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);