Deserializing json array using gson

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.开发者_开发技巧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...

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.

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.

According to Gson user guide:

Serializing and Deserializing Generic Types
When you call toJson(obj), Gson calls obj.getClass() to get information about the fields to serialize. Similarly, you can typically pass MyClass.class object in the fromJson(json, MyClass.class) method. This works fine as long as the object is a non-generic type. However, if the object is of a generic type, then the generic type information is lost because of Java Type Erasure. Here is an example illustrating the point:

List<String> myStrings = new List<String>();
gson.toJson(myStrings); // Will cause a runtime exception

gson.fromJson(json, myStrings.getClass());

The above call results in a runtime exception because Gson invokes myStrings.getClass() to get its class information, but this method returns a raw class, List.class. This means that Gson has no way of knowing that this is a list of Strings, and not plain objects.

You can solve this problem by specifying the correct parameterized type for your generic type. You can do this by using the TypeToken class.

Type listType = new TypeToken<List<String>>() {}.getType();
gson.toJson(myStrings, listType);

gson.fromJson(json, listType);

The idiom used to get listType actually defines an anonymous local inner class containing a method getType() that returns the fully parameterized type.

Hope this helps.

I think we'd need more details, such as what you wrote for the deserialization.