JSON String to Java object using GSON

Question

I am trying to parse json to java.

I have the following string that is valid json according to jsonlint.com

private final static String LOC_JSON = 
         "["
        +"{"
        +"  \"lat1\": 39.737567,"
        +"  \"lat2\": 32.7801399,"
        +"  \"long1\": -104.98471790000002,"
        +"  \"long2\": -96.80045109999998"
        +"},"
        +"  ["
        +"      {"
        +"          \"lat\": {"
        +"              \"b\": 38.88368709500021,"
        +"              \"d\": 40.620468491667026"
        +"          },"
        +"          \"long\": {"
        +"          \"b\": -105.75306170749764,"
        +"          \"d\": -104.675854661387"
        +"          }"
        +"      }"
        +"  ]"
        +"]";

I am trying to parse it into an object and I get the following error. "Expected BEGIN_OBJECT but was BEGIN_ARRAY at line 1 column 2"

            Gson gson = new Gson();
        BoxSearch b = gson.fromJson( LOC_JSON, BoxSearch.class ); 

BoxSearch consists of this.

private Number lat1;
private Number lat2;
private Number long1;
private Number long2;
private Boxes[] boxes;

Boxes is a Latitude object and a Longitude object which are both defined identical.

private String b;
private String d;

I can parse the higher level attributes (lat1,lat2,long1 and long2) into a more simple BoxSearch object that only has those 4 attributes. The trouble comes when the json and the object are more complex. Is it even possible to do what I am trying?

I hope I have provided enough information to get some help. I would be happy to provide more info or even a test project if need be. I am running this as a junit test.

Thanks.

Answer 1

Gson gson = new Gson();
gson.fromJson(jsonStr,YourClass.class);

very easy.

Answer 2

The reason for the error is that your JSON at the top level is an array, not an object. That is covered by GSON throwing "Expected BEGIN_OBJECT but was BEGIN_ARRAY"?.

However, the solution there won't work for your JSON because you have an array of mixed types (an object and an array) rather than an array of a single type of object. For that you're going to have to write a custom deserializer (See The section of the Gson user's guide that covers this) or use Gson's JsonParser class directly and extract the data from the parse tree.

Edit from comments above:

If you're the one creating the JSON, it looks like what you want is an array of BoxSearch objects?

Based on your Java BoxSearch class, you'd need JSON structured like:

[
    {
        "lat1" : 39.737567,
        "lat2" : 32.7801399,
        "long1" : -104.98471790000002,
        "long2" : -96.80045109999998,
        "boxes" : [ 
                    {
                      "lat": {
                          "b": 38.88368709500021,
                          "d": 40.620468491667026
                      },
                      "long": {
                          "b": -105.75306170749764,
                          "d": -104.675854661387
                      }
                    }
                  ]
    }
]

However, the way you have Boxes class defined won't work for that. (Did you mean to have an array of them?). As-is it would need to look like:

class Boxes {
    Box lat;
    @SerializedName("long")
    Box lon;
}

class Box {
   String b;
   String d;
}

Now you have an array containing one type of object (BoxSearch) which you could deserialize with:

Type collectionType = new TypeToken<Collection<BoxSearch>>(){}.getType();
Collection<BoxSearch> boxSearchCollection = gson.fromJson(json, collectionType);

If you really don't need an array of these, get rid of the outer array and simply do:

gson.fromJson(json, BoxSearch.class);