.date = date;
    }
}

Now you can do the following example.

Gson gson = new Gson();

CurrencyData data = new CurrencyData();
data.setBase("USD");
data.setDate("2019-01-01");

Map<String, BigDecimal> ratesMap = new HashMap<>();
ratesMap.put("SEK", new BigDecimal("9.1"));
ratesMap.put("DKK", new BigDecimal("8.2"));
data.setRates(ratesMap);

String json = gson.toJson(data);

System.out.println(json);

Which prints:

{"base":"USD","date":"2019-01-01","rates":{"DKK":8.2,"SEK":9.1}}

You can also reverse the process like so.

CurrencyData parsedData = gson.fromJson(json, CurrencyData.class);
// Prints only the "rates"
System.out.println(parsedData.getRates().toString());

Which prints:

{DKK=8.2, SEK=9.1}