Deserializing json array using gson

Question

Continuing from this question.

I'm having trouble deserializing the following json array (Sorry for the size):

"geometry": { "type": "Polygon", "coordinates": [ [ [ 771230.894373, 4422896.962001 ],
 [ 804804.852796, 4451159.130080 ], [ 876828.563339, 4417873.954498 ], [ 959794.979827, 
4430944.287708 ], [ 910992.515063, 4372980.866944 ], [ 932488.308736, 4357684.778349 ], 
[ 918573.372386, 4115663.286966 ], [ 834059.614976, 4013708.358795 ], [ 929360.231044, 
3833522.241529 ], [ 1008029.715188, 3776446.653183 ], [ 1061663.445852, 3533717.758754      
], [ 1035703.740599, 3519308.069656 ], [ 1095348.723766, 3396028.487184 ], [ 
1108462.159782, 3230455.268230 ], [ 1083571.121640, 3163122.508021 ], [ 1103953.720405, 
3082716.041755 ], [ 1045722.494771, 3020215.642212 ], [ 1117367.719045, 2915275.458735 
], [ 1141268.013718, 2827405.304519 ], [ 1286729.192338, 2790314.754276 ], [ 
1334329.406601, 2695307.513404 ], [ 829417.592210, 2374337.277646 ], [ 647042.870444, 
2207530.090128 ], [ 370914.873531, 2152159.656850 ], [ 346669.488436, 2173360.227237 ], 
[ 359905.375891, 2251757.174668 ], [ 199905.871774, 2309591.361246 ], [ 129963.835709, 
2361036.252651 ], [ 130208.738589, 2404106.913263 ], [ -964785.432600, 3159802.671416 
], [ -964829.960396, 3338713.127631 ], [ -851005.781060, 3424742.002477 ], [ -
616522.405653, 3491025.523892 ], [ -547749.224241, 3569019.334331 ], [ -403724.067052, 
3628920.873754 ], [ -423973.082428, 3724062.779415 ], [ -333893.350478, 3741450.793542 
], [ -317696.364567, 3774909.265404 ], [ -131414.328674, 3777826.527844 ], [ -
112467.751341, 3830221.719769 ], [ -185682.580436, 3930014.456814 ], [ -194499.084106, 
4129581.855629 ], [ -245950.952751, 4175549.526399 ], [ -42303.076294, 4287174.981681 
], [ -11222.674464, 4271148.905617 ], [ 131633.628071, 4371332.547494 ], [ 
433220.392528, 4427574.250017 ], [ 593119.709103, 4389089.571176 ], [ 719645.442339, 
4451856.882422 ], [ 771230.894373, 4422896.962001 ] ] ] }

If I paste it into a json-viewer, I get this structure:

[geometry]
...
[coordinates] => Array
    (
        [0] => Array
            (
                [0] => Array
                    (
                        [0] => 771230.894373
                        [1] => 4422896.962001
                    )
                [1] => Array
                    (
                        [0] => 804804.852796
                        [1] => 4451159.13008
                    )
                ...
                [n] => Array
        [n] => Array

Now, the array containing the arrays with the coordinates has a variable size. So I figured that in java, this whole object whould be an array, containing a collection of arrays, with each array containing a Collection<double[]>. Something like Collection<double[]>[][].

But gson does not accept this. I get the following error message:

Exception in thread "main" com.google.gson.JsonParseException: Expecting object but     
found array: 2.963610

Which seems weird, as 2.963610 doesnt look like an array to me. But it might have confused me to the point where I'm lost, more or less...

Answer 1

I think I know where your problem comes from, reading the Gson API :

If the object that your are serializing/deserializing is a ParameterizedType (i.e. contains at least one type parameter and may be an array) then you must use the toJson(Object, Type) or fromJson(String, Type) method. Here is an example for serializing and deserialing a ParameterizedType:

Type listType = new TypeToken<LinkedList>() {}.getType();
List target = new LinkedList();
target.add("blah");

Gson gson = new Gson();
String json = gson.toJson(target, listType);
List target2 = gson.fromJson(json, listType);

Knowing that

Type typeOfCollectionOfFoo = new TypeToken<Collection<Foo>>(){}.getType() 

Hope this helps.

Answer 2

The coordinates in JSON is a three-dimensional matrix. With Collection<double[]>[][] you're going one dimension too far. The Collection itself is already one dimension, so you've basically declared a four-dimensional matrix.

With the error message, Gson is basically telling you that it is expecting an object for the fourth dimension, but it instead encountered a double.

The following represent valid three-dimensional matrices which should be perfectly handled by Gson:

• private double[][][] coordinates; (recommended)
• private Collection<double[]>[] coordinates;
• private Collection<double[][]> coordinates;
• private Collection<Collection<double[]>> coordinates;
• private Collection<Collection<Collection<Double>>> coordinates;

---

That said, I would prefer List above Collection in this particular case. With a List you can guarantee that it's been filled with insertion order and you'll be able to get elements by index.