java - How can I prevent gson from converting integers to doubles

Question

I've got integers in my json, and I do not want gson to convert them to doubles. The following does not work:

@Test
public void keepsIntsAsIs(){
    String json="[{"id":1,"quantity":2,"name":"apple"},{"id":3,"quantity":4,"name":"orange"}]";
    GsonBuilder gsonBuilder = new GsonBuilder();
    gsonBuilder.registerTypeAdapter(Double.class,  new DoubleSerializerAsInt());
    Gson gson = gsonBuilder.create();
    List<Map<String, Object>> l = gson.fromJson(json, List.class);
    for(Map<String, Object> item : l){
        System.out.println(item);
    }
}

private static class DoubleSerializerAsInt implements JsonSerializer<Double>{

    @Override
    public JsonElement serialize(Double aDouble, Type type, JsonSerializationContext jsonSerializationContext) {
        int value = (int)Math.round(aDouble);
        return new JsonPrimitive(value);
    }
}

The output is not what I want:

{id=1.0, quantity=2.0, name=apple}
{id=3.0, quantity=4.0, name=orange}

Is there a way to have Integers instead of Doubles in my Map?

{id=1, quantity=2, name=apple}
{id=3, quantity=4, name=orange}

Edit: not all my fields are integer. I've modified my example accordingly. I've read quite a few examples online, including some answers on this site, but it does not work in this particular case.

Answer 1

1) You have to create custom JsonDeserializer and not JsonSerializer like in your question.

2) I don't think this behavior comes from Double deserializer. it is more like json object/map problem

Here is from source code:

case NUMBER:
      return in.nextDouble();

So you can try approach with custom deserializer for Map<String, Object> (or some more generic map if you want) :

public static class MapDeserializerDoubleAsIntFix implements JsonDeserializer<Map<String, Object>>{

    @Override  @SuppressWarnings("unchecked")
    public Map<String, Object> deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
        return (Map<String, Object>) read(json);
    }

    public Object read(JsonElement in) {

        if(in.isJsonArray()){
            List<Object> list = new ArrayList<Object>();
            JsonArray arr = in.getAsJsonArray();
            for (JsonElement anArr : arr) {
                list.add(read(anArr));
            }
            return list;
        }else if(in.isJsonObject()){
            Map<String, Object> map = new LinkedTreeMap<String, Object>();
            JsonObject obj = in.getAsJsonObject();
            Set<Map.Entry<String, JsonElement>> entitySet = obj.entrySet();
            for(Map.Entry<String, JsonElement> entry: entitySet){
                map.put(entry.getKey(), read(entry.getValue()));
            }
            return map;
        }else if( in.isJsonPrimitive()){
            JsonPrimitive prim = in.getAsJsonPrimitive();
            if(prim.isBoolean()){
                return prim.getAsBoolean();
            }else if(prim.isString()){
                return prim.getAsString();
            }else if(prim.isNumber()){

                Number num = prim.getAsNumber();
                // here you can handle double int/long values
                // and return any type you want
                // this solution will transform 3.0 float to long values
                if(Math.ceil(num.doubleValue())  == num.longValue())
                   return num.longValue();
                else{
                    return num.doubleValue();
                }
           }
        }
        return null;
    }
}

To use it you will have to give proper TypeToken to registerTypeAdapter and gson.fromJson function:

String json="[{"id":1,"quantity":2,"name":"apple"}, {"id":3,"quantity":4,"name":"orange"}]";

GsonBuilder gsonBuilder = new GsonBuilder();

gsonBuilder.registerTypeAdapter(new TypeToken<Map <String, Object>>(){}.getType(),  new MapDeserializerDoubleAsIntFix());

Gson gson = gsonBuilder.create();
List<Map<String, Object>> l = gson.fromJson(json, new TypeToken<List<Map<String, Object>>>(){}.getType() );

for(Map<String, Object> item : l)
    System.out.println(item);

String serialized = gson.toJson(l);
System.out.println(serialized);

Result:

{id=1, quantity=2, name=apple}
{id=3, quantity=4, name=orange}
Serialized back to: [{"id":1,"quantity":2,"name":"apple"},{"id":3,"quantity":4,"name":"orange"}]

---

PS: It is just one more option you can try. Personally i feel like creating custom object for your json instead of List<Map<String, Integer>> is much cooler and easier to read way