Convert complex Java object to Json using Gson

Question

I am using GSON to serialize Java object.

I have a Java class with following properties.

String property1;
Map<String, HashMap> property2 = new HashMap<>();
Map<String, ArrayList<String>> property3 = new HashMap<>();
Map<String, String[]> property4 = new HashMap<>();

I want to convert this to Json. Because of the maps with HashMaps inside, it has become difficult. I know I can get Json of a map with gsonObject.toJson(map). But I want all these properties in the Json Object. (all in one. not Concatenating many objects)

Can anyone help me to get this done?

It cannot be. <String, int> is not legal. You probably mean <String , Integer>. Also, such a set of embedded collections smells fishy... — fge
– fge, Commented Mar 16, 2014 at 16:44

Sotirios Delimanolis · Accepted Answer · 2014-03-16 16:48:01Z

1

I don't see what the problem is. Gson can serialize Maps just fine.

Assuming your class is named Test

Test test = new Test();

test.property1 = "some value";

HashMap<String, Integer> map = new HashMap<>();
map.put("one", 1);
map.put("fourty two", 42);
test.property2.put("property2-key", map);

ArrayList<String> strings = new ArrayList<>(Arrays.asList("string1",
            "string2", "string3"));
test.property3.put("property3-key", strings);

String[] stringArray = { "array1", "array2", "array3" };
test.property4.put("property4-key", stringArray);

Gson gson = new Gson();
String json = gson.toJson(test);
System.out.println(json);

It generates the following

{
    "property1": "some value",
    "property2": {
        "property2-key": {
            "fourty two": 42,
            "one": 1
        }
    },
    "property3": {
        "property3-key": [
            "string1",
            "string2",
            "string3"
        ]
    },
    "property4": {
        "property4-key": [
            "array1",
            "array2",
            "array3"
        ]
    }
}

answered Mar 16, 2014 at 16:48

Sotirios Delimanolis

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7 Comments

harish Over a year ago

The property2 is of type Map<String, HashMap>. The HashMap inside it is of <String, Integer>.

Sotirios Delimanolis Over a year ago

@harsh That is not a problem.

harish Over a year ago

It works only for simple Maps. Does not work for Maps inside Maps.

Sotirios Delimanolis Over a year ago

@harsh If you look at the generated JSON, you have a map inside a map in property2. property2 is a Map and it contains a key property2-key which has a value that is a Map.

harish Over a year ago

It gives an error to me starting with Exception in thread "main" java.lang.IllegalArgumentException: Argument is not an array at java.lang.reflect.Array.getLength(Native Method) at com.google.gson.internal.bind.ArrayTypeAdapter.write(ArrayTypeAdapter.java:91) at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68) at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.write(MapTypeAdapterFactory.java:209)

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